Qt：利用事件过滤器在QLabel上画框

柔情只为你懂 2022-10-24 11:44 181阅读 0赞

![ac840771d75814fd26a587515f83f5fd846.jpg][]、

第一：对目标对象调用installEventFilter()来注册监视对象(事件过滤器);  
第二：重写监视对象的eventFilter()函数处理目标对象的事件。

1、在mainwindows中添加：

public:
        bool eventFilter(QObject *watched, QEvent *event);
    private:
        bool mouseIsdown;
        QPointF startPos;
        QPointF endPos;

2、构造函数中安装事件过滤器：

mouseIsdown = false;

ui->label->installEventFilter(this);

3、在 mainwindows.cpp中添加

#include <QPainter>
    #include <QMouseEvent>
    bool MainWindow::eventFilter(QObject *watched, QEvent *event)
    {
        if(watched == ui->label )
        {
            if(event->type() == QEvent::MouseButtonPress)
            {
                QMouseEvent *mouseEvent = (QMouseEvent *)event;
                if(mouseEvent->buttons()&Qt::LeftButton)
                {
                     mouseIsdown = true;
                     startPos = mouseEvent->pos();
                }
            }
            if(event->type() == QEvent::MouseMove)
            {
                QMouseEvent *mouseEvent=(QMouseEvent *)event;
                if(mouseIsdown)
                {
                    endPos = mouseEvent->pos();
                    ui->label->update();
                }
             }
            if(event->type() == QEvent::MouseButtonRelease)
            {
                QMouseEvent *mouseEvent=(QMouseEvent *)event;
                endPos = mouseEvent->pos();
                mouseIsdown = false;
            }
            if(event->type() == QEvent::Paint)
            {
                magicTime();
                QPainter painter(ui->label);
                QPen pen = painter.pen();
                pen.setWidth(3);
                pen.setColor(QColor(173, 255, 47));
                painter.setRenderHint(QPainter::Antialiasing, true);
                painter.setPen(pen);
                QRect rect(startPos.x(),startPos.y(),endPos.x(), endPos.y());
                painter.drawRect(rect);
            }
    
        }
        return QMainWindow::eventFilter(watched, event); //将事件传递给父类
    }

参考：[https://blog.csdn.net/qq\_31607215/article/details/53964594][https_blog.csdn.net_qq_31607215_article_details_53964594]

[ac840771d75814fd26a587515f83f5fd846.jpg]: /images/20221024/4b8e2daf852446b9a406f8522946519d.png
[https_blog.csdn.net_qq_31607215_article_details_53964594]: https://www.oschina.net/action/GoToLink?url=https%3A%2F%2Fblog.csdn.net%2Fqq_31607215%2Farticle%2Fdetails%2F53964594