QML / C++ best practice

Here are some hints what to do and not to do from my experience when creating QML applications with C++ backend, also see the “official” qt best practice page

Command Query Segregation :

Functions that change state should not return values and functions that return values should not change state.

see http://cqrs.nu/Faq/command-query-responsibility-segregation

Do:

• declare readonly attributes in C++ & bind to them in QML
• use Q_INVOKABLEs for actions which change state like this ctrl.actionWhichChangesX(42)

Dont:

• set C++ attributes from QML like this ctrl.x = 42

Signal names:

reflect events, something that has happened, so write in past tense

Do:

• signals:
  void stageCleared(…)

Dont:

• signals:
  void clearStage(…)

Binding model properties to interaction elements

If you want to bind a model property directly to a UI element which is used to edit this property, e.g. a line edit there’s a problem that the binding gets lost when editing has finished.

TextField {
   text : myModel.myTextProperty 
   onEditingFinished : controller.setMyTextProperty(text) 
}

There are several solutions for this, you can for example reestablish the binding after editing using Qt.binding like this:

onEditingFinished: {
    controller.setMyTextProperty(text)
    text = Qt.binding(function() { return myModel.myTextProperty }
}

or dont bind at all but work with an additional property like this :

TextField {

   property string myText : myModel.myTextProperty 
   onMyTextChanged: text = myText 
   onEditingFinished : controller.setMyTextProperty(text) 
}

Debugging broken bindings

New in Qt 5.10: Diagnostics when breaking QML bindings

set QT_LOGGING_RULES=”qt.qml.binding.removal.info=true”