Document qmlRegisterRevision()

Short summary

QDeclarativeItem has implicitWidth and implicitHeight declared with Q_PROPERTY( ... REVISION 1)

With the following class "ObjectItem" it's inherited properties implicitWidth and implicitHeight can not be accessed from QML:

class ObjectItem : public QDeclarativeItem
{
Q_OBJECT
};

Long Description

In 4.8.0 rc1 there is an issue where QML extention c++ classes which are Q_OBJECT's do not expose Q_PROPERTY( ... REVISON 1 ) properties to the QML context.

There is a workaround that further reveals the issue, declare a QML extention c++ class that IS NOT a Q_OBJECT and register that with qmlRegisterType< X, 1 >. Then QML can access the Q_OBJECT derived classes REVISON 1 properties.

Below example main.cpp that exhibits the issue:

------------------------------------------

#include <QtDeclarative>

class WorkaroundItem : public QDeclarativeItem {};

class ObjectItem : public QDeclarativeItem
{
Q_OBJECT
};

int main(int argc, char **argv)
{
QApplication app(argc, argv);

qmlRegisterType< ObjectItem, 1 >( "MyQML", 1, 0, "ObjectItem" );
/* qmlRegisterType< WorkaroundItem, 1 >( "MyQML", 1, 0, "WorkaroundItem" ); */

QDeclarativeView view( QUrl( "main.qml" ) );
view.show();

return app.exec();
}

------------------------------------------

If 'main.qml' contains the following:

import MyQML 1.0;

ObjectItem
{
width: implicitWidth * 0.25
height: implicitHeight * 0.25
}

---------------------------------------

The following console output is obtained:

file:///.../main.qml:6: ReferenceError: Can't find variable: implicitHeight
file:///.../main.qml:5: ReferenceError: Can't find variable: implicitWidth

If the commented registration of WorkaroundItem is uncommented then no ReferenceError is reported (and the binds function as expected).

Further note, if ObjectItem is registered with no revison information:

qmlRegisterType< ObjectItem >( "MyQML", 1, 0, "ObjectItem" );

and the WorkaroundItem is uncommented then NO ReferenceErrors are reported; however if WorkaroundItem is registered without revision information then ReferenceErrors ARE reported.