[Development] Property bindings in Qt 6

[Simon Hausmann]

Hi,

Earlier this year, Olivier, Samuel, Auri and I worked on a project to re-evaluate how we could bring the declarative Qt Quick approach of doing user interfaces closer to C++, in order to allow building and running user interfaces in very memory and processor-power constrained environments. There were many different outcomes of this. One of them was that we figured out a way to compile QML binding expressions down to full C++, without any run-time interpretation. This required building a new way of defining properties and their relationships, a new property binding system. The results were so convincing that the plan was born to productize this for Qt 6 in multiple layers and steps. I'd like to initiate a first step in that direction by proposing API and functionality for Qt 6 and briefly outline how we see the building blocks apply to QML and Qt Quick:

In QML, today, properties consist of a type, a setter function and a getter function, and the functions are implemented by the developer. There is also a change signal that needs to be emitted when the value changes.

Binding expressions declared in .qml files are created behind the scenes and the QML engine makes sure to call the getter functions during the evaluation and the setter function to write the result. Through a connection to the change signal, bindings are automatically re-evaluated when properties change and the new values are passed to the setter functions. It's pretty magic and it works, but it requires a fair amount of indirection and side-loading of data structures.

I would like to propose an API that replaces the setter and getter functions on objects with a new property template class that encapsulates the property value instead, and the ability to tie binding expressions to these properties for automatic updates. In short, it looks like this:

    QProperty<QString> surname("John");
    QProperty<QString> lastname("Smith");

    QProperty<QString> fullname;
    fullname.setBinding([&]() { return surname() + " " + lastname(); });

    qDebug() << fullname(); // Prints "John Smith"

    surname = "Emma"; // Marks binding expression as dirty

    qDebug() << fullname(); // Re-evaluates the binding expression and prints "Emma Smith"


You can see a work-in-progress patch for this in Gerrit at

    https://codereview.qt-project.org/c/qt/qtbase/+/275352


The basic data structure behind this is the property value itself as well as doubly linked lists to track dependencies between properties and binding expressions. Due to the encapsulation of the data itself in a class, it is possible to do a lazy evaluation of bindings. (Credit goes in particular to Olivier for the idea and first implementation in our project)


Once this class and its documentation is complete, the next step is to build a bridge to the QML engine and the moc, so that it's possible to associate binding expressions in .qml files with properties declared this way. Similarly, it needs to be possible to access such properties through the meta-call, if they are placed inside Q_OBJECT classes.

The next step is to begin applying this to the implementation of Qt Quick. Some of which may require shims for the public Qt Quick API (to keep it Q_PROPERTY based), and for the private Qt Quick types the idea would be to start using QProperty.

Finally, once all the pieces are in place, we hope to extend the qml tooling to compile the binding expressions in .qml files to C++ that uses this more light-weight property system whenever possible. Ulf has been working towards this from the QML engine direction (see the recent email about moc and meta-type extraction) and Fabian has been working on the QML linter as a starting point towards a compilation model for QML.


This is our rough plan of how we'd like to address one aspect of QML and Qt Quick today. We are looking forward to any feedback and questions to help us review and refine this design.


Simon
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