def closeEvent(self, e):
        """Qt slot when the window is closed."""
        if self._closed:
            return
        res = self.emit('close')
        # Discard the close event if False is returned by one of the callback
        # functions.
        if False in res:  # pragma: no cover
            e.ignore()
            return
        super(GUI, self).closeEvent(e)
        self._closed = True
Qt slot when the window is closed.