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采用双缓冲实现界面实时响应鼠标的拖动绘制。

思想如下：首先需要两张画布pix和tempPix，他们都是QPixmap实例；pix用来保存初始界面或上一阶段以完成的绘制；tempPix用来作为鼠标拖动时的实时界面绘制；当鼠标左键按下后拖动时每次都将将pix赋值给tempPix，然后用tempPix进行绘制，最后鼠标左键释放时将tempPix赋值给pix。

widget.h代码如下

#ifndef WIDGET_H
#define WIDGET_H#include <QWidget>QT_BEGIN_NAMESPACE
namespace Ui { class Widget; }
QT_END_NAMESPACEclass Widget : public QWidget
{Q_OBJECTpublic:Widget(QWidget *parent = nullptr);~Widget();protected:void mousePressEvent(QMouseEvent *event);void mouseMoveEvent(QMouseEvent *event);void mouseReleaseEvent(QMouseEvent *event);void paintEvent(QPaintEvent *event);
private:Ui::Widget *ui;QPixmap pix;QPixmap tempPix;QPoint startPoint;QPoint endPoint;bool isDrawing;
};
#endif // WIDGET_H

widget.cpp代码如下：

#include "widget.h"
#include "ui_widget.h"
#include <QMouseEvent>
#include <QPainter>Widget::Widget(QWidget *parent): QWidget(parent), ui(new Ui::Widget)
{ui->setupUi(this);pix = QPixmap(400, 300);//宽，高pix.fill(Qt::white);tempPix = pix;isDrawing = false;
}void Widget::mousePressEvent(QMouseEvent *event)
{if(event->button() == Qt::LeftButton) {startPoint = event->pos();isDrawing = true;}
}void Widget::mouseMoveEvent(QMouseEvent *event)
{if(event->buttons() & Qt::LeftButton) {endPoint = event->pos();tempPix = pix;update(); //调用paintEvent();}
}void Widget::mouseReleaseEvent(QMouseEvent *event)
{if(event->button() == Qt::LeftButton) {endPoint = event->pos();isDrawing = false;update();}
}void Widget::paintEvent(QPaintEvent *)
{int x = startPoint.x();int y = startPoint.y();int width = endPoint.x() - x;int height = endPoint.y() - y;QPainter painter;painter.begin(&tempPix);painter.drawRect(x, y, width, height);painter.end();painter.begin(this);painter.drawPixmap(0, 0, tempPix);if(!isDrawing) {pix = tempPix;}
}Widget::~Widget()
{delete ui;
}

效果展示：