自学QT之事件过滤

QT的事件过滤是通过重写函数来实现的，这个函数是：eventFilter()

以下是一个通过单击不同的标签，实现标签上图片的变化的例子。

首先新建一个项目，基于对话框

头文件：

#ifndef EVENTFILTER_H
#define EVENTFILTER_H
#include <QDialog>
#include <QImage>
#include <QLabel>
#include <QEvent>

class EventFilter : public QDialog
{
    Q_OBJECT

public:
    EventFilter(QWidget *parent = 0);
    ~EventFilter();
public slots:
    bool eventFilter(QObject *, QEvent *);

private:
    QLabel *label1;
    QLabel *label2;

    QLabel *label3;
    QLabel *stateLabel;
    QImage image1;
    QImage image2;
    QImage image3;
};

#endif // EVENTFILTER_H

.cpp文件：

#include "eventfilter.h"
#include <QGridLayout>
#include <QMouseEvent>


EventFilter::EventFilter(QWidget *parent)
    : QDialog(parent)
{
    setWindowTitle(tr("事件过滤"));
    label1=new QLabel;
    image1.load(":/new/prefix1/image/2.png");
    label1->setAlignment(Qt::AlignHCenter|Qt::AlignVCenter);
    label1->setPixmap(QPixmap::fromImage(image1));
    label2=new QLabel;
    image2.load(":/new/prefix1/image/2.png");
    label2->setAlignment(Qt::AlignHCenter|Qt::AlignVCenter);
    label2->setPixmap(QPixmap::fromImage(image2));
    label3=new QLabel;
    image3.load(":/new/prefix1/image/2.png");
    label3->setAlignment(Qt::AlignHCenter|Qt::AlignVCenter);
    label3->setPixmap(QPixmap::fromImage(image3));
    stateLabel=new QLabel;
    stateLabel->setText("鼠标按下标志");
    stateLabel->setAlignment(Qt::AlignCenter);
    QHBoxLayout *Layout=new QHBoxLayout;
    Layout->addWidget(label1);
    Layout->addWidget(label2);
    Layout->addWidget(label3);
    QVBoxLayout *mainLayout=new QVBoxLayout(this);
    mainLayout->addLayout(Layout);
    mainLayout->addWidget(stateLabel);
    label1->installEventFilter(this);
    label2->installEventFilter(this);
    label3->installEventFilter(this);


}

bool EventFilter::eventFilter(QObject *watched, QEvent *event)
{
    if(watched==label1)
    {
        if(event->type()==QEvent::MouseButtonPress)
        {
            QMouseEvent *mouseEvent=(QMouseEvent *)event;
            {
                if(mouseEvent->button()==Qt::LeftButton)
                {
                    stateLabel->setText("左键被按下左边图片");
                }
                else if(mouseEvent->button()==Qt::RightButton)
                {
                    stateLabel->setText("右键被按下左边图片");

                }
                else if(mouseEvent->button()==Qt::MidButton)
                {
                    stateLabel->setText("中健被按下左边图片");
                }
                QMatrix matrix;
                matrix.scale(1.8,1.8);
                QImage tmpimg=image1.transformed(matrix);
                label1->setPixmap(QPixmap::fromImage(tmpimg));
            }

        }
        if(event->type()==QEvent::MouseButtonRelease)
        {
            stateLabel->setText("鼠标释放左边图片");
            label1->setPixmap(QPixmap::fromImage(image1));
        }
    }
    else if(watched==label2)
    {
        if(event->type()==QEvent::MouseButtonPress)
        {
            QMouseEvent *mouseEvent=(QMouseEvent *)event;
            {
                if(mouseEvent->button()==Qt::LeftButton)
                {
                    stateLabel->setText("左键被按下中间图片");
                }
                else if(mouseEvent->button()==Qt::RightButton)
                {
                    stateLabel->setText("右键被按下中间图片");

                }
                else if(mouseEvent->button()==Qt::MidButton)
                {
                    stateLabel->setText("中健被按下中间图片");
                }
                QMatrix matrix;
                matrix.scale(1.8,1.8);
                QImage tmpimg=image2.transformed(matrix);
                label2->setPixmap(QPixmap::fromImage(tmpimg));
            }

        }
        if(event->type()==QEvent::MouseButtonRelease)
        {
            stateLabel->setText("鼠标释放中间图片");
            label2->setPixmap(QPixmap::fromImage(image2));
        }

    }
    else if(watched==label3)
    {
        if(event->type()==QEvent::MouseButtonPress)
        {
            QMouseEvent *mouseEvent=(QMouseEvent *)event;
            {
                if(mouseEvent->button()==Qt::LeftButton)
                {
                    stateLabel->setText("左键被按下右边图片");
                }
                else if(mouseEvent->button()==Qt::RightButton)
                {
                    stateLabel->setText("右键被按下右边图片");

                }
                else if(mouseEvent->button()==Qt::MidButton)
                {
                    stateLabel->setText("中健被按下右边图片");
                }
                QMatrix matrix;
                matrix.scale(1.8,1.8);
                QImage tmpimg=image3.transformed(matrix);
                label3->setPixmap(QPixmap::fromImage(tmpimg));
            }

        }
        if(event->type()==QEvent::MouseButtonRelease)
        {
            stateLabel->setText("鼠标释放右边图片");
            label3->setPixmap(QPixmap::fromImage(image3));
        }
    }
    return QDialog::eventFilter(watched,event);
}
EventFilter::~EventFilter()
{

}

如图，通过单击不同的标签，标签显示不同的大小的图片，并且最下的标签也跟着改变。这里的实现原理就是事件过滤，过滤标签上的单击事件。