when use QGraphicsView::setScene to show another scene, the scene is not actived

i only define one view to display different scene, when use setScene to show another scene, the scene is not actived;
when i send a QEvent::WindowActivate to the scene, it's ok;
just like this below:

void MyWidget::slot_clicked1()
{
graphicsView_1->setScene( scene_1 );
QEvent e( QEvent::WindowActivate );
QApplication::sendEvent( scene_1, &e );
}

void MyWidget::slot_clicked2()
{
graphicsView_1->setScene( scene_2 );
QEvent e( QEvent::WindowActivate );
QApplication::sendEvent( scene_2, &e );
}