運行截圖如下：

代碼如下：

widget.h

#ifndef WIDGET_H #define WIDGET_H#include <QWidget>class QMouseEvent;namespace Ui { class Widget; }class Widget : public QWidget {Q_OBJECTpublic:explicit Widget(QWidget *parent = 0);~Widget();protected:void paintEvent(QPaintEvent *event)override;void mousePressEvent(QMouseEvent *event)override;void mouseMoveEvent(QMouseEvent *event)override;void mouseReleaseEvent(QMouseEvent *event)override;private:Ui::Widget *ui;QPixmap pix;QPoint lastPoint;QPoint endPoint;QPixmap tempPix;QPixmap tempPix2;bool isDrawing;bool isChange;int originalWidth;int originalHeight;bool winSizeIsChanged(); };#endif // WIDGET_H

widget.cpp

#include "widget.h" #include "ui_widget.h"#include <QComboBox> #include <QMouseEvent> #include <QPainter> #include <QDebug>Widget::Widget(QWidget *parent) :QWidget(parent),ui(new Ui::Widget) {ui->setupUi(this);this->setWindowTitle("CSDN IT1995");pix=QPixmap(this->width(),this->height());pix.fill(Qt::white);isDrawing=false;isChange=false;originalWidth=this->width();originalHeight=this->height();}Widget::~Widget() {delete ui; }void Widget::paintEvent(QPaintEvent *event){Q_UNUSED(event);int x,y,w,h;x=lastPoint.x();y=lastPoint.y();w=endPoint.x()-x;h=endPoint.y()-y;QPainter painter(this);if(isDrawing){tempPix=pix;QPainter pp(&tempPix);if(ui->comboBox->currentIndex()==0){pp.setPen(Qt::red);pp.drawRect(x,y,w,h);}if(ui->comboBox->currentIndex()==1){pp.setPen(Qt::green);pp.drawEllipse(x,y,w,h);}if(ui->comboBox->currentIndex()==2){pp.setPen(Qt::blue);pp.drawRoundRect(x,y,w,h);}painter.drawPixmap(0,0,tempPix);}else{QPainter pp(&pix);if(ui->comboBox->currentIndex()==0&&isChange){pp.setPen(Qt::red);pp.drawRect(x,y,w,h);}if(ui->comboBox->currentIndex()==1&&isChange){pp.setPen(Qt::green);pp.drawEllipse(x,y,w,h);}if(ui->comboBox->currentIndex()==2&&isChange){pp.setPen(Qt::blue);pp.drawRoundRect(x,y,w,h);}painter.drawPixmap(0,0,pix);}if(winSizeIsChanged()){pix=QPixmap(this->width(),this->height());pix.fill(Qt::white);painter.drawPixmap(0,0,pix);}isChange=false; }bool Widget::winSizeIsChanged(){if(this->width()!=originalWidth||this->height()!=originalHeight){originalHeight=this->height();originalWidth=this->width();return true;}return false; }void Widget::mousePressEvent(QMouseEvent *event){if(event->button()==Qt::LeftButton){lastPoint=event->pos();endPoint=lastPoint;isDrawing=true;}}void Widget::mouseMoveEvent(QMouseEvent *event){if(event->buttons()&Qt::LeftButton){endPoint=event->pos();update();} }void Widget::mouseReleaseEvent(QMouseEvent *event){if(event->button()==Qt::LeftButton){isChange=true;endPoint=event->pos();isDrawing=false;update();} }

main.cpp

#include "widget.h" #include <QApplication>int main(int argc, char *argv[]) {QApplication a(argc, argv);Widget w;w.show();return a.exec(); }

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