Qt 6.8 LTS Released!

We are thrilled to announce the release of Qt 6.8, packed with support for new desktop, mobile, and embedded platforms, hundreds of improvements, and exciting new features to boost your development experience and meet the needs of demanding applications.

Check out the highlights here:

Qt Design Studio 4.6 Released

Qt Design Studio 4.6 is here! This month, we present to you an exciting new edition of the Qt Design Studio, further uniting developers and designers. Let's take a deeper look!

 

Qt Design Studio 4.6.1 Released

We fixed the bug that was causing unexpected crashes when importing Figma projects. We recommend updating to this latest version to ensure a smoother experience.

 Read more in the bug report for the details. 


Qt 6.7.3 Released

We have released Qt 6.7.3. As a patch release, Qt 6.7.3 does not introduce new features but contains more than 400 bug fixes, security updates, and other improvements to the top of the Qt 6.7.2 release. See more information about the most important changes and bug fixes from Qt 6.7.3 release note.

Introduction to the QGraphics framework — Creating vector interfaces using the QGraphics View framework

The Qt Graphics View Framework allows you to develop fast and efficient 2D vector graphic scenes. Scenes can contain millions of items, each with their own features and behaviors. By using the Graphics View via PySide6 you get access to this highly performant graphics layer in Python. Whether you're integrating vector graphics views into an existing PySide6 application, or simply want a powerful vector graphics interface for Python, Qt's Graphics View is what you're looking for.

Some common uses of the Graphics View include data visualization, mapping applications, 2D design tools, modern data dashboards and even 2D games.

In this tutorial we'll take our first steps looking at the Qt Graphics View framework, building a scene with some simple vector items. This will allow us to familiarize ourselves with the API and coordinate system, which we'll use later to build more complex examples.

The Graphics View Framework

The Graphics View framework consists of 3 main parts QGraphicsView, QGraphicsScene, and QGraphicsItem, each with different responsibilities.

The framework can be interpreted using the Model-View paradigm, with the QGraphicsScene as the Model and the QGraphicsView as the View. Each scene can have multiple views. The QGraphicsItems within the scene can be considered as items within the model, holding the visual data that the scene combines to define the complete image.

QGraphicsScene is the central component that glues everything together. It acts as a whiteboard on which all items are drawn (circles, rectangles, lines, pixmaps, etc). The QGraphicsView has the responsibility of rendering a given scene -- or part of it, with some transformation (scaling, rotating, shearing) -- to display it to the user. The view is a standard Qt widget and can be placed inside any Qt layout.

QGraphicsScene provides some important functionalities out of the box, so we can use them to develop advanced applications without struggling with low-level details. For example --

  • Collision Detection, detect a graphics item is collided with another item.
  • Item Selection, gives us the ability to deal with multiple items at the same time, for example, the user can select multiple items, and when pressing delete, a function asks the scene to give the list for all selected items, and then delete them.
  • Items discovery, the scene can tell us what items are present (or part of them) at a specific point or inside some defined region, for example, if the user adds an item that intersects with a forbidden area, the program will detect them and give them another (mostly red) color.
  • Events Propagation, the scene receives the events and then propagates them to items.

To define a QGraphicsScene you define it's boundaries or sceneRect which defines the x & y origins and dimensions of the scene. If you don't provide a sceneRect it will default to the minimum bounding rectangle for all child items -- updating as items are added, moved or removed. This is flexible but less efficient.

Items in the scene are represented by QGraphicsItem objects. These are the basic building block of any 2D scene, representing a shape, pixmap or SVG image to be displayed in the scene. Each item has a relative position inside the sceneRect and can have different transformation effects (scale, translate, rotate, shear).

Finally, the QGraphicsView is the renderer of the scene, taking the scene and displaying it -- either wholly or in part -- to the user. The view itself can have transformations (scale, translate, rotate and shear) applied to modify the display without affecting the underlying scene. By default the view will forward mouse and keyboard events to the scene allowing for user interaction. This can be disabled by calling view.setInteractive(False).

A simple scene

Let's start by creating a simple scene. The following code creates QGraphicsScene, defining a 400 x 200 scene, and then displays it in a QGraphicsView.

python
import sys
from PySide6.QtWidgets import QGraphicsScene, QGraphicsView, QApplication

app = QApplication(sys.argv)

# Defining a scene rect of 400x200, with it's origin at 0,0.
# If we don't set this on creation, we can set it later with .setSceneRect
scene = QGraphicsScene(0, 0, 400, 200)

view = QGraphicsView(scene)
view.show()
app.exec()

If you run this example you'll see an empty window.

Empty Scene displayed in a window The empty graphics scene, shown in a QGraphicsView window.

Not very exciting yet -- but this is our QGraphicsView displaying our empty scene.

As mentioned earlier, QGraphicsView is a widget. In Qt any widgets without a parent display as windows. This is why our QGraphicsView appears as a window on the desktop.

Adding items

Let's start adding some items to the scene. There are a number of built-in graphics items which you can customize and add to your scene. In the example below we use QGraphicsRectItem which draws a rectangle. We create the item passing in it's dimensions, and then set it's position pen and brush before adding it to the scene.

python
import sys
from PySide6.QtWidgets import QGraphicsScene, QGraphicsView, QGraphicsRectItem, QApplication
from PySide6.QtGui import QBrush, QPen
from PySide6.QtCore import Qt

app = QApplication(sys.argv)

# Defining a scene rect of 400x200, with it's origin at 0,0.
# If we don't set this on creation, we can set it later with .setSceneRect
scene = QGraphicsScene(0, 0, 400, 200)

# Draw a rectangle item, setting the dimensions.
rect = QGraphicsRectItem(0, 0, 200, 50)

# Set the origin (position) of the rectangle in the scene.
rect.setPos(50, 20)

# Define the brush (fill).
brush = QBrush(Qt.red)
rect.setBrush(brush)

# Define the pen (line)
pen = QPen(Qt.cyan)
pen.setWidth(10)
rect.setPen(pen)

scene.addItem(rect)

view = QGraphicsView(scene)
view.show()
app.exec()

Running the above you'll see a single, rather ugly colored, rectangle in the scene.

A single rectangle in the scene A single rectangle in the scene

Adding more items is simply a case of creating the objects, customizing them and then adding them to the scene. In the example below we add an circle, using QGraphicsEllipseItem -- a circle is just an ellipse with equal height and width.

python
import sys
from PySide6.QtWidgets import QGraphicsScene, QGraphicsView, QGraphicsRectItem, QGraphicsEllipseItem, QApplication
from PySide6.QtGui import QBrush, QPen
from PySide6.QtCore import Qt

app = QApplication(sys.argv)

# Defining a scene rect of 400x200, with it's origin at 0,0.
# If we don't set this on creation, we can set it later with .setSceneRect
scene = QGraphicsScene(0, 0, 400, 200)

# Draw a rectangle item, setting the dimensions.
rect = QGraphicsRectItem(0, 0, 200, 50)

# Set the origin (position) of the rectangle in the scene.
rect.setPos(50, 20)

# Define the brush (fill).
brush = QBrush(Qt.red)
rect.setBrush(brush)

# Define the pen (line)
pen = QPen(Qt.cyan)
pen.setWidth(10)
rect.setPen(pen)

ellipse = QGraphicsEllipseItem(0, 0, 100, 100)
ellipse.setPos(75, 30)

brush = QBrush(Qt.blue)
ellipse.setBrush(brush)

pen = QPen(Qt.green)
pen.setWidth(5)
ellipse.setPen(pen)

# Add the items to the scene. Items are stacked in the order they are added.
scene.addItem(ellipse)
scene.addItem(rect)


view = QGraphicsView(scene)
view.show()
app.exec()

The above code will give the following result.

Scene with two items A scene with two items

The order you add items affects the stacking order in the scene -- items added later will always appear on top of items added first. However, if you need more control you can set the stacking order using .setZValue.

python
ellipse.setZValue(500)
rect.setZValue(200)

Now the circle (ellipse) appears above the rectangle.

Using Zvalue to order items in the scene Using Zvalue to order items in the scene

Try experimenting with setting the Z value of the two items -- you can set it before or after the items are in the scene, and can change it at any time.

Z in this context refers to the Z coordinate. The X & Y coordinates are the horizontal and vertical position in the scene respectively. The Z coordinate determines the relative position of items toward the front and back of the scene -- coming "out" of the screen towards the viewer.

There are also the convenience methods .stackBefore() and .stackAfter() which allow you to stack your QGraphicsItem behind, or in front of another item in the scene.

python
ellipse.stackAfter(rect)

Making items moveable

Our two QGraphicsItem objects are currently fixed in position where we place them, but they don't have to be! As already mentioned Qt's Graphics View framework allows items to respond to user input, for example allowing them to be dragged and dropped around the scene at will. Simple functionality like is actually already built in, you just need to enable it on each QGraphicsItem. To do that we need to set the flag QGraphicsItem.GraphicsItemFlags.ItemIsMoveable on the item.

The full list of graphics item flags is available here.

python
import sys
from PySide6.QtWidgets import QGraphicsScene, QGraphicsView, QGraphicsItem, QGraphicsRectItem, QGraphicsEllipseItem, QApplication
from PySide6.QtGui import QBrush, QPen
from PySide6.QtCore import Qt

app = QApplication(sys.argv)

# Defining a scene rect of 400x200, with it's origin at 0,0.
# If we don't set this on creation, we can set it later with .setSceneRect
scene = QGraphicsScene(0, 0, 400, 200)

# Draw a rectangle item, setting the dimensions.
rect = QGraphicsRectItem(0, 0, 200, 50)

# Set the origin (position) of the rectangle in the scene.
rect.setPos(50, 20)

# Define the brush (fill).
brush = QBrush(Qt.red)
rect.setBrush(brush)

# Define the pen (line)
pen = QPen(Qt.cyan)
pen.setWidth(10)
rect.setPen(pen)

ellipse = QGraphicsEllipseItem(0, 0, 100, 100)
ellipse.setPos(75, 30)

brush = QBrush(Qt.blue)
ellipse.setBrush(brush)

pen = QPen(Qt.green)
pen.setWidth(5)
ellipse.setPen(pen)

# Add the items to the scene. Items are stacked in the order they are added.
scene.addItem(ellipse)
scene.addItem(rect)

ellipse.setFlag(QGraphicsItem.ItemIsMovable)

view = QGraphicsView(scene)
view.show()
app.exec()

In the above example we've set ItemIsMovable on the ellipse only. You can drag the ellipse around the scene -- including behind the rectangle -- but the rectangle itself will remain locked in place. Experiment with adding more items and configuring the moveable status.

If you want an item to be selectable you can enable this by setting the ItemIsSelectable flag, for example here using .setFlags() to set multiple flags at the same time.

python
ellipse.setFlags(QGraphicsItem.ItemIsMovable | QGraphicsItem.ItemIsSelectable)

If you click on the ellipse you'll now see it surrounded by a dashed line to indicate that it is selected. We'll look at how to use item selection in more detail in a later tutorial.

A selected item A selected item in the scene, highlighted with dashed lines

Another way to create objects.

So far we've been creating items by creating the objects and then adding them to the scene. But you can also create an object in the scene directly by calling one of the helper methods on the scene itself, e.g. scene.addEllipse(). This creates the object and returns it so you can modify it as before.

python
import sys
from PySide6.QtWidgets import QGraphicsScene, QGraphicsView, QGraphicsRectItem, QApplication
from PySide6.QtGui import QBrush, QPen
from PySide6.QtCore import Qt

app = QApplication(sys.argv)

scene = QGraphicsScene(0, 0, 400, 200)

rect = scene.addRect(0, 0, 200, 50)
rect.setPos(50, 20)

# Define the brush (fill).
brush = QBrush(Qt.red)
rect.setBrush(brush)

# Define the pen (line)
pen = QPen(Qt.cyan)
pen.setWidth(10)
rect.setPen(pen)

view = QGraphicsView(scene)
view.show()
app.exec()

Feel free to use whichever form you find most comfortable in your code.

You can only use this approach for the built-in QGraphicsItem object types.

Building a more complex scene

So far we've built a simple scene using the basic QGraphicsRectItem and QGraphicsEllipseItem shapes. Now let's use some other QGraphicsItem objects to build a more complex scene, including lines, text and QPixmap (images).

python
from PySide6.QtCore import QPointF, Qt
from PySide6.QtWidgets import QGraphicsRectItem, QGraphicsScene, QGraphicsView, QApplication
from PySide6.QtGui import QBrush, QPainter, QPen, QPixmap, QPolygonF
import sys

app = QApplication(sys.argv)

scene = QGraphicsScene(0, 0, 400, 200)

rectitem = QGraphicsRectItem(0, 0, 360, 20)
rectitem.setPos(20, 20)
rectitem.setBrush(QBrush(Qt.red))
rectitem.setPen(QPen(Qt.cyan))
scene.addItem(rectitem)

textitem = scene.addText("QGraphics is fun!")
textitem.setPos(100, 100)

scene.addPolygon(
    QPolygonF(
        [
            QPointF(30, 60),
            QPointF(270, 40),
            QPointF(400, 200),
            QPointF(20, 150),
        ]),
    QPen(Qt.darkGreen),
)

pixmap = QPixmap("cat.jpg")
pixmapitem = scene.addPixmap(pixmap)
pixmapitem.setPos(250, 70)

view = QGraphicsView(scene)
view.setRenderHint(QPainter.Antialiasing)
view.show()

app.exec()

If you run the example above you'll see the following scene.

Scene with multiple items Scene with multiple items including a rectangle, polygon, text and a pixmap.

Let's step through the code looking at the interesting bits.

Polygons are defined using a series of QPointF objects which give the coordinates relative to the items position. So, for example if you create a polygon object with a point at 30, 20 and then move this polygon object X & Y coordinates 50, 40 then the point will be displayed at 80, 60 in the scene.

Points inside an item are always relative to the item itself, and item coordinates are always relative to the scene -- or the item's parent, if it has one. We'll take a closer look at the Graphics View coordinate system in the next tutorial.

To add an image to the scene we can open it from a file using QPixmap(). This creates a QPixmap object, which can then in turn add to the scene using scene.addPixmap(pixmap). This returns a QGraphicsPixmapItem which is the QGraphicsItem type for the pixmap -- a wrapper than handles displaying the pixmap in the scene. You can use this object to perform any changes to item in the scene.

The multiple layers of objects can get confusing, so it's important to choose sensible variable names which make clear the distinction between, e.g. the pixmap itself and the pixmap item that contains it.

Finally, we set the flag RenderHint,Antialiasing on the view to smooth the edges of diagonal lines. You almost always want to enable this on your views as otherwise any rotated objects will look very ugly indeed. Below is our scene without antialiasing enabled, you can see the jagged lines on the polygon.

Scene with multiple items Scene with antialiasing disabled.

Antialiasing has a (small) performance impact however, so if you are building scenes with millions of rotated items it may in some cases make sense to turn it off.

Adding graphics views to Qt layouts

The QGraphicsView is subclassed from QWidget, meaning it can be placed in layouts just like any other widget. In the following example we add the view to a simple interface, with buttons which perform a basic effect on the view -- raising and lowering selected item's ZValue. This has the effect of allowing us to move items in front and behind other objects.

The full code is given below.

python
import sys

from PySide6.QtCore import Qt
from PySide6.QtGui import QBrush, QPainter, QPen
from PySide6.QtWidgets import (
    QApplication,
    QGraphicsEllipseItem,
    QGraphicsItem,
    QGraphicsRectItem,
    QGraphicsScene,
    QGraphicsView,
    QHBoxLayout,
    QPushButton,
    QSlider,
    QVBoxLayout,
    QWidget,
)


class Window(QWidget):
    def __init__(self):
        super().__init__()

        # Defining a scene rect of 400x200, with it's origin at 0,0.
        # If we don't set this on creation, we can set it later with .setSceneRect
        self.scene = QGraphicsScene(0, 0, 400, 200)

        # Draw a rectangle item, setting the dimensions.
        rect = QGraphicsRectItem(0, 0, 200, 50)
        rect.setPos(50, 20)
        brush = QBrush(Qt.red)
        rect.setBrush(brush)

        # Define the pen (line)
        pen = QPen(Qt.cyan)
        pen.setWidth(10)
        rect.setPen(pen)

        ellipse = QGraphicsEllipseItem(0, 0, 100, 100)
        ellipse.setPos(75, 30)

        brush = QBrush(Qt.blue)
        ellipse.setBrush(brush)

        pen = QPen(Qt.green)
        pen.setWidth(5)
        ellipse.setPen(pen)

        # Add the items to the scene. Items are stacked in the order they are added.
        self.scene.addItem(ellipse)
        self.scene.addItem(rect)

        # Set all items as moveable and selectable.
        for item in self.scene.items():
            item.setFlag(QGraphicsItem.ItemIsMovable)
            item.setFlag(QGraphicsItem.ItemIsSelectable)

        # Define our layout.
        vbox = QVBoxLayout()

        up = QPushButton("Up")
        up.clicked.connect(self.up)
        vbox.addWidget(up)

        down = QPushButton("Down")
        down.clicked.connect(self.down)
        vbox.addWidget(down)

        rotate = QSlider()
        rotate.setRange(0, 360)
        rotate.valueChanged.connect(self.rotate)
        vbox.addWidget(rotate)

        view = QGraphicsView(self.scene)
        view.setRenderHint(QPainter.Antialiasing)

        hbox = QHBoxLayout(self)
        hbox.addLayout(vbox)
        hbox.addWidget(view)

        self.setLayout(hbox)

    def up(self):
        """ Iterate all selected items in the view, moving them forward. """
        items = self.scene.selectedItems()
        for item in items:
            z = item.zValue()
            item.setZValue(z + 1)

    def down(self):
        """ Iterate all selected items in the view, moving them backward. """
        items = self.scene.selectedItems()
        for item in items:
            z = item.zValue()
            item.setZValue(z - 1)

    def rotate(self, value):
        """ Rotate the object by the received number of degrees """
        items = self.scene.selectedItems()
        for item in items:
            item.setRotation(value)


app = QApplication(sys.argv)

w = Window()
w.show()

app.exec()

If you run this, you will get a window like that shown below. By selecting an item in the graphics view and then clicking either the "Up" or "Down" button you can move items up and down within the scene -- behind and in front of one another. The items are all moveable, so you can drag them around too. Clicking on the slider will rotate the currently selected items by the set number of degrees.

A graphics scene with some custom controls A graphics scene with some custom controls

The raising and lowering is handled by our custom methods up and down, which work by iterating over the currently selected items in the scene -- retrieved using scene.selectedItems() and then getting the items z value and increasing or decreasing it respectively.

python
    def up(self):
        """ Iterate all selected items in the view, moving them forward. """
        items = self.scene.selectedItems()
        for item in items:
            z = item.zValue()
            item.setZValue(z + 1)

While rotation is handled using the item.setRotation method. This receives the current angle from the QSlider and again, applies it to any currently selected items in the scene.

python

    def rotate(self, value):
        """ Rotate the object by the received number of degrees. """
        items = self.scene.selectedItems()
        for item in items:
            item.setRotation(value)

Take a look at the QGraphicsItem documentation for some other properties you can control with widgets and try extending the interface to allow you to change them dynamically.

Hopefully this quick introduction to the Qt Graphics View framework has given you some ideas of what you can do with it. In the next tutorials we'll look at how events and user interaction can be handled on items and how to create custom & compound items for your own scenes.

KD Reports 2.3.0

We’re pleased to announce the release of KD Reports 2.3.0, the latest version of our reporting tool for Qt applications. This marks our first major update in two years, bringing several bug fixes and new features that further improve the experience of generating reports.

What is KD Reports?

KD Reports is a versatile tool for generating reports directly from Qt applications. It supports creating printable and exportable reports using code or XML, featuring elements like text, tables, charts, headers, and footers. Whether for visualizing database content, generating invoices, or producing formatted printouts, KD Reports makes it easy to create structured reports within your Qt projects.

What’s New in KD Reports 2.3.0?

The new release includes essential bug fixes and feature enhancements that make KD Reports even more robust and user-friendly.

Bug Fixes

The 2.3.0 release addresses several important issues to improve stability and compatibility. One major fix resolves an infinite loop and other problems caused by changes in QTextFormat behavior in Qt 6.7. Right-aligned tabs, which previously didn’t work when paragraph margins were set, have also been corrected. High-DPI rendering has been improved to eliminate blurriness in displays where the device pixel ratio (DPR) is not equal to 1. Furthermore, an issue with result codes being overwritten in the KDReportsPreviewDialog has been fixed. Finally, table borders, which were lost after upgrading to Qt 6.8, now behave as expected, maintaining their cell borders throughout.

New Features

KD Reports 2.3.0 introduces several new features aimed at providing more customization and flexibility in report generation. For instance, the AutoTableElement now supports customization of header styling via the new setHorizontalHeaderFormatFunction and setVerticalHeaderFormatFunction, which are demonstrated in the PriceList example. Additionally, individual table cell formatting has been enhanced with the setCellFormatFunction, allowing for customization of borders and padding. Text alignment within table cells has also been improved with the new setVerticalAlignment feature, making it easy to vertically center or top-align text when using different font sizes within the same row.

The AbstractTableElement now allows setting column constraints while leaving some columns without constraints—just pass {} for unconstrained columns. This feature is particularly useful when setting constraints for columns further to the right. Also, the TableElement has gained rowCount() and columnCount() methods, which can be used in dynamic scenarios, such as applying alternate background colors to rows.

Lastly, you can now disable the progress dialog during printing or PDF export using setProgressDialogEnabled(false). This is useful for applications that generate multiple documents or handle progress tracking internally, offering more control over the user interface during these operations.

You can explore all the new features and improvements in KD Reports 2.3.0 on its GitHub page. Download the latest release and check out the detailed changes to see how they can enhance your reporting tasks. Feel free to share your feedback or report any issues you encounter along the way.

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Implementing an Audio Mixer, Part 2

Recap

In Part 1, we covered PCM audio and superimposing waveforms, and developed an algorithm to combine an arbitrary number of audio streams into one.

Now we need to use these ideas to finish a full implementation using Qt Multimedia.

Using Qt Multimedia for Audio Device Access

So what do we need? Well, we want to use a single QAudioOutput, which we pass an audio device and a supported audio format.

We can get those like this:

const QAudioDeviceInfo &device = QAudioDeviceInfo::defaultOutputDevice();
const QAudioFormat &format = device.preferredFormat();

Let’s construct our QAudioOutput object using the device and format:

static QAudioOutput audioOutput(device, format);

Now, to use it to write data, we have to call start on the audio output, passing in a QIODevice *.

Normally we would use the QIODevice subclass QBuffer for a single audio buffer. But in this case, we want our own subclass of QIODevice, so we can combine multiple buffers into one IO device.

We’ll call our subclass MixerStream. This is where we will do our bufferCombine, and keep our member list of streams mStreams.

We will also need another stream type for mStreams. For now let’s just call it DecoderStream, forward declare it, and worry about its implementation later.

One thing that’s good to know at this point is DecoderStream objects will get the data buffers we need by decoding audio data from a file. Because of this, we’ll need to keep our audio format from above to as a data member mFormat. Then we can pass it to decoders when they need it.

Implementing MixerStream

Since we are subclassing QIODevice, we need to provide reimplementations for these two protected virtual functions:

virtual qint64 QIODevice::readData(char *data, qint64 maxSize);
virtual qint64 QIODevice::writeData(const char *data, qint64 maxSize);

We also want to provide a way to open new streams that we’ll add to mStreams, given a filename. We’ll call this function openStream. We can also allow looping a stream multiple times, so let’s add a parameter for that and give it a default value of 1.

Additionally, we’ll need a user-defined destructor to delete any pointers in the list that might remain if the MixerStream is abruptly destructed.

// mixerstream.h

#pragma once

#include <QAudioFormat>
#include <QAudioOutput>
#include <QIODevice>

class DecodeStream;

class MixerStream : public QIODevice
{
    Q_OBJECT

public:
    explicit MixerStream(const QAudioFormat &format);
    ~MixerStream();

    void openStream(const QString &fileName, int loops = 1);

protected:
    qint64 readData(char *data, qint64 maxSize) override;
    qint64 writeData(const char *data, qint64 maxSize) override;

private:
    QAudioFormat mFormat;
    QList<DecodeStream *> mStreams;
};

Notice that combineSamples isn’t in the header. It’s a pretty basic function that doesn’t require any members, so we can just implement it as a free function.

Let’s put it in a header mixer.h and wrap it in a namespace:

// mixer.h

#pragma once

#include <QtGlobal>

#include <limits>

namespace Mixer
{
inline qint16 combineSamples(qint32 samp1, qint32 samp2)
{
    const auto sum = samp1 + samp2;
    if (std::numeric_limits<qint16>::max() < sum)
        return std::numeric_limits<qint16>::max();
    
    if (std::numeric_limits<qint16>::min() > sum)
        return std::numeric_limits<qint16>::min();
    
    return sum;
}
} // namespace Mixer

There are some very basic things we can get out of the way quickly in the MixerStream cpp file. Recall that we must implement these member functions:

explicit MixerStream(const QAudioFormat &format);
~MixerStream();

void openStream(const QString &fileName, int loops = 1);

qint64 readData(char *data, qint64 maxSize) override;
qint64 writeData(const char *data, qint64 maxSize) override;

The constructor is very simple:

MixerStream::MixerStream(const QAudioFormat &format)
    : mFormat(format)
{
    setOpenMode(QIODevice::ReadOnly);
}

Here we use setOpenMode to automatically open our device in read-only mode, so we don’t have to call open() directly from outside the class.

Also, since it’s going to be read-only, our reimplementation of QIODevice::writeData will do nothing:

qint64 MixerStream::writeData([[maybe_unused]] const char *data,
                              [[maybe_unused]] qint64 maxSize)
{
    Q_ASSERT_X(false, "writeData", "not implemented");
    return 0;
}

The custom destructor we need is also quite simple:

MixerStream::~MixerStream()
{
    while (!mStreams.empty())
        delete mStreams.takeLast();
}

readData will be almost exactly the same as the implementation we did earlier, but returning qint64. The return value is meant to be the amount of data written, which in our case is just the maxSize argument given to it, as we write fixed-size buffers.

Additionally, we should call qAsConst (or std::as_const) on mStreams in the range-for to avoid detaching the Qt container. For more on qAsConst and range-based for loops, see Jesper Pederson’s blog post on the topic.

qint64 MixerStream::readData(char *data, qint64 maxSize)
{
    memset(data, 0, maxSize);

    constexpr qint16 bitDepth = sizeof(qint16);
    const qint16 numSamples = maxSize / bitDepth;

    for (auto *stream : qAsConst(mStreams))
    {
        auto *cursor = reinterpret_cast<qint16 *>(data);
        qint16 sample;

        for (int i = 0; i < numSamples; ++i, ++cursor)
            if (stream->read(reinterpret_cast<char *>(&sample), bitDepth))
                *cursor = Mixer::combineSamples(sample, *cursor);
    }

    return maxSize;
}

That only leaves us with openStream. This one will require us to discuss DecodeStream and its interface.

The function should construct a new DecodeStream on the heap, which will need a file name and format. DecodeStream, as implied by its name, needs to decode audio files to PCM data. We’ll use a QAudioDecoder within DecodeStream to accomplish this, and for that, we need to pass mFormat to the constructor. We also need to pass loops to the constructor, as each stream can have a different number of loops.

Now our constructor call will look like this:

DecodeStream(fileName, mFormat, loops);

We can then use operator<< to add it to mStreams.

Finally, we need to remove it from the list when it’s done. We’ll give it a Qt signal, finished, and connect it to a lambda expression that will remove the stream from the list and delete the pointer.

Our completed openStream function now looks like this:

void MixerStream::openStream(const QString &fileName, int loops)
{
    auto *decodeStream = new DecodeStream(fileName, mFormat, loops);
    mStreams << decodeStream;
    
    connect(decodeStream, &DecodeStream::finished, this, [this, decodeStream]() {
        mStreams.removeAll(decodeStream);
        decodeStream->deleteLater();
    });
}

Recall from earlier that we call read on a stream, which takes a char * to which the read data will be copied and a qint64 representing the size of the data.

This is a QIODevice function, which will internally call readData. Thus, DecoderStream also needs to be a QIODevice.

Getting PCM Data for DecodeStream

In DecodeStream, we need readData to spit out PCM data, so we need to decode our audio file to get its contents in PCM format. In Qt Multimedia, we use a QAudioDecoder for this. We pass it an audio format to decode to, and a source device, in this case a QFile file handle for our audio file.

When a QAudioDecoder‘s start method is called, it will begin decoding the source file in a non-blocking manner, emitting a signal bufferReady when a full buffer of decoded PCM data is available.

On that signal, we can call the decoder’s read method, which gives us a QAudioBuffer. To store in a data member in DecodeStream, we use a QByteArray, which we can interact with using QBuffers to get a QIODevice interface for reading and writing. This is the ideal way to work with buffers of bytes to read or write in Qt.

We’ll make two QBuffers: one for writing data to the QByteArray (we’ll call it mInputBuffer), and one for reading from the QByteArray (we’ll call it mOutputBuffer). The reason for using two buffers rather than one read/write buffer is so the read and write positions can be independent. Otherwise, we will encounter more stuttering.

So when we get the bufferReady signal, we’ll want to do something like this:

const QAudioBuffer buffer = mDecoder.read();
mInputBuf.write(buffer.data<char>(), buffer.byteCount());

We’ll also need to have some sort of state enum. The reason for this is that when we are finished with the stream and emit finished(), we remove and delete the stream from a connected lambda expression, but read might still be called before that has completed. Thus, we want to only read from the buffer when the state is Playing.

Let’s update mixer.h to put the enum in namespace Mixer:

#pragma once

#include <QtGlobal>

#include <limits>

namespace Mixer
{
enum State
{
    Playing,
    Stopped
};

inline qint16 combineSamples(qint32 samp1, qint32 samp2)
{
    const auto sum = samp1 + samp2;

    if (std::numeric_limits<qint16>::max() < sum)
        return std::numeric_limits<qint16>::max();

    if (std::numeric_limits<qint16>::min() > sum)
        return std::numeric_limits<qint16>::min();

    return sum;
}
} // namespace Mixer

Implementing DecodeStream

Now that we understand all the data members we need to use, let’s see what our header for DecodeStream looks like:

// decodestream.h

#pragma once

#include "mixer.h"

#include <QAudioDecoder>
#include <QBuffer>
#include <QFile>

class DecodeStream : public QIODevice
{
    Q_OBJECT

public:
    explicit DecodeStream(const QString &fileName, const QAudioFormat &format, int loops);

protected:
    qint64 readData(char *data, qint64 maxSize) override;
    qint64 writeData(const char *data, qint64 maxSize) override;

signals:
    void finished();

private:
    QFile mSourceFile;
    QByteArray mData;
    QBuffer mInputBuf;
    QBuffer mOutputBuf;
    QAudioDecoder mDecoder;
    QAudioFormat mFormat;
    Mixer::State mState;
    int mLoopsLeft;
};

In the constructor, we’ll initialize our private members, open the DecodeStream in read-only (like we did earlier), make sure we open the QFile and QBuffers successfully, and finally set up our QAudioDecoder.

DecodeStream::DecodeStream(const QString &fileName, const QAudioFormat &format, int loops)
    : mSourceFile(fileName)
    , mInputBuf(&mData)
    , mOutputBuf(&mData)
    , mFormat(format)
    , mState(Mixer::Playing)
    , mLoopsLeft(loops)
{
    setOpenMode(QIODevice::ReadOnly);

    const bool valid = mSourceFile.open(QIODevice::ReadOnly) && 
                       mOutputBuf.open(QIODevice::ReadOnly) &&
                       mInputBuf.open(QIODevice::WriteOnly);

    Q_ASSERT(valid);

    mDecoder.setAudioFormat(mFormat);
    mDecoder.setSourceDevice(&mSourceFile);
    mDecoder.start();

    connect(&mDecoder, &QAudioDecoder::bufferReady, this, [this]() {
        const QAudioBuffer buffer = mDecoder.read();
        mInputBuf.write(buffer.data<char>(), buffer.byteCount());
    });
}

Once again, our QIODevice subclass is read-only, so our writeData method looks like this:

qint64 DecodeStream::writeData([[maybe_unused]] const char *data,
                               [[maybe_unused]] qint64 maxSize)
{
    Q_ASSERT_X(false, "writeData", "not implemented");
    return 0;
}

Which leaves us with the last part of the implementation, DecodeStream‘s readData function.

We zero out the char * with memset to avoid any noise if there are areas that are not overwritten. Then we simply read from the QByteArray into the char * if mState is Mixer::Playing.

We check to see if we finished reading the file with QBuffer::atEnd(), and if we are, we decrement the loops remaining. If it’s zero now, that was the last (or only) loop, so we set mState to stopped, and emit finished(). Either way we seek back to position 0. Now if there are loops left, it starts reading from the beginning again.

qint64 DecodeStream::readData(char *data, qint64 maxSize)
{
    memset(data, 0, maxSize);

    if (mState == Mixer::Playing)
    {
        mOutputBuf.read(data, maxSize);
        if (mOutputBuf.size() && mOutputBuf.atEnd())
        {
            if (--mLoopsLeft == 0)
            {
                mState = Mixer::Stopped;
                emit finished();
            }
            
            mOutputBuf.seek(0);
        }
    }

    return maxSize;
}

Now that we’ve implemented DecodeStream, we can actually use MixerStream to play two audio files at the same time!

Using MixerStream

Here’s an example snippet that shows how MixerStream can be used to route two simultaneous audio streams into one system mixer channel:

const auto &device = QAudioDeviceInfo::defaultOutputDevice();
const auto &format = device.preferredFormat();

auto mixerStream = std::make_unique<MixerStream>(format);

auto *audioOutput = new QAudioOutput(device, format);
audioOutput->setVolume(0.5);
audioOutput->start(mixerStream.get());

mixerStream->openStream(QStringLiteral("/path/to/some/sound.wav"));
mixerStream->openStream(QStringLiteral("/path/to/something/else.mp3"), 3);


Final Remarks

The code in this series of posts is largely a reimplementation of Lova Widmark’s project QtMixer. Huge thanks to her for a great and lightweight implementation. Check the project out if you want to use something like this for a GPL-compliant project (and don’t mind that it uses qmake).

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Implementing an Audio Mixer, Part 1

Motivation

When using Qt Multimedia to play audio files, it’s common to use QMediaPlayer, as it supports a larger variety of formats than QSound and QSoundEffect. Consider a Qt application with several audio sources; for example, different notification sounds that may play simultaneously. We want to avoid cutting notification sounds off when a new one is triggered, and we don’t want to construct a queue for notification sounds, as sounds will play at the incorrect time. We instead want these sounds to overlap and play simultaneously.

Ideally, an application with audio has one output stream to the system mixer. This way in the mixer control, different applications can be set to different volume levels. However, a QMediaPlayer instance can only play one audio source at a time, so each notification would have to construct a new QMediaPlayer. Each player in turn opens its own stream to the system.

The result is a huge number of streams to the system mixer being opened and closed all the time, as well as QMediaPlayers constantly being constructed and destructed.

To resolve this, the application needs a mixer of its own. It will open a single stream to the system and combine all the audio into the one stream.

Before we can implement this, we first need to understand how PCM audio works.

PCM

As defined by Wikipedia:

Pulse-code modulation (PCM) is a method used to digitally represent sampled analog signals. It is the standard form of digital audio in computers, compact discs, digital telephony and other digital audio applications. In a PCM stream, the amplitude of the analog signal is sampled at uniform intervals, and each sample is quantized to the nearest value within a range of digital steps.

Here you can see how points are sampled in uniform intervals and quantized to the closest number that can be represented.

Sampling and quantization of a signal (red) for 4-bit LPCM over a time domain at specific frequency.

Image Source: Wikipedia

Description from Wikipedia: Sampling and quantization of a signal (red) for 4-bit LPCM over a time domain at specific frequency.

Think of a PCM stream as a humongous array of bytes. More specifically, it’s an array of samples, which are either integer or float values and a certain number of bytes in size. The samples are these discrete amplitude values from a waveform, organized contiguously. Think of the each element as being a y-value of a point along the wave, with the index representing an offset from x=0 at a uniform time interval.

Here is a graph of discretely sampled points along a sinusoidal waveform similar to the one above:

A discrete waveform

Image Source: Wikimedia Commons

Description from Wikimedia Commons: Image of a discrete time sinusoid

Let’s say we have an audio waveform that is a simple sine wave, like the above examples. Each point taken at discrete intervals along the curve here is a sample, and together they approximate a continuous waveform. The distance between the samples along the x-axis is a time delta; this is the sample period. The sample rate is the inverse of this, the number of samples that are played in one second. The typical standard sample rate for audio on CDs is 44100 Hz – we can’t really hear that this data is discrete (plus, the resultant sound wave from air movement is in fact a continuous waveform).

We also have to consider the y-axis here, which represents the amplitude of the waveform at each sampled point. In the image above, amplitude A is normalized such that A\in[−1,1]. In digital audio, there are a few different ways to represent amplitude. We can’t represent all real numbers on a computer, so the representation of the range of values varies in precision.

For example, let’s say we have two different representations of the wave above: 8-bit signed integer and 16-bit signed integer. The normalized value 1 from the image above maps to (2^{8}\div{2})−1=127 with 8-bit representation and (2^{16}\div2)−1=32767 with 16-bit. Therefore, with 16-bit representation, we have 128 times as many possible values to represent the same range; it is more precise, but the required size to store each 16-bit sample is double that of 8-bit samples.

We call the chosen representation, and thus the size of each sample, the bitdepth. Some common bitdepths are 16-bit int, 24-bit int, and 32-bit float, but there are many others in existence.

Let’s consider a huge stream of 16-bit samples and a sample rate of 44100 Hz. We write samples to the audio device periodically with a fixed-size buffer; let’s say it is 4096 bytes. The device will play each sample in the buffer at the aforementioned rate. Since each sample is a contiguous 2-byte short, we can fit 2048 samples into the buffer at once. We need to write 44100 samples in one second, so the whole buffer will be written around 21.5 times per second.

What if we have two different waveforms though, and what if one starts halfway through the other one? How do we mix them so that this buffer contains the data from both sources?

Waveform Superimposition

In the study of waves, you can superimpose two waves by adding them together. Let’s say we have two different discrete wave approximations, each represented by 20 signed 8-bit integer values. To superimpose them, for each index, add the values at that index. Some of these sums will exceed the limits of 8-bit representation, so we clamp them at the end to avoid signed integer overflow. This is known as hard clipping and is the phenomenon responsible for digital overdrive distortion.

x Wave 1 (y_1) Wave 2 (y_2) Sum (y_1+y_2) Clamped Sum
0 +60 −100 −40 −40
1 −120 +80 −40 −40
2 +40 +70 +110 +110
3 −110 −100 −210 −128
4 +50 −110 −60 −60
5 −100 +60 −40 −40
6 +70 +50 +120 +120
7 −120 −120 −240 −128
8 +80 −100 −20 −20
9 −80 +40 −40 −40
10 +90 +80 +170 +127
11 −100 −90 −190 −128
12 +60 −120 −60 −60
13 −120 +70 −50 −50
14 +80 −120 −40 −40
15 −110 +80 −30 −30
16 +90 −100 −10 −10
17 −110 +90 −20 −20
18 +100 −110 −10 −10
19 −120 −120 −240 −128

Now let’s implement this in C++. We’ll start small, and just combine two samples.

Note: we will use qint types here, but qint16 will be the same as int16_t and short on most systems, and similarly qint32 will correspond to int32_t and int.

qint16 combineSamples(qint32 samp1, qint32 samp2)
{
    const auto sum = samp1 + samp2;

    if (std::numeric_limits<qint16>::max() < sum)
        return std::numeric_limits<qint16>::max();

    if (std::numeric_limits<qint16>::min() > sum)
        return std::numeric_limits<qint16>::min();

    return sum;
}

This is quite a simple implementation. We use a function combineSamples and pass in two 16-bit values, but they will be converted to 32-bit as arguments and summed. This sum is clamped to the limits of 16-bit integer representation using std::numeric_limits in the <limits> header of the standard library. We then return the sum, at which point it is re-converted to a 16-bit value.

Combining Samples for an Arbitrary Number of Audio Streams

Now consider an arbitrary number of audio streams n. For each sample position, we must sum the samples of all n streams.

Let’s assume we have some sort of audio stream type (we’ll implement it later), and a list called mStreams containing pointers to instances of this stream type. We need to implement a function that loops through mStreams and makes calls to our combineSamples function, accumulating a sum into a new buffer.

Assume each stream in mStreams has a member function read(char *, qint64). We can copy one sample into a char * by passing it to read, along with a qint64 representing the size of a sample (bitdepth). Remember that our bitdepth is 16-bit integer, so this size is just sizeof(qint16).

Using read on all the streams in mStreams and calling combineSamples to accumulate a sum might look something like this:

qint16 accumulatedSum = 0;

for (auto *stream : mStreams)
{
    // call stream->read(char *, qint64)
    // to read a sample from the stream into streamSample
    qint16 streamSample;
    stream->read(reinterpret_cast<char *>(&streamSample), sizeof(qint16)));
    
    // accumulate
    accumulatedSum = combineSamples(sample, accumulatedSum);
}

The first pass will add samples from the first stream to zero, effectively copying it to accumulatedSum. When we move to another stream, the samples from the second stream will be added to those copied values from the first stream. This continues, so the call to combineSamples for a third stream would combine the third stream’s sample with the sum of the first two. We continue to add directly into the buffer until we have combined all the streams.

Combining All Samples for a Buffer

Now let’s use this concept to add all the samples for a buffer. We’ll make a function that takes a buffer char *data and its size qint64 maxSize. We’ll write our accumulated samples into this buffer, reading all samples from the streams and adding them using the method above.

The function signature looks like this:

void readData(char *data, qint64 maxSize);

Let’s achieve more efficiency by using a constexpr variable for the bitdepth:

constexpr qint16 bitDepth = sizeof(qint16);

There’s no reason to call sizeof multiple times, especially considering sizeof(qint16) can be evaluated as a literal at compile-time.

With the size of each sample and the size of the buffer, we can get the total number of samples to write:

const qint16 numSamples = maxSize / bitDepth;

For each stream in mStreams we need to read each sample up to numSamples. As the sample index increments, a pointer to the buffer data needs to also be incremented, so we can write our results at the correct location in the buffer.

That looks like this:

void readData(char *data, qint64 maxSize)
{
    // start with 0 in the buffer
    memset(data, 0, maxSize);

    constexpr qint16 bitDepth = sizeof(qint16);
    const qint16 numSamples = maxSize / bitDepth;

    for (auto *stream : mStreams)
    {
        // this pointer will be incremented across the buffer
        auto *cursor = reinterpret_cast<qint16 *>(data);
        qint16 sample;

        for (int i = 0; i < numSamples; ++i, ++cursor)
            if (stream->read(reinterpret_cast<char *>(&sample), bitDepth))
                *cursor = combineSamples(sample, *cursor);
    }
}

The idea here is that we can start playing new audio sources by adding new streams to mStreams. If we add a second stream halfway through a first stream playing, the next buffer for the first stream will be combined with the first buffer of this new stream. When we’re done playing a stream, we just drop it from the list.

 

Next Steps

In Part 2, we’ll use Qt Multimedia to fully implement our mixer, connect to our audio device, and test it on some audio files.

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Release 4.2.0: New Felgo Hot Reload, OAuth 2.0 Plugin, Universal Links, Android 14 & Qt Creator 14

The Felgo 4.2.0 update adds many new features and improvements for Felgo QML Hot Reload. These include support for singletons, better handling of JavaScript and bindings changes, and improved debugging and error-handling capabilities. The release also adds a new RFC-conform OAuth 2.0 plugin, universal app link handling for your app, and updates to Felgo Plugins, Android 14 Support, and Qt Creator 14 for the latest Google Play Store requirements. Read on to learn more about the release.

Qt and Trivial Relocation (Part 5)

In the previous posts of this series (if you’ve missed them: parts 1, 2, 3, and 4), we have learned about relocation and trivial relocation.

We have explored what relocation means, what trivial relocation means, and how it can be used to optimize the implementation of certain data structures, such as the reallocation of a vector-like container (std::vector, QVector and so on).

Furthermore, we have explored how trivial relocation is connected to move assignments and how some types may be trivially relocatable for move construction but not for assignments. This property can be used to further optimize other operations, such as swaps, erasure, and swap-based algorithms such as std::sort or std::rotate.

In this blog post, we’ll have a look at trivial relocation from a “Standard C++” point of view.

Is trivial relocation allowed in Standard C++?

That’s probably a question we should have asked as soon as we started this journey. Of course, the answer is no, it is not allowed!

Remember how trivial relocation works: we use memcpy a source object(‘s representation) into some storage, and claim that operation realizes the equivalent of move-constructing the source into that storage, plus destroying the source.

The problem is that one can’t just put data into some storage and pretend that an object exists in there. This is only allowed for a specific set of types, such as trivially copyable types. (Note that if a type is trivially copyable, then Qt automatically considers it trivially relocatable.)

However, as we have discussed, many interesting types (QString, std::vector, std::unique_ptr, …) are not trivially copyable, but they would still benefit from trivial relocatability.

Qt simply ignores the Standard wording and just uses trivial relocatability because it makes our code faster, reduces template bloat, reduces compilation times, and so on. I call this “Undefined Behavior That Works In Practice”: yes, the operation is illegal, but so are many others. Qt is in good company here; many other popular libraries employ the same “illegal” optimization — to name a few: Folly, EASTL, BSL, and possibly others; a survey is available here.

Towards standardization

A few years ago, P1144 (“std::is_trivially_relocatable”) emerged. This proposal (which, by the way, has reached its eleventh revision at the time of this writing) was inspired by the work in many existing libraries (including Qt) and introduced the necessary language and library facilities in order to give well-defined semantics to relocation and trivial relocation. Despite its many iterations, it never made it over the “finish line”, voted into C++.

Some time after P1144’s initial revision, P2786 (“Trivial Relocatability For C++26”) was proposed. This was an alternative design, w.r.t. P1144, with different relocation semantics and a different set of enablers.

P1144 and P2786 “competed” for a little while; then, during the ISO C++ meeting in Tokyo (March 2024), the Evolution Working Group voted to adopt P2786 for C++26.

That’s great news, right?

Well, not really. After some analysis, it turned out that P2786’s design is limiting and not user-friendly, to the point that there have been serious concerns that existing libraries may not make use of it at all. In particular, P2786’s relocation semantics do not match Qt’s. With my Qt hat on, it soon became clear that we could not have used P2786 in Qt to replace our own implementation of trivial relocation. This fact raised some serious concerns.

For this reason, I co-authored a “petition paper” (P3236), together with many other library authors, asking EWG to reconsider its decision of going ahead with P2786.

I also have analyzed in detail the problems that I have with P2786’s design in a separate paper (P3233). I don’t want to go through the complete list of issues in a blog post — please read the paper and send feedback. 🙂

In June 2024, during the ISO C++ meeting in St. Louis, I presented P3233 to EWG. If you’re interested and/or want a summary of the issues I raised, check out the slides that I used during my presentation.

P3278R0 (“Analysis of interaction between relocation, assignment, and swap”) was also presented, making many of the remarks that I’ve also made in my paper; I consider a good sign that different authors independently reached the same conclusions.

Eventually, EWG voted to take P2786 back, given the issues raised. I consider it a victory in the face of the danger of standardizing something that does not match the current practices. Still, I understand the frustration at pouring a tremendous amount of work into getting a feature into C++, only to get asked to go back to the drawing board.

So what now?

I hope I can find the time in the next few months to work more on trivial relocation, and present more papers at the next ISO C++ meeting in Wrocław. I think the goal to have trivial relocatability in C++26 is doable; we “just” have to iron out the details.

Regarding the two competing proposals (P1144 versus P2786), in P3236 I actually make an argument that they should be “merged”: each one has some design characteristics that are missing in the other.

Therefore, the trivial relocation story is anything but over. Stay tuned for more updates, hopefully in not the too distant future. 🙂

In the meanwhile, thank you for reading so far.

Overview about all installments:

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Formatting Selected Text in QML

Motivation

Let’s say we’re working on a QML project that involves a TextEdit.

There’s some text in it:

here is some text

We want to select part of this text and hit ctrl+B to make it bold:

here is some text

In Qt Widgets, this is trivial, but not so much in QML – we can get font.bold of the entire TextEdit, but not of just the text in the selection. We have to implement formattable selections manually.

To do this, there are two approaches we’ll look at:

  1. The first is to hack it together by getting the formatted text from the selection and editing this. Rather than setting properties of selected text, this solution actually inserts or removes formatting symbols from the underlying rich text source.
  2. The other way to do this is to create a QML object that is implemented in C++ and exposed to TextEdit as a property. This way we can make use of QTextDocument and QTextCursor to actually set text properties within the selection area. This more closely follows the patterns expected in Qt.

In Qt 6.7, the TextEdit QML element does have a cursorSelection property that works in this way, and by dissecting its implementation, we can write a pseudo-backport for other Qt versions.

Before we do this, let’s take a look at the hacky QML/JS solution.

Hacky Approach

We start by focusing on just making ctrl+B bold shortcuts work:

TextEdit {
    id: txtEdit

    anchors.fill: parent
    selectByMouse: true
    textFormat: TextEdit.RichText
}

Shortcut {
    sequence: StandardKey.Bold
    onActivated: {
        if (txtEdit.selectedText.length > 0)
        {
            const start = txtEdit.selectionStart
            const end = txtEdit.selectionEnd
            let sel = txtEdit.getFormattedText(start, end)
                             .split("<!--StartFragment-->")[1]
                             .split("<!--EndFragment-->")[0]
            txtEdit.remove(start, end)
            if (sel.includes("font-weight:600;"))
                sel = sel.replace("font-weight:600;", "")
            else
                sel = "<b>" + sel + "</b>"
            txtEdit.insert(txtEdit.cursorPosition, sel)
            txtEdit.select(start, end)
        }
    }
}

Notice that we actually remove and replace the selected text, and reselect the insertion manually.

We can set up similar shortcuts for italics and underline trivially, but what if we want to set font properties of only the text in the selected area?

To keep things simple, let’s see what happens if we want to set just the font family and size:

FontDialog {
    id: fontDlg
}

Shortcut {
    id: fontShortcut

    property string sel: ""
    property int start: 0
    property int end: 0

    sequence: StandardKey.Find
    onActivated: {
        if (txtEdit.selectedText.length > 0)
        {
            start = txtEdit.selectionStart
            end = txtEdit.selectionEnd
            sel = txtEdit.getFormattedText(start, end)
                         .split("<!--StartFragment-->")[1]
                         .split("<!--EndFragment-->")[0]
            fontDlg.open()
        }
    }
}

Connections {
    target: fontDlg

    function onAccepted() {
        txtEdit.remove(fontShortcut.start, fontShortcut.end)
        if (fontShortcut.sel.includes("font-family:")) {
            let fontToReplace = fontShortcut.sel.split("font-family:'")[1].split("';")[0]
            fontShortcut.sel = fontShortcut.sel.replace(fontToReplace, fontDlg.font.family)
        } else {
            fontShortcut.sel = "<span style=\"font-family: '"
                             + fontDlg.font.family + "'; font-size:"
                             + (fontDlg.font.pixelSize ? fontDlg.font.pixelSize
                                                       : fontDlg.font.pointSize)
                             + "\">" + fontShortcut.sel + "</span>"
        }
        txtEdit.insert(txtEdit.cursorPosition, fontShortcut.sel)
        txtEdit.select(fontShortcut.start, fontShortcut.end)
    }
}

If we start messing with other font style properties like italic, bold, spacing, etc., we will end up with almost unreadably nasty string manipulation here.

This solution is overall hacky, as we replace HTML-formatted text from a snipped out section. It would be more Qt-idiomatic to retrieve QFont info from a selection and set the properties without editing raw rich text. Furthermore, it’s better to do as much logic as possible in C++ rather than with JavaScript in QML.

Implementation of cursorSelection in Qt 6.7 QML

Let’s take a look at the cursorSelection property of QtQuick TextEdit in Qt 6.7.

By looking at its property declaration in qquicktextedit_p.h, the type of cursorSelection is QQuickTextSelection.

This type is very basic. It has four read/write properties.

Here is the header qquicktextselection_p.h:

class Q_QUICK_EXPORT QQuickTextSelection : public QObject
{
    Q_OBJECT

    Q_PROPERTY(QString text READ text WRITE setText NOTIFY textChanged FINAL)
    Q_PROPERTY(QFont font READ font WRITE setFont NOTIFY fontChanged FINAL)
    Q_PROPERTY(QColor color READ color WRITE setColor NOTIFY colorChanged FINAL)
    Q_PROPERTY(Qt::Alignment alignment READ alignment WRITE setAlignment NOTIFY alignmentChanged FINAL)

    QML_ANONYMOUS
    QML_ADDED_IN_VERSION(6, 7)

public:
    explicit QQuickTextSelection(QObject *parent = nullptr);

    QString text() const;
    void setText(const QString &text);

    QFont font() const;
    void setFont(const QFont &font);

    QColor color() const;
    void setColor(QColor color);

    Qt::Alignment alignment() const;
    void setAlignment(Qt::Alignment align);

Q_SIGNALS:
    void textChanged();
    void fontChanged();
    void colorChanged();
    void alignmentChanged();

private:
    QTextCursor cursor() const;
    void updateFromCharFormat(const QTextCharFormat &fmt);
    void updateFromBlockFormat();

private:
    QTextCursor m_cursor;
    QTextCharFormat m_charFormat;
    QTextBlockFormat m_blockFormat;
    QQuickTextDocument *m_doc = nullptr;
    QQuickTextControl *m_control = nullptr;
};

Notice we’ve got these private data members:

QTextCursor m_cursor;
QTextCharFormat m_charFormat;
QTextBlockFormat m_blockFormat;
QQuickTextDocument *m_doc = nullptr;
QQuickTextControl *m_control = nullptr;

The m_doc and m_control are retrieved from the TextEdit which parents the selection object. The object is always constructed by a QQuickTextEdit, so in the constructor, the parent is cast to one using qmlobject_cast. Then we set these two fields.

QQuickTextSelection::QQuickTextSelection(QObject *parent)
    : QObject(parent)
{
    // When QQuickTextEdit creates its cursorSelection, it passes itself as the parent
    if (auto *textEdit = qmlobject_cast<QQuickTextEdit *>(parent)) {
        m_doc = textEdit->textDocument();
        m_control = QQuickTextEditPrivate::get(textEdit)->control;
        // ...
        // ...

Now what are m_charFormat and m_blockFormat?

Text documents are composed of a list of text blocks, which can be paragraphs, lists, tables, images, etc. Thus, a block format represents an individual block’s alignment formatting. Char format contains formatting information at the character level, like font family, weight, style, size, color, and so forth.

To initialize these, we need to get the cursor from the text control.

QTextCursor QQuickTextSelection::cursor() const
{
    if (m_control)
        return m_control->textCursor();
    return m_cursor;
}

The cursor will give us a char format and a block format, which we use to get the font / color / alignment at the cursor’s location.

QFont QQuickTextSelection::font() const
{
    return cursor().charFormat().font();
}

// ...

QColor QQuickTextSelection::color() const
{
    return cursor().charFormat().foreground().color();
}

// ...

Qt::Alignment QQuickTextSelection::alignment() const
{
    return cursor().blockFormat().alignment();
}

currentCharFormatChanged is emitted by QQuickTextControl when the cursor moves or the document’s contents change. If this format is indeed different from the fields of the selection object, we must update them and emit the selection’s signals, just as we would in setters. Since we keep track of block alignment too, we have to do the same when the cursor moves and block format is different.

QQuickTextSelection::QQuickTextSelection(QObject *parent)
    : QObject(parent)
{
    // When QQuickTextEdit creates its cursorSelection, it passes itself as the parent
    if (auto *textEdit = qmlobject_cast<QQuickTextEdit *>(parent)) {
        m_doc = textEdit->textDocument();
        m_control = QQuickTextEditPrivate::get(textEdit)->control;
        connect(m_control, &QQuickTextControl::currentCharFormatChanged,
                this, &QQuickTextSelection::updateFromCharFormat);
        connect(m_control, &QQuickTextControl::cursorPositionChanged,
                this, &QQuickTextSelection::updateFromBlockFormat);
    }
}

// ...
// ...
// ...

inline void QQuickTextSelection::updateFromCharFormat(const QTextCharFormat &fmt)
{
    if (fmt.font() != m_charFormat.font())
        emit fontChanged();
    if (fmt.foreground().color() != m_charFormat.foreground().color())
        emit colorChanged();

    m_charFormat = fmt;
}

inline void QQuickTextSelection::updateFromBlockFormat()
{
    QTextBlockFormat fmt = cursor().blockFormat();

    if (fmt.alignment() != m_blockFormat.alignment())
        emit alignmentChanged();

    m_blockFormat = fmt;
}

Here are the setters for the properties, which use the cursor to access and mutate the character or block properties at its position.

void QQuickTextSelection::setText(const QString &text)
{
    auto cur = cursor();
    if (cur.selectedText() == text)
        return;

    cur.insertText(text);
    emit textChanged();
}

// ...

void QQuickTextSelection::setFont(const QFont &font)
{
    auto cur = cursor();
    if (cur.selection().isEmpty())
        cur.select(QTextCursor::WordUnderCursor);

    if (font == cur.charFormat().font())
        return;

    QTextCharFormat fmt;
    fmt.setFont(font);
    cur.mergeCharFormat(fmt);
    emit fontChanged();
}

// ...

void QQuickTextSelection::setColor(QColor color)
{
    auto cur = cursor();
    if (cur.selection().isEmpty())
        cur.select(QTextCursor::WordUnderCursor);

    if (color == cur.charFormat().foreground().color())
        return;

    QTextCharFormat fmt;
    fmt.setForeground(color);
    cur.mergeCharFormat(fmt);
    emit colorChanged();
}

// ...

void QQuickTextSelection::setAlignment(Qt::Alignment align)
{
    if (align == alignment())
        return;

    QTextBlockFormat format;
    format.setAlignment(align);
    cursor().mergeBlockFormat(format);
    emit alignmentChanged();
}

Now, we want to do something like this in our code. The issue is that this implementation resides in the Qt source code itself, and cursorSelection is a property of QQuickTextEdit. If we want to do something like this without changing Qt source code, we have to use attached properties.

Implementing an Attached Property

Using CursorSelection as an attached property for a TextEdit in QML might look something like this:

Item {
    // ...
    // ...
    // ...
    
    Shortcut {
        // ctrl+B to toggle bold / not bold for selection
        sequence: StandardKey.Bold
        onActivated: {
            txtEdit.CursorSelection.font = Qt.font({
                bold: txtEdit.CursorSelection.font.bold !== true
            })
        }
    }

    TextEdit {
        id: txtEdit

        // ...
        
        CursorSelection.font {
            bold: false
            italic: false
            underline: false
        }
    }
}

To create our own attached property, we have to create two classes: CursorSelectionAttached and CursorSelection.

CursorSelectionAttached will contain the implementation of the selection, while CursorSelection serves as the attaching type, using the qmlAttachedProperties() method to expose the signals and properties of an instance of CursorSelectionAttached to the parent to which it is attached.

CursorSelection also needs the QML_ATTACHED() macro in its header declaration, and we must specify that it has an attached property with the macro QML_DECLARE_TYPEINFO() outside the class scope.

Thus, CursorSelection will just look like this:

// CursorSelection.h

class CursorSelection : public QObject
{
    Q_OBJECT
    QML_ATTACHED(CursorSelectionAttached)
    QML_ELEMENT

public:
    static CursorSelectionAttached *qmlAttachedProperties(QObject *object);
};

QML_DECLARE_TYPEINFO(CursorSelection, QML_HAS_ATTACHED_PROPERTIES)

Where the entire implementation is just this function definition:

// CursorSelection.cpp

CursorSelectionAttached *CursorSelection::qmlAttachedProperties(QObject *object)
{
    if (auto *textEdit = qobject_cast<QQuickTextEdit *>(object))
        return new CursorSelectionAttached(textEdit);
    return nullptr;
}

Notice that we perform the qobject_cast here and forward the result as the parent of the attached object. This way we only construct an attached object if we can cast the parent object to a TextEdit.

Now, let’s see how CursorSelectionAttached should be implemented. We begin with the constructor:

// we know that parent will be a QQuickTextEdit *
CursorSelectionAttached::CursorSelectionAttached(QQuickTextEdit *parent) noexcept
    : QObject(parent)
    , mEdit(parent)            // this is the TextEdit we are attached to 
{
    // make sure the QTextDocument exists
    const auto *const quickDoc = mEdit->textDocument(); // QQuickTextDocument *
    auto *doc = quickDoc->textDocument();               // QTextDocument *
    Q_ASSERT(doc != nullptr);

    // retrieve QTextCursor from the QTextDocument
    mCursor = QTextCursor(doc);

    // When deselecting, the cursor position and anchor are
    // set to the TextEdit's cursor position
    connect(mEdit, &QQuickTextEdit::selectedTextChanged,
            this, &CursorSelectionAttached::moveAnchorIfDeselected);
    
    connect(mEdit, &QQuickTextEdit::cursorPositionChanged,
            this, &CursorSelectionAttached::updatePosition);
    
    // if we set a format with no selection, we keep it in an optional
    // then when new text is added, it will have this formatting
    // for example, with no selection we press ctrl+B and then start
    // typing. we expect the text to be bold.
    connect(mEdit->textDocument()->textDocument(),
            &QTextDocument::contentsChange,
            this,
            &CursorSelectionAttached::applyFormatToNewTextIfNeeded);
}

Note that we connect to these three slots:

  • moveAnchorIfDeselected
  • updatePosition
  • applyFormatToNewTextIfNeeded

Let’s investigate the purpose of these.

moveAnchorIfDeselected is invoked when the TextEdit’s selected text changes. A QTextCursor has an anchor, which controls selection area. If text is being selected, the anchor is fixed in place where the selection is started, and the cursor position moves independently of the anchor. The selection area is located between the two positions. When a cursor moves without selecting anything, the anchor is located at and moves along with the cursor position.

Thus, when a cursor’s position is moved, we need to know if the anchor should be moved with it.

Since we invoke moveAnchorIfDeselected when the selected text changes, we know that if the selection is now empty, this means there was a selection that has been deselected. Thus, the cursor and anchor should be equal to one another.

void CursorSelectionAttached::moveAnchorIfDeselected()
{
    if (mEdit->selectedText().isEmpty())
        mCursor.setPosition(mEdit->cursorPosition(), QTextCursor::MoveAnchor);
}

updatePosition is invoked when the TextEdit’s cursor position changes. Depending on the TextEdit’s selection start and end positions, there are a few ways the cursor could be updated.

If there is no selected area in the TextEdit, the cursor and anchor should move together. If a selection’s start and end position both change, we must move the cursor twice: once to the start position, with the anchor moving, and once to the end position, with the anchor fixed in place. If the selection area is being resized, for example by dragging or using Shift+ArrowKeys, the cursor should move with the anchor fixed in place.

void CursorSelectionAttached::updatePosition()
{
    // if there's no selection, just move the cursor & anchor
    if (mEdit->selectionEnd() == mEdit->selectionStart())
    {
        mCursor.setPosition(mEdit->cursorPosition(), QTextCursor::MoveAnchor);
    }

    // if both the start and end need to be updated:
    // move cursor and anchor to selection start, and
    // move cursor to selection end while keeping anchor at start
    //
    // we have to make sure the anchor is moved correctly so the
    // whole selection matches up -- otherwise cursor selection 
    // start or end might be in the middle of the actual
    // selection, wherever the anchor is
    else if (mEdit->selectionStart() != mCursor.selectionStart() &&
             mEdit->selectionEnd() != mCursor.selectionEnd())
    {
        mCursor.setPosition(mEdit->selectionStart(), QTextCursor::MoveAnchor);
        mCursor.setPosition(mEdit->selectionEnd(), QTextCursor::KeepAnchor);
    }

    // these two cases are for selection dragging, only start or
    // end will move, so anchor stays in place
    else if (mEdit->selectionStart() != mCursor.selectionStart())
    {
        mCursor.setPosition(mEdit->selectionStart(), QTextCursor::KeepAnchor);
    }
    else if (mEdit->selectionEnd() != mCursor.selectionEnd())
    {
        mCursor.setPosition(mEdit->selectionEnd(), QTextCursor::KeepAnchor);
    }
}

applyFormatToNewTextIfNeeded is invoked when the contents of the text document change. This is because font properties might be set without an active selection. In this case, the expected behavior is for the characters added afterwards will have these properties.

For example, if the font family is changed with no selection, and we start typing, we expect our text to be in this new font. To do this, we need an optional in which we can save a format to apply to new text if needed, or otherwise contains nullopt. We will call it mOptFormat. It can be set in property setters, which you will see later. For now, we just make sure to use it when the text document content changes and there exists a value in the optional.

void CursorSelectionAttached::applyFormatToNewTextIfNeeded(int from, int charsRemoved, int charsAdded)
{
    if (charsAdded && mOptFormat)
    {
        mCursor.setPosition(mCursor.position() - 1, QTextCursor::KeepAnchor);
        mCursor.mergeCharFormat(mOptFormat.value());
        mOptFormat.reset();
    }
}

Now, let’s take a look at the properties to expose to QML, and how they can be retrieved and set using the cursor. Like the QQuickTextSelection implementation, we will have properties text and font. We can implement the others as well, but for the sake of brevity, we will just focus on these two.

Q_PROPERTY(QString text READ text WRITE setText NOTIFY textChanged FINAL)
Q_PROPERTY(QFont font READ font WRITE setFont NOTIFY fontChanged FINAL)

We’ll need to declare and define these getters and setters, and declare the signals:

Getters:

[[nodiscard]] QString text() const;
[[nodiscard]] QFont font() const;

Setters:

void setText(const QString &text);
void setFont(const QFont &font);

Signals:

void textChanged();
void fontChanged();

The getter and setter implementations will look very similar to the previous implementations shown for QQuickTextSelection, with some minor differences.

Getter implementations:

QString CursorSelectionAttached::text() const
{
    return mCursor.selectedText();
}

QFont CursorSelectionAttached::font() const
{
    // simply get the font at the cursor position using charFormat
    auto ret = mCursor.charFormat().font();

    // if the cursor is at the start of a selection, we need to take the font
    // at the position right in front of it. otherwise, the font will refer to the 
    // character at the position right before the selection begins
    if (mCursor.hasSelection() && mCursor.position() == mCursor.selectionStart())
    {
        auto cur = mCursor;
        cur.setPosition(cur.position() + 1);
        ret = cur.charFormat().font();
    }
    return ret;
}

Setter implementations:

void CursorSelectionAttached::setText(const QString &text)
{
    if (mCursor.selectedText() == text)
        return;

    mCursor.insertText(text);
    emit textChanged();
}

void CursorSelectionAttached::setFont(const QFont &font)
{
    if (font == mCursor.charFormat().font())
        return;

    QTextCharFormat fmt = mCursor.charFormat();
    fmt.setFont(font, QTextCharFormat::FontPropertiesSpecifiedOnly);

    // when no selection, formatting must be set on the next insertion
    if (mCursor.selection().isEmpty())
        mOptFormat = fmt;
    else
        mCursor.mergeCharFormat(fmt);

    emit fontChanged();
}

The only thing that needs to be done now is override the destructor, which can just be set to default:

~CursorSelectionAttached() override = default;

Now we have all the implementation we need to use the attached property. If we put the two classes in one header file, it will look like this:

#pragma once

#include <QObject>
#include <QTextCursor>
#include <QtQml>
#include <optional>

class QQuickTextEdit;

class CursorSelectionAttached : public QObject
{
    Q_OBJECT
    Q_PROPERTY(QString text READ text WRITE setText NOTIFY textChanged FINAL)
    Q_PROPERTY(QFont font READ font WRITE setFont NOTIFY fontChanged FINAL)
    QML_ANONYMOUS

public:
    explicit CursorSelectionAttached(QQuickTextEdit *parent) noexcept;
    ~CursorSelectionAttached() override = default;
    [[nodiscard]] QString text() const;
    [[nodiscard]] QFont font() const;
    void setText(const QString &text);
    void setFont(const QFont &font);

signals:
    void textChanged();
    void fontChanged();

private slots:
    void moveAnchorIfDeselected();
    void updatePosition();
    void applyFormatToNewTextIfNeeded(int from, int charsRemoved, int charsAdded);

private:
    QTextCursor mCursor;
    QQuickTextEdit *mEdit;
    std::optional<QTextCharFormat> mOptFormat;
};

class CursorSelection : public QObject
{
    Q_OBJECT
    QML_ATTACHED(CursorSelectionAttached)
    QML_ELEMENT

public:
    static CursorSelectionAttached *qmlAttachedProperties(QObject *object);
};

QML_DECLARE_TYPEINFO(CursorSelection, QML_HAS_ATTACHED_PROPERTIES)

With this header, an implementation file containing the definitions, and a call to qmlRegisterUncreatableType<CursorSelection> in your main.cpp, the attached property can be used in QML.

Final Remarks

Though this is not a perfect backport, this code allows us to set font properties for selected text in QML in a nearly identical way to its implementation in Qt 6.7. This is especially useful to implement any kind of richtext editing in a QML application, where this functionality is severely lacking in any Qt version prior to 6.7. Hopefully this is a helpful guide to backporting features, implementing attached properties, and doing more sane text editing in QML apps. 🙂

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Behind the Scenes of Embedded Updates

An over-the-air (OTA) update capability is an increasingly critical part of any embedded product to close cybersecurity vulnerabilities, allow just-in-time product rollouts, stomp out bugs, and deliver new features. We’ve talked about some of the key structural elements that go into an embedded OTA architecture before. But what about the back end? Let’s address some of those considerations now.

The challenges of embedded connectivity

The ideal of a constant Internet connection is more aspiration than reality for many embedded devices. Sporadic connections, costly cellular or roaming charges, and limited bandwidth are common hurdles. These conditions necessitate smart management of update payloads and robust retry strategies that can withstand interruptions, resuming where they left off without getting locked in a continually restarting update cycle.

There are other ways to manage spotty connections. Consider using less frequent update schedules or empower users to initiate updates. These strategies however have trade-offs, including the potential to miss critical security patches. One way to strike a balance is to implement updates as either optional or mandatory, or flag updates as mandatory only when critical, allowing users to pace out updates when embedded connectivity isn’t reliable.

To USB or not to USB

When network access is very unreliable, or even just plain absent, then USB updates are indispensable for updating device software. These updates can also serve as effective emergency measures or for in-field support. While the process of downloading and preparing a USB update can often be beyond a normal user’s capability, it’s a critical fallback and useful tool for technical personnel.

OTA servers: SaaS or self-hosted

Deciding between software as a service (SaaS) and self-hosted options for your OTA server is a decision that impacts not just the update experience but also compliance with industry and privacy regulations. While SaaS solutions can offer ease and reliability, certain scenarios may necessitate on-premise servers. If you do need to host an OTA server yourself, you’ll need to supply the server hardware and assign a maintenance team to manage it. But you may not have to build it all from scratch – you can still call in the experts with proven experience in setting up self-hosted OTA solutions.

Certificates: The bedrock of OTA security

SSL certificates are non-negotiable for genuine and secure OTA updates. They verify your company as the authentic source of updates. Choosing algorithms with the longest (comparatively equivalent) key lengths will extend the reliable lifespan of these certificates. However, remember that certificates do expire; having a game plan in place to deal with expired certificates will allow you to avoid the panic of an emergency scramble if it should happen unexpectedly.

Accurate timekeeping is also essential for validating SSL certificates. A functioning and accurate real-time clock that is regularly NTP/SNTP synchronized is critical. If timekeeping fails, your certificates won’t be validated properly, causing all sorts of issues. (We recommend reading our OTA best practice guide for advice on what to do proactively and reactively with invalidated or expired certificates.

Payload encryption: Non-negotiable

Encrypted update payloads are imperative as a safeguard against reverse-engineering and content tampering. This is true for OTA updates as well as any USB or offline updates. Leveraging the strongest possible encryption keys that your device can handle will enhance security significantly.

Accommodating the right to repair

The growing ‘right to repair’ movement and associated legislation imply that devices should support updates outside of your organization’s tightly controlled processes. This may mean that you need to provide a manual USB update to meet repair requirements without exposing systems to unauthorized OTA updates. To prevent your support team from struggling with amateur software updates, you’ll want to configure your device to set a flag when unauthorized software has been loaded. This status can be checked by support teams to invalidate support or warranty agreements.

Summary

By carefully navigating the critical aspects of OTA updates, such as choosing the right hosting option and managing SSL certificates and encryption protocols, your embedded systems can remain up-to-date and secure under any operating conditions. While this post introduces the issues involved in embedded-system updates, there is much more to consider for a comprehensive strategy. For a deeper exploration and best practices in managing an embedded product software update strategy, please visit our best practice guide, Updates Outside the App Store.

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The post Behind the Scenes of Embedded Updates appeared first on KDAB.

PyQt6 & PySide6 Books updated for 2024 — Extended and updated with new examples, demos including Model View Controller architecture

Hello! Today I have released new digital updates to my book Create GUI Applications with Python & Qt.

This update brings all versions up to date with the latest developments in PyQt6 & PySide6. As well as corrections and additions to existing chapters, there are new sections dealing with form layouts, built-in dialogs and developing Qt applications using a Model View Controller (MVC) architecture. The same corrections and additions have been made to the PyQt5 & PySide2 editions.

As always, if you've previously bought a copy of the book you get these updates for free! Just go to the downloads page and enter the email you used for the purchase.

You can buy the latest editions below --

If you bought the book elsewhere (in paperback or digital) you can register to get these updates too. Email your receipt to register@pythonguis.com

Enjoy!

The Smarter Way to Rust

If you’ve been following our blog, you’re likely aware of Rust’s growing presence in embedded systems. While Rust excels in safety-by-design, it’s also common to find it integrated with C++. This strategic approach leverages the strengths of both languages, including extensive C++ capabilities honed over the years in complex embedded systems. Let’s delve into some key concepts for integrating Rust and C++.

Adding Rust to C++

If you’re adding Rust to an existing C++ project, you need to start in the right place. Begin by oxidizing (that is, converting code to Rust) areas that are bug-prone, difficult to maintain, or with security vulnerabilities. These are where Rust can offer immediate improvements. Focus on modules that are self-contained, have clean interfaces, and are primarily procedural rather than object-oriented. For example, libraries that handle media or image processing can be prime candidates for rewriting in Rust as these are often vulnerable to memory safety issues. Parsers and input handling routines also stand to benefit from Rust’s guarantees of safety.

Deciding between Rusting outside-in or inside-out

As your project scales, weigh the merits of maintaining a C++ core with Rust components versus a Rust-centric application with C++ libraries. For smaller, newer projects, starting with Rust may help you avoid the complexities of dealing with C foreign function interfaces (FFIs). This decision may hinge on your safety priorities: if your project’s core tenant is safety, then a Rust-centric approach may be preferable. Conversely, if safety is needed only in certain areas of C++ project, keeping the core in C++ could be more practical.

Another consideration is how your project handles multi-threading. Mixing threading and memory ownership between Rust and C++ is very complex and prone to mistakes. Depending on how your application uses threads, this may tilt the decision in the direction of either C++ or Rust as the main “host” application.

Keeping C++ where it excels

While Rust offers many advantages, particularly in safety, C++ has its own merits that shouldn’t be hastily dismissed. The decision to rewrite should be strategic, based on actual needs rather than a pursuit of language purity since the risk of introducing new bugs through rewriting well-tested and stable C++ code outweigh the benefits of a Rust rewrite. Time-tested C++ code, particularly in areas like signal processing or cryptography, might be best left as is. Such code is often highly optimized, stable, and less prone to memory-related issues. As the saying goes, if it’s not broken, don’t “fix” it.

Navigating Rust limitations

Despite its growing ecosystem, Rust is still relatively young. Relying on packages maintained by small teams or single individuals carries inherent risks. Moreover, as Rust is still in a period of rapid language evolution, this could result in frequent updates, posing challenges for large-scale or long-lived projects. In certain scenarios, such as very large codebases, specific embedded support requirements, or projects with long development cycles, C++ may remain the more practical choice. It is wise to use C++ where stability and longevity are important, and Rust where safety is critical but some development fluidity is acceptable.

Summary

By combining the reliability of C++ with the safety of Rust, developers can engineer systems that endure while minimizing the risk of common programming pitfalls. If you’re interested in reading more about this topic, you’ll want to read our best practice guide on Rust/C++ integration, which was created in collaboration with Ferrous Systems co-founder Florian Gilcher.

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