Qt画图

mainwindow.h

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>
#include<QMouseEvent>
#include<Qpainter>
#include<QDebug>

QT_BEGIN_NAMESPACE
namespace Ui { class MainWindow; }
QT_END_NAMESPACE

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    MainWindow(QWidget *parent = nullptr);
    ~MainWindow();

protected:
    // 画图事件
    void paintEvent(QPaintEvent *event);
    // 鼠标按下事件
    void mousePressEvent(QMouseEvent *event);
    // 鼠标移动事件
    void mouseMoveEvent(QMouseEvent *event);
    // 鼠标释放事件
    void mouseReleaseEvent(QMouseEvent *event);
    // 窗口重置事件
    virtual void resizeEvent(QResizeEvent *event) override;


private:
    Ui::MainWindow *ui;
    QPixmap pix,temp;
    QPoint lastPoint;
    QPoint endPoint;
    qreal scale;
    bool flag;
};
#endif // MAINWINDOW_H

mainwindow.cpp

#include "mainwindow.h"
#include "./ui_mainwindow.h"

MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
    , ui(new Ui::MainWindow)
{
    ui->setupUi(this);
    QRect geometry = this->geometry();
    int w = geometry.width();
    int h = geometry.height();
    pix = QPixmap(w,h);
    pix.fill(Qt::white);
    flag = false;
}

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


void MainWindow::paintEvent(QPaintEvent *){
    int x,y,w,h;
    x = lastPoint.x();
    y = lastPoint.y();
    w = endPoint.x()-x;
    h = endPoint.y()-y;
    QPainter painter(this);
    if(flag){
        temp = pix;
        QPainter ppainter(&temp);
        ppainter.drawRect(x,y,w,h);
        painter.drawPixmap(0,0,temp);
    }else{
        QPainter ppainter(&pix);
        ppainter.drawRect(x,y,w,h);
        painter.drawPixmap(0,0,pix);
    }
}

void MainWindow::mousePressEvent(QMouseEvent *event){
    if(event->button()==Qt::LeftButton){
        lastPoint = event->pos();
        flag = true;
    }

}

void MainWindow::mouseMoveEvent(QMouseEvent *event){
    if(event->button()&Qt::LeftButton){
        endPoint = event->pos();
        update();
    }

}

void MainWindow::mouseReleaseEvent(QMouseEvent *event){
    if(event->button()==Qt::LeftButton){
        endPoint = event->pos();
        flag = false;
        update();
    }
}

void MainWindow::resizeEvent(QResizeEvent *){
    QRect geometry = this->geometry();
    int w = geometry.width();
    int h = geometry.height();
    pix = QPixmap(w,h);
    pix.fill(Qt::white);
}