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_NAMESPACE

class Widget : public QWidget
{
    Q_OBJECT

public:
    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：