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How to use `Q_ARG` and `Q_RETURN_ARG` with `QMetaObject.invokeMethod()` in PySide6

2 min read

pyside6 Q_ARG Q_RETURN_ARG invokeMethod

Source

  • an (over-complicated) example that creates an object in the thread of the RemoteControl object
class RemoteControl(QObject):
 
    __thread: QThread
 
    def create_object_in_thread(self, object_type: type[_RemoteObjectT], *args, **kwargs) -> _RemoteObjectT:
        """
        Create a RemoteObject derived object in the thread where the RemoteControl object lives.
 
        Parameters
        ----------
        object_type : type[_RemoteObjectT]
            The type of the object to create
        args : Any
            Positional arguments for the object constructor
        kwargs : Any
            Keyword arguments for the object constructor
 
        Returns
        -------
        _RemoteObjectT
            The created object
        """
 
        return self._create_object_in_thread(object_type, args, kwargs)
 
    @Slot("QVariant", "QVariantList", "QVariantMap", result=QObject)  # type: ignore[arg-type]
    def _create_object_in_thread(self, object_type: type[QObject], args, kwargs) -> QObject:
        """
        Helper function to allow calling `create_object_in_thread()` in another thread via `QMetaObject.invokeMethod()`
        and correctly map `args` and `kwargs` to the QMetaType system and back.
        """
 
        if QThread.currentThread() != self.__thread:
            obj: QObject = QMetaObject.invokeMethod(
                self,
                "_create_object_in_thread",  # type: ignore[arg-type]
                Qt.ConnectionType.BlockingQueuedConnection,
                Q_RETURN_ARG(QObject),
                Q_ARG("QVariant", object_type),
                Q_ARG("QVariantList", args),
                Q_ARG("QVariantMap", kwargs),
            )
            return obj
 
        return object_type(*args, **kwargs)
  • this is, by the way, way too complicated and can be solved by a much simpler implementation
class RemoteControl(QObject):
 
    __thread: QThread
 
    def create_object_in_thread(self, object_type: type[_RemoteObjectT], *args, **kwargs) -> _RemoteObjectT:
        """
        Create a RemoteObject derived object in the thread where the RemoteControl object lives.
 
        Parameters
        ----------
        object_type : type[_RemoteObjectT]
            The type of the object to create
        args : Any
            Positional arguments for the object constructor
        kwargs : Any
            Keyword arguments for the object constructor
 
        Returns
        -------
        _RemoteObjectT
            The created object
        """
 
        obj = object_type(*args, **kwargs)
        obj.moveToThread(self.__thread)
        return obj

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