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//虚函数， 事件分发
bool event(QEvent *e);

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keyPressEvent();
keyReleaseEvent();
mouseDoubleClickEvent();
mouseReleaseEvent();

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selfWidget->setMouseTracking(true);

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class CustomButton : public QPushButton
{
    Q_OBJECT
public:
    CustomButton(QWidget *parent = 0);

protected:
   void mousePressEvent(QMouseEvent *event);	//重写mousePressEvent事件处理函数

private:
    void onButtonCliecked();
};

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CustomButton::CustomButton(QWidget *parent) :
    QPushButton(parent)
{
    connect(this, &CustomButton::clicked,
            this, &CustomButton::onButtonCliecked);
}

//重写mousePressEvent 处理函数
//当重写事件回调函数时，时刻注意是否需要通过调用父类的同名函数来确保原有实现仍能进行！
//下述重写后，上面的connect中的"clicked"信号，永远不会发生，这个信号连接的槽函数也就永远不会被执行。
void mousePressEvent(QMouseEvent *event)
{
	if(event->button() == Qt::LeftButton)
	{
		qDebug()<<"Left";
	}
	else
	{
		QPushButton::mousePressEvent(event);
	}
}


void CustomButton::onButtonCliecked()
{
    qDebug() << "You clicked this!";
}

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int main(int argc, char *argv[])
{
    QApplication a(argc, argv);

    CustomButton btn;
    btn.setText("This is a Button!");
    btn.show();

    return a.exec();
}

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void MainWindow::closeEvent(QCloseEvent *event)
{
   bool exit = QMessageBox::question(this,
                     tr("Quit"),
                     tr("Are you sure to quit this application?"),
                     QMessageBox::Yes | QMessageBox::No,
                     QMessageBox::No) == QMessageBox::Yes);
	if (exit) 
	{
	        event->accept();
	} 
	else 
	{
	         event->ignore();
	}
}

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bool CustomWidget::event(QEvent *e)  //QEvnet 对象作为参数，即需要转发的事件对象
{
    if (e->type() == QEvent::KeyPress) {
        QKeyEvent *keyEvent = static_cast<QKeyEvent *>(e);  //强制类型转换，将QEvent转成QKeyEvent
        //另一种转换
        //QKeyEvent *keyEvent = (QKeyEvent *)e;
        if (keyEvent->key() == Qt::Key_Tab) {
            qDebug() << "You press tab.";
            return true;
        }
    }
    return QWidget::event(e);
}

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bool CustomTextEdit::event(QEvent *e)
{
   if (e->type() == QEvent::KeyPress) 
   {
        QKeyEvent *keyEvent = static_cast<QKeyEvent *>(e);
       if (keyEvent->key() == Qt::Key_Tab) 
       {
            qDebug() << "You press tab.";
            return true;						//处理完Tab 按键消息，就直接返回，表示已经
            								    //对Tab键按下事件进行了处理
       }
    }
    
    return false; 						//对于其他事件，由于直接返回false, Qt不转发，其父类	
                                        //收不到其他事件，导致这个自定义的Widget只能处理Tab事件
}

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//!!! Qt5
bool QObject::event(QEvent *e)
{
    switch (e->type()) {
    case QEvent::Timer:
        timerEvent((QTimerEvent*)e);
        break;

    case QEvent::ChildAdded:
    case QEvent::ChildPolished:
    case QEvent::ChildRemoved:
        childEvent((QChildEvent*)e);
        break;
    // ...
    default:
        if (e->type() >= QEvent::User) {
            customEvent(e);
            break;
        }
        return false;
    }
    return true;
}

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virtual bool QObject::eventFilter ( QObject * watched, QEvent * event );

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 class MainWindow : public QMainWindow
 {
 public:
     MainWindow();

 protected:
     bool eventFilter(QObject *obj, QEvent *ev) override;

 private:
     QTextEdit *textEdit;
 };

 MainWindow::MainWindow()
 {
     textEdit = new QTextEdit;
     setCentralWidget(textEdit);

     textEdit->installEventFilter(this);			//安装过滤器
 }

 bool MainWindow::eventFilter(QObject *obj, QEvent *event)
 {
     /*
      * In your reimplementation of this function, if you want to filter the event out,
      * i.e. stop it being handled further, return true; otherwise return false.
      */
     if (obj == textEdit) {									//过滤特定Widget上的事件，判断obj对象
         if (event->type() == QEvent::KeyPress) {
             QKeyEvent *keyEvent = static_cast<QKeyEvent*>(event);
             qDebug() << "Ate key press" << keyEvent->key();
             return true;							//stop it being handled further
         } else {
             return false;							//return false, let it being handled further
         }
     } else {
         // pass the event on to the parent class
         return QMainWindow::eventFilter(obj, event);
     }
 }

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void QObject::installEventFilter ( QObject * filterObj )