Dynamic Qt Meta Objects

Small documented code example to test dynamic meta objects in Qt 5.

! Dependency to core-private is needed !

Random Notes:

• When using this with QML (as in the example), any meta objects added after the QML file/component is loaded will not be reflected on the QML side (the metaobject will not be "reloaded" as it should be static...).
• When adding a property, add a notify signal too or it will raise a QML error at runtime.
• Everything is not handled in this example.
• Don't forget to update the class name and super class when building with QMetaObjectBuilder.
• Lots of code is copy-pasted from generated moc file...

QVariant Data Pointers:

Extracted from Qt Devs Days PDF, see below)

• Returning values:

• Known type: *reinterpret_cast<T*>(argv[0]) = value,

• QVariant: QMetaType::construct(typeid, argv[0], value.data()).

• Receiving values:

• Known type: *reinterpret_cast<T*>(argv[0]),

• QVariant: QVariant(typeid, argv[0]).

• Special case: QVariant

• argv[0] points to QVariant directly in this case.

• Properties:

  • argv[0] is a pointer to the value data,
  • argv[2] is an int* for the return value of setProperty().

• Calling Methods

• Arguments:

• argv[0]: return value,

• argv[i]: i-th argument,

• QMetaType::create/destroy can be handy to manage copies.

• Common task: turn void** arguments into a QVariantList

• Never store argv itself, most likely invalid after you return!

• For each argument:

• Determine its type id,

• Your meta object should have that,

• QMetaType::type() is useful if type is only available as a string,

• Alternative for incoming signals: sender() + senderSignalIndex().

• Create a copy in a QVariant: QVariant(typeid, argv[i]).

• Emitting Signals

• QMetaObject::activate(this, metaObject(), id, argv);

Documentation:

https://www.qtdeveloperdays.com/sites/default/files/QtDevDays2014US-DIY-moc.pdf (a bit outdated, some things are wrong now, like base classe call in qt_metacall)

https://doc.qt.io/archives/qq/qq16-dynamicqobject.html (no QML but dynamic signal-slot connections)