Question

I would like to execute a method which can only be called once my QApplication is displayed, i.e. when it has entered its main event loop exec_(). I’m new to Qt4 (using PyQt4): i was hoping to have a on_start()-like callback, but didn’t find one.
Do i need to create a thread or a timer? Or is there some callback included in the API already?

Answer 1

For now i have chosen to use QThread this way:

class MyThread(QtCore.QThread):
    def run(self):
    ''' reinplemented from parent '''
        # make thread sleep to make sure
        # QApplication is running before doing something
        self.sleep(2)
        do_something()

class MyWidget(QtGui.QWidget):
    def __init__(self, parent=None):
        QtGui.QWidget.__init__(self, parent)
        self.attr = 'foo'
        self.thread = MyThread(self)
        self.thread.start()

def main():
    app = QtGui.QApplication(sys.argv)
    w = MyWidget()
    w.show()
    sys.exit(app.exec_())

It works for my purpose because my thread in my (real) code actually looks for the application window in X display and will try doing so until it finds it. But otherwise that’s not an elegant way to solve the problem.
It would be nicer if there was a signal emitted by QApplication when entering the event-loop. JAB proposed the Qt.ApplicationActivate but it doesn’t seem to be emitted by QApplication, and even if it was, because the MyWidget() is not instantiated as a child of QApplication, i wouldn’t know how to pass the signal from app to w

I’ll wait for a better answer, if any, before accepting my answer as the chosen solution.