QT notes - EventFilter event filter

Qt The framework provides us with a series of event processing mechanisms , When a window event occurs , The event will go through ： The incident was distributed ->
Event filtering -> Event distribution -> Several stages of event handling .

every last Qt Each application corresponds to a unique QApplication Application object , Then call the object's exec() function , such Qt
The event detection inside the framework starts （ The program will enter an event loop to listen for the application's events ）.

int main(int argc, char *argv[])
{
    
    QApplication a(argc, argv);
    MainWindow w;
    w.show();
    return a.exec();
}

Event distribution process ：

When the event occurs ,Qt Call with application object notify() Function to send the event to the specified window ：

[override virtual] bool QApplication::notify(QObject *receiver, QEvent *e);

Events can be filtered through the event filter during sending , By default, any generated events are not filtered ：

//  You need to install a filter on the window first ,  This event will trigger 
void QObject::installEventFilter(QObject *filterObj)		// Event filter 
[virtual] bool QObject::eventFilter(QObject *watched, QEvent *event)	

Help document ：

When the event is sent to the specified window , The event distributor of the window will classify the received events :

[override virtual protected] bool QWidget::event(QEvent *event);

Help document ：

The event dispatcher will classify the events after classification （ Mouse events 、 Keyboard events 、 Graphic Events ...） Distribute to the corresponding event handler function for processing , Each event handler function has a default processing action （ We can also rewrite these event handler functions ）, such as ： Mouse events ：

//  The mouse click 
[virtual protected] void QWidget::mousePressEvent(QMouseEvent *event);
//  Mouse release 
[virtual protected] void QWidget::mouseReleaseEvent(QMouseEvent *event);
//  Mouse movement 
[virtual protected] void QWidget::mouseMoveEvent(QMouseEvent *event);

An example of an event filter is as follows ：
If an event filter is installed on the component , that , Filters can capture events before components , So as to carry out corresponding treatment .

First step ： Define our event filter events

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

The second step ： Bind the object to be bound to the event filter

ui.label->installEventFilter(this);
ui.label_move->installEventFilter(this);

The third step ： Rewrite our event filter event

bool QtEventFilter::eventFilter(QObject* watched, QEvent* event)
{
    
	// Objects to filter 
	if (watched == ui.label)
	{
    
		// Events to be filtered 
		if (event->type() == QEvent::MouseMove)
		{
    
			QMouseEvent* mouseEvent = (QMouseEvent*)event;
			QPoint p = mouseEvent->pos();
			QString s = QString("x=%1,y=%2").arg(QString::number(p.x())).arg(QString::number(p.y()));
			ui.lineEdit_move->setText(s);

		}
		if (event->type() == QEvent::MouseButtonPress)
		{
    
			QMouseEvent* mousePressEvent = (QMouseEvent*)event;
			if (mousePressEvent->buttons() & Qt::LeftButton)
			{
    
				QPoint p1 = mousePressEvent->pos();
				QString s1 = QString("x=%1,y=%2").arg(QString::number(p1.x())).arg(QString::number(p1.y()));
				ui.lineEdit_left->setText(s1);
			}	
		}
		if (event->type() == QEvent::MouseButtonDblClick)
		{
    
			qDebug() << QStringLiteral("label Use MouseButtonDblClick");
		}
	}
	if (ui.label_move == watched)
	{
    	
		if (event->type() == QEvent::MouseMove)
		{
    
			QMouseEvent* mouseEvent = static_cast<QMouseEvent*>(event);
			qDebug() << QStringLiteral(" This is the filter  MouseMove");
			// Intercept   Don't continue to pass down 
			return true;
		}
		if (event->type() == QEvent::MouseButtonPress)
		{
    
			QMouseEvent* mouseEvent = static_cast<QMouseEvent*>(event);
			qDebug() << QStringLiteral(" This is the filter  MouseButtonPress ");
			// Don't intercept , Keep delivering 
			return false;
		}	
	}
	// Pass other events to the base class 
	return QWidget::eventFilter(watched, event);	
}

MyLabel.cpp Customize Label Mouse press and mouse move events

#include "MyLabel.h"

MyLabel::MyLabel(QWidget *parent)
	: QLabel(parent)
{
    
	this->setMouseTracking(true);	// Set mouse tracking 
}

MyLabel::~MyLabel()
{
    
}

void MyLabel::mousePressEvent(QMouseEvent *e)
{
    
	qDebug() << QStringLiteral(" This is the subcomponent ：mousePressEvent");
	if (e->buttons() == Qt::LeftButton)
	{
    
		qDebug() << QStringLiteral(" Left mouse button ");
	}
	else if(e->buttons() == Qt::RightButton)
	{
    
		qDebug() << QStringLiteral(" Right click the mouse ");
	}
	e->ignore();
}

void MyLabel::mouseMoveEvent(QMouseEvent *e)
{
    
	qDebug() << QStringLiteral(" This is the subcomponent ：mouseMoveEvent x=%1,y=%2").arg(e->x()).arg(e->y());
	e->ignore();
}

MyLabel.h

#pragma once

#include <QLabel>
#include <QMouseEvent>
#include <QDebug>
class MyLabel : public QLabel
{
    
	Q_OBJECT

public:
	MyLabel(QWidget *parent);
	~MyLabel();
protected:
	void mousePressEvent(QMouseEvent *);
	void mouseMoveEvent(QMouseEvent *);
};

Main window code ：
QtEventFilter.cpp

#include "QtEventFilter.h"
#include <QDebug>
QtEventFilter::QtEventFilter(QWidget *parent)
	: QWidget(parent)
{
    
	ui.setupUi(this);
	// Install event filter 
	ui.label->installEventFilter(this);
	ui.label_move->installEventFilter(this);
	ui.label->setMouseTracking(true);
}
QtEventFilter::~QtEventFilter()
{
    

}
bool QtEventFilter::eventFilter(QObject* watched, QEvent* event)
{
    
	if (watched == ui.label)
	{
    
		if (event->type() == QEvent::MouseMove)
		{
    
			//m_SerPort = Port_Screen;
			QMouseEvent* mouseEvent = (QMouseEvent*)event;
			QPoint p = mouseEvent->pos();
			QString s = QString("x=%1,y=%2").arg(QString::number(p.x())).arg(QString::number(p.y()));
			ui.lineEdit_move->setText(s);

		}
		if (event->type() == QEvent::MouseButtonPress)
		{
    
			//m_SerPort = Port_Left;
			QMouseEvent* mousePressEvent = (QMouseEvent*)event;
			if (mousePressEvent->buttons() & Qt::LeftButton)
			{
    
				QPoint p1 = mousePressEvent->pos();
				QString s1 = QString("x=%1,y=%2").arg(QString::number(p1.x())).arg(QString::number(p1.y()));
				ui.lineEdit_left->setText(s1);
			
			}
			
		}
		if (event->type() == QEvent::MouseButtonDblClick)
		{
    
			qDebug() << QStringLiteral("label Use MouseButtonDblClick");
		}
	}
	if (ui.label_move == watched)
	{
    
		
		if (event->type() == QEvent::MouseMove)
		{
    
			QMouseEvent* mouseEvent = static_cast<QMouseEvent*>(event);
			qDebug() << QStringLiteral(" This is the filter  MouseMove");
			// Intercept 
			return true;
		}
		if (event->type() == QEvent::MouseButtonPress)
		{
    
			QMouseEvent* mouseEvent = static_cast<QMouseEvent*>(event);
			qDebug() << QStringLiteral(" This is the filter  MouseButtonPress ");
			// Don't intercept , Keep delivering 
			return false;
		}	
	}
	// Pass on other events 
	return QWidget::eventFilter(watched, event);
	
}
void QtEventFilter::mouseDoubleClickEvent(QMouseEvent* event)
{
    
	if (event->type() == QEvent::MouseButtonDblClick)
	{
    
		qDebug() << "QEvent::MouseButtonDblClick"; 
	}
}

void QtEventFilter::mousePressEvent(QMouseEvent* event)
{
    
	if (event->type() == QEvent::MouseButtonPress)
	{
    
		qDebug() << QStringLiteral(" This is the parent component  QEvent::MouseButtonPress");
	}
}

void QtEventFilter::mouseMoveEvent(QMouseEvent* event)
{
    
	if (event->type() == QEvent::MouseMove)
	{
    
		qDebug() << QStringLiteral(" This is the parent component  QEvent::MouseMove");
	}
}

QtEventFilter.h

#include <QtWidgets/QWidget>
#include "ui_QtEventFilter.h"
#include <QRect>
#include <QEvent>
#include <QMouseEvent>
#include <QKeyEvent>
class QtEventFilter : public QWidget
{
    
	Q_OBJECT

public:
	QtEventFilter(QWidget *parent = Q_NULLPTR);
	~QtEventFilter();
protected:
	bool eventFilter(QObject* watched, QEvent* event);
	void mouseDoubleClickEvent(QMouseEvent* event);
	void mousePressEvent(QMouseEvent* event);
	void mouseMoveEvent(QMouseEvent* event);

private:
	Ui::QtEventFilterClass ui;
};

Let's look at the event filter

true： The ui.lable_move Object's mouseMove The event was intercepted , Do not pass down execution MyLabel Rewrite the mousemove event
false： The ui.lable_move Object's mousepress The event is not intercepted , Continue to pass down execution MyLabel Rewrite the mousepress event
Pictured ：

It is proved that the event filter filters MyLabel Rewrite the mousemove event . If there's no filtering , prints “ This is the subcomponent ：mouseMoveEvent x=%1,y=%2”.

When the sub component receives mousepress When an event is ：
e->ignore();// Ignore this event , Continue to pass on （ Passed to its parent class ）
e->accept();// Accept this event , Don't continue to pass down （ The next object passed is its parent , Is not the base class ）

Proved ： After the sub component accepts the event , If four e->ignore The corresponding event that will continue to be passed to the parent component

There is no effect on other events except the filtered events , Because the event dispatcher function of the parent class is called at the end of the rewritten event dispatcher function

// The event dispatcher function of the parent class , Other events are distributed according to the default distribution process 
return QWidget::eventFilter(watched, event);	

This ensures that other events are distributed according to the default distribution process , And was eventually disposed of by the window .

ui chart ：

Reference blog ：
https://subingwen.cn/qt/event_handler/
https://blog.csdn.net/fengjliu/article/details/48572089