qt重写mouseMoveEvent后不显示标签解决办法

void myview::mouseMoveEvent(QMouseEvent* event)
{

	viewPoint = event->pos();
	scenePoint = mapToScene(viewPoint);
	scenePointDispaly.setText("Coordinate:(" + QString::number(scenePoint.x()) + "," + QString::number(scenePoint.y()) + ")");
	scenePointDispaly.setGeometry(QRect(1650, 0, 300, 30));
	//qDebug() << "QString::number(scenePoint.x()) = " << QString::number(scenePoint.x()) << endl;
	scenePointDispaly.setParent(this);
}

项目遇到一个将光标在scene的坐标显示到label，然后label加到scene中。如图已经重写了mouseMoveEvent事件，debug的时候也可以将位置信息xy打印出来，但调试的时候就是看不到显示坐标的标签，myview的构造函数里面也加上了setMouseTracking(true);搞不懂，后面发现把设置text等语句加到myview的构造函数里面就可以了（代码如下），不是很明白原理，在此记录一下

myview::myview(QWidget* parent)
	: QGraphicsView(parent)
{
	setMouseTracking(true);

	scenePointDispaly.setText("Coordinate:(" + QString::number(scenePoint.x()) + "," + QString::number(scenePoint.y()) + ")");
	scenePointDispaly.setGeometry(QRect(1650, 0, 300, 30));
	//qDebug() << "QString::number(scenePoint.x()) = " << QString::number(scenePoint.x()) << endl;
	scenePointDispaly.setParent(this);


}