[QT] QT event handling

Event handling

QT in , Event as an object , Inherited from QEvent class , Common keyboard events QKeyEvent、 Mouse events QMouseEvent And timer events QTimerEvent etc. .QT in , whatever QObject Subclass examples can receive and process events . In the actual programming, we usually realize the paintEvent()、mousePressEvent() And other event handling functions to handle specific events of specific parts .

Of every program main Functions are called at the end QApplication Class exec() function , It will make QT The application goes to The event loop , Make the application run Receive various events . Once something happens ,QT A corresponding QEvent Subclass object to represent it , And pass it on to QObject Object or subobject .

---

Mouse events

Rewrite the mouse implementation to achieve the functions you want to achieve .

mouseevent.h

#ifndef MOUSEEVENT_H
#define MOUSEEVENT_H

#include <QMainWindow>
#include<QLabel>
#include<QMouseEvent>
namespace Ui {
class MouseEvent;
}

class MouseEvent : public QMainWindow
{
    Q_OBJECT

public:
    explicit MouseEvent(QWidget *parent = 0);
    ~MouseEvent();
protected:
    void mousePressEvent(QMouseEvent *event);// The mouse click 
    void mouseMoveEvent(QMouseEvent *event);// Mouse movement 
    void mouseReleaseEvent(QMouseEvent *event);// Mouse release 
private:
    Ui::MouseEvent *ui;
    QLabel *m_statusLabel;
    QLabel* m_posLabel;
};

#endif // MOUSEEVENT_H

mouseevent.cpp

#include "mouseevent.h"
#include "ui_mouseevent.h"

MouseEvent::MouseEvent(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MouseEvent)
{
    ui->setupUi(this);
    setWindowTitle(QString(" Mouse events "));
    m_statusLabel = new QLabel(QString(" The current position :"));
    m_statusLabel->setFixedWidth(100);

    m_posLabel = new QLabel(QString(""));
    m_posLabel->setFixedWidth(100);

    statusBar()->addPermanentWidget(m_statusLabel);// Add permanent parts to the status bar 
    statusBar()->addPermanentWidget(m_posLabel);
}

MouseEvent::~MouseEvent()
{
    delete ui;
}

void MouseEvent::mousePressEvent(QMouseEvent *event)
{
    QString str = "("+QString::number(event->x())+","+QString::number(event->y())+")";
    if(event->button() == Qt::LeftButton)
    {
        statusBar()->showMessage(QString(" left-click :") +str );
    }
    else if(event->button() == Qt::MidButton)
    {
        statusBar()->showMessage(QString(" In the key :")+str);
    }
    else if(event->button() == Qt::RightButton)
    {
        statusBar()->showMessage(QString(" Right click ")+str);
    }
}

void MouseEvent::mouseMoveEvent(QMouseEvent *event)
{
    QString strPos;
    strPos = "(" + QString::number(event->x())+","+QString::number(event->y())+")";
    m_posLabel->setText(strPos);
}

void MouseEvent::mouseReleaseEvent(QMouseEvent *event)
{
    QString strPos;
    strPos = "(" + QString::number(event->x())+","+QString::number(event->y())+")";
    statusBar()->showMessage(QString(" Release in :")+strPos,3000);
}

---

Keyboard events

Rewrite the keyboard events to achieve the desired effect .

keyevent.h

#ifndef KEYEVENT_H
#define KEYEVENT_H

#include <QWidget>
#include<QKeyEvent>
namespace Ui {
class KeyEvent;
}

class KeyEvent : public QWidget
{
    Q_OBJECT

public:
    explicit KeyEvent(QWidget *parent = 0);
    ~KeyEvent();
    void drawPix();
    void keyPressEvent(QKeyEvent *event) override;
    void paintEvent(QPaintEvent* event)override;

private:
    Ui::KeyEvent *ui;
    QPixmap *m_pix;
    QImage m_image;
    int m_startX;      // Icon vertex position 
    int m_startY;
    int m_width;       // The width of the interface , Height 
    int m_height;
    int m_step;         // step 

};

#endif // KEYEVENT_H

keyevent.cpp

#include "keyevent.h"
#include "ui_keyevent.h"
#include<QPainter>
#include<QPen>
KeyEvent::KeyEvent(QWidget *parent) :
   QWidget(parent),
   ui(new Ui::KeyEvent)
{
   ui->setupUi(this);
   setWindowTitle(QString(" Keyboard events "));
   setAutoFillBackground(true);
   setFixedSize(521,256);
   m_width = size().width();
   m_height = size().height();
   m_pix = new QPixmap(m_width,m_height);
   m_pix->fill(Qt::white);
   m_image.load("picture.jpg");
   m_startX = 100;
   m_startY = 100;
   m_step = 20;
   drawPix();

}

KeyEvent::~KeyEvent()
{
   delete ui;
}

void KeyEvent::drawPix()
{
   m_pix->fill(Qt::white);
   QPainter painter(this);
   QPen pen(Qt::DotLine);
   // Draw vertical gridlines in steps 
   //bagin And end Pairs appear 
   for(int i = m_step;i < m_width; i+=m_step)
   {
       painter.begin(m_pix);// Appoint m_pix For drawing equipment 
       painter.setPen(pen);// Set pen 
       painter.drawLine(QPoint(i,0),QPoint(i,height()));
       painter.end();
   }
   // Draw horizontal gridlines in steps 
   for(int j = m_step;j < m_height;j += m_step)
   {
       painter.begin(m_pix);// Appoint m_pix For drawing equipment 
       painter.setPen(pen);// Set pen 
       painter.drawLine(QPoint(0,j),QPoint(m_width,j));
       painter.end();
   }
   // Draw pictures 
   painter.begin(m_pix);
   painter.drawImage(QPoint(m_startX,m_startY),m_image);
   painter.end();
}

void KeyEvent::keyPressEvent(QKeyEvent *event)
{
   // Press down ctrl, Every move is 1 Pixel 
   if(event->modifiers() == Qt::ControlModifier)
   {
        if(event->key() == Qt::Key_Left)
        {
            m_startX = (m_startX -1)<0 ? m_startX : m_startX-1;
        }
        if(event->key() == Qt::Key_Right)
        {
            m_startX = (m_startX +1+m_image.width()) > m_width ? m_startX :m_startX+1 ;
        }
        if(event->key() == Qt::Key_Up)
        {
           m_startY = (m_startY - 1) < 0 ? m_startY:m_startY -1;
        }
        if(event->key() == Qt::Key_Down)
        {
           m_startY = (m_startY +1 +m_image.height()) > m_height ? m_startY:m_startY+1;
        }
   }
   else// I didn't press ctrl key , Each move is a step 
   {
       // Adjust the upper left corner of the icon to the vertex of the mesh 
       m_startX = m_startX - m_startX % m_step;
       m_startY  =m_startY - m_startY % m_step;
       if(event->key() == Qt::Key_Left)
       {
           m_startX = (m_startX -m_step)<0 ? m_startX : m_startX-m_step;
       }
       if(event->key() == Qt::Key_Right)
       {
           m_startX = (m_startX +m_step+m_image.width()) > m_width ? m_startX :m_startX+m_step ;
       }
       if(event->key() == Qt::Key_Up)
       {
          m_startY = (m_startY - m_step) < 0 ? m_startY:m_startY -m_step;
       }
       if(event->key() == Qt::Key_Down)
       {
          m_startY = (m_startY +m_step +m_image.height()) > m_height ? m_startY:m_startY+m_step;
       }
   }
   drawPix();// According to the adjusted icon position, reset the icon in m_pix Draw an image on 
   update();// Trigger window redrawing 
}

void KeyEvent::paintEvent(QPaintEvent *event)
{
   QPainter painter;
   painter.begin(this);
   painter.drawPixmap(QPoint(0,0),*m_pix);
   painter.end();
}

---

Event filtering

Specify what events an object handles .

** Example ：** Press the mouse button to zoom the specified picture

dialog.h

#ifndef DIALOG_H
#define DIALOG_H

#include <QDialog>
#include<QLabel>
#include<QImage>
#include<QHBoxLayout>
#include<QEvent>
#include<QMouseEvent>

class Dialog : public QDialog
{
    Q_OBJECT

public:
    Dialog(QWidget *parent = 0,Qt::WindowFlags f = 0);
    ~Dialog();
public slots:
    bool eventFilter(QObject*watched ,QEvent *event)override;
private:
    QLabel *m_label1;
    QLabel *m_label2;
    QLabel *m_label3;
    QLabel *m_stateLabel;

    QImage m_image1;
    QImage m_image2;
    QImage m_image3;


};

#endif // DIALOG_H

dialog.cpp

#include "dialog.h"
#include<QHBoxLayout>

Dialog::Dialog(QWidget *parent,Qt::WindowFlags f)
    : QDialog(parent,f)
{
    setWindowTitle(QString(" Event filtering "));

    m_label1 = new QLabel;
    m_label2 =new QLabel;
    m_label3 = new QLabel;
    m_stateLabel = new QLabel(QString(" Press the mouse button to mark "));



    m_stateLabel->setAlignment(Qt::AlignHCenter);// Horizontal center 

    m_image1.load("fly1.png");
    m_image2.load("fly2.png");
    m_image3.load("fly3.png");

    m_label1->setPixmap(QPixmap::fromImage(m_image1));
    m_label2->setPixmap(QPixmap::fromImage(m_image2));
    m_label3->setPixmap(QPixmap::fromImage(m_image3));
    // Add horizontal layout 
    QHBoxLayout *layout = new QHBoxLayout;
    layout->addWidget(m_label1);
    layout->addWidget(m_label2);
    layout->addWidget(m_label3);


    QVBoxLayout*mainLayout = new QVBoxLayout(this);
    mainLayout->addLayout(layout);
    mainLayout->addWidget(m_stateLabel);

    resize(m_image2.width() * 3,m_image2.height() *2);

    // Install event filtering for the picture label part , And specify the whole form as the object for monitoring events .
    m_label1->installEventFilter(this);
    m_label2->installEventFilter(this);
    m_label3->installEventFilter(this);


}

Dialog::~Dialog()
{

}

// object - event 
bool Dialog::eventFilter(QObject *watched, QEvent *event)
{
    QMatrix matrix;// enlarge scale 
    QImage tmpImg;// Save the processed image 

    // Enlarge picture 
    matrix.scale(2.0,2.0);
    // object 
    if(watched == m_label1)
    {
        // Handling of mouse down events 
        if(event->type() == QEvent::MouseButtonPress)
        {
            // Convert event types Wie Mouse events 
            QMouseEvent* mouseEvent = (QMouseEvent*)event;
            if(mouseEvent->button() & Qt::LeftButton)
            {
                m_stateLabel->setText(QString(" Left click on the picture 1"));
            }
            if(mouseEvent->button() & Qt::MidButton)
            {
                m_stateLabel->setText(QString(" Middle press the picture 1"));
            }
            if(mouseEvent->button() & Qt::RightButton)
            {
                m_stateLabel->setText(QString(" Right click on the picture 1"));
            }
            // Zoom the original image with tmpImg Temporary storage , Set pictures 
            tmpImg = m_image1.transformed(matrix);
            m_label1->setPixmap(QPixmap::fromImage(tmpImg));
        }
        // Mouse release , Restore picture size 
        if(event->type() == QEvent::MouseButtonRelease)
        {
            m_stateLabel->setText(QString(" Mouse release picture 1"));
            m_label1->setPixmap(QPixmap::fromImage(m_image1));
        }
    }
  else if(watched ==m_label2)
  {
        if(event->type() == QEvent::MouseButtonPress)
        {
            QMouseEvent* mouseEvent = (QMouseEvent*)event;
            if(mouseEvent->button() & Qt::LeftButton)
            {
                m_stateLabel->setText(QString(" Left click on the picture 2"));
            }
            if(mouseEvent->button() & Qt::MidButton)
            {
                m_stateLabel->setText(QString(" Middle press the picture 2"));
            }
            if(mouseEvent->button() & Qt::RightButton)
            {
                m_stateLabel->setText(QString(" Right click on the picture 2"));
            }
            tmpImg = m_image2.transformed(matrix);
            m_label2->setPixmap(QPixmap::fromImage(tmpImg));
        }
        if(event->type() == QEvent::MouseButtonRelease)
        {
            m_stateLabel->setText(QString(" Mouse release picture 2"));
            m_label2->setPixmap(QPixmap::fromImage(m_image2));
        }
  }
  else if(watched ==m_label3)
  {
      if(event->type() == QEvent::MouseButtonPress)
      {
          QMouseEvent* mouseEvent = (QMouseEvent*)event;
          if(mouseEvent->button() & Qt::LeftButton)
          {
              m_stateLabel->setText(QString(" Left click on the picture 3"));
          }
          if(mouseEvent->button() & Qt::MidButton)
          {
              m_stateLabel->setText(QString(" Middle press the picture 3"));
          }
          if(mouseEvent->button() & Qt::RightButton)
          {
              m_stateLabel->setText(QString(" Right click on the picture 3"));
          }
          tmpImg = m_image3.transformed(matrix);
          m_label3->setPixmap(QPixmap::fromImage(tmpImg));
      }
      if(event->type() == QEvent::MouseButtonRelease)
      {
          m_stateLabel->setText(QString(" Mouse release picture 3"));
          m_label3->setPixmap(QPixmap::fromImage(m_image3));
      }
  }

    // The event is handed over to the upper dialog box for processing 
    return QDialog::eventFilter(watched,event);
}