QPainter(QPaintDevice) not working

[wolfseifert]

According to Qt 6 documentation

 void MyWidget::paintEvent(QPaintEvent *)
 {
     QPainter p;
     p.begin(this);
     p.drawLine(drawingCode);        // drawing code
     p.end();
 }

and

 void MyWidget::paintEvent(QPaintEvent *)
 {
     QPainter p(this);
     p.drawLine(drawingCode);        // drawing code
 }

should be the same. Translated to QtJambi this means:

var p = new QPainter();
p.begin(glWidget);
//...
p.end();

and

var p = new QPainter(glWidget);
//...

should be the same. But the second approach with QPainter(QPaintDevice) is not working. Line 3286 in QPainter.java

    @io.qt.QtUninvokable
    private void initialize(QPaintDevice device, boolean inConstructor){
        if(device instanceof io.qt.core.QObject){
            io.qt.core.QObject object = (io.qt.core.QObject)device;
            if(object.isWidgetType() || device instanceof QPaintDeviceWindow) {
                java.util.List<QPainter> painters = __paintedDevices.get(device);
                if(painters==null){
                    if(inConstructor)
3286                    throw new QPaintingOutsidePaintEventException();
                }else if(painters.size()==0){
                    threadCheck((io.qt.core.QObject)device);
                    painters = java.util.Collections.singletonList(this);
                    __paintedDevices.put(device, painters);
                }else{
                    threadCheck((io.qt.core.QObject)device);
                    if(painters.size()==1){
                        painters = new java.util.LinkedList<>(painters);
                        __paintedDevices.put(device, painters);
                    }
                    painters.add(this);
                }
            }else {
                threadCheck((io.qt.core.QObject)device);
            }
        }else if(device==null){
            java.util.Objects.requireNonNull(device, "Argument 'QPaintDevice': null not expected.");
        }
    }

throws an exception. The painters are also null in the first approach, but because I am not in a constructor it works fine.

My workaround for now is to use the first approach.

[omix]

Qt does not accept paining on widgets outside of paintEvent(QPaintEvent *). Thus, QtJambi checks if it is within paintEvent method when calling QPainter.begin(device) and new QPainter(device).

I thought there is a unit test checking these cases. And I also thought my own program does these things as well. I'll check it.

[wolfseifert]

This happens within paintEvent. Substituting approach two with approach one resolves the issue (nothing else changed).

[omix]

So, using begin(device) works fine but not using constructor?

[omix]

Actually, there is a test scenario in unit tests:
io.qt.autotests.TestPaintOnWidget
and here:

• testPaintingInsideViaConstructor() and
• testPaintingInsideViaBegin()
  These tests succeed.

Now as I see the tests, I can tell you QPainter.begin(device) never throws an exception. It simply returns false if painting is not allowed.
Thus, new QPainter(device) will throw exception in all scenarios where QPainter.begin(device) will return false.

[omix]

Did you call paintEvent directly? In this case, the exception is thrown because paintEvent needs to be called by event loop only.

[wolfseifert]

I checked again: approach one (begin) works, approach two (constructor) fails within the same context.The context is overriden paintGL() of QOpenGLWidget and called by the framework.

[omix]

Ok, I'll add QOpenGLWidget::paintGL() as scenario to TestPaintOnWidget.

[omix]

This issue is solved with QtJambi 6.2.2.