QGraphicsView的paintEvent双缓存绘画

MyGraphicsView.h

#ifndef MYGRAPHICSVIEW_H
#define MYGRAPHICSVIEW_H

#include <QGraphicsView>
#include <QMouseEvent>
#include <QPaintEvent>
#include <QKeyEvent>
#include <QPainter>
#include <QPixmap>
#include <QDebug>

class MyGraphicsView : public QGraphicsView
{
    Q_OBJECT

public:
    MyGraphicsView(QWidget *parent);
    ~MyGraphicsView();

protected:
    void mousePressEvent(QMouseEvent *event);
    void mouseDoubleClickEvent(QMouseEvent *event);
    void mouseMoveEvent(QMouseEvent *event);
    void mouseReleaseEvent(QMouseEvent *event);
    void paintEvent(QPaintEvent *event);
    void keyPressEvent(QKeyEvent *event);

private:
    QPixmap pix;
    QPoint lasetPoint;
    QPoint endPoint;
    QPixmap tempPix;
    bool isDrawing;
    bool isDoubleClick;
};

#endif // MYGRAPHICSVIEW_H

MyGraphicsView.cpp

#include "MyGraphicsView.h"

MyGraphicsView::MyGraphicsView(QWidget *parent)
    : QGraphicsView(parent)
{
    pix = QPixmap(1024, 768);
    pix.fill(Qt::white);
    isDrawing = false;
    isDoubleClick = false;
}

MyGraphicsView::~MyGraphicsView()
{

}

void MyGraphicsView::mousePressEvent(QMouseEvent *event)
{
    if (event->button() == Qt::LeftButton)
    {
        lasetPoint = event->pos();
        isDrawing = true;
        isDoubleClick = false;
    }

    QGraphicsView::mousePressEvent(event);
}

void MyGraphicsView::mouseDoubleClickEvent(QMouseEvent *event)
{
    if (event->button() == Qt::LeftButton)
    {
        isDoubleClick = true;
    }

    QGraphicsView::mouseDoubleClickEvent(event);
}

void MyGraphicsView::mouseMoveEvent(QMouseEvent *event)
{
    if (event->buttons() & Qt::LeftButton)
    {
        if (!isDoubleClick)
        {
            endPoint = event->pos();
            this->viewport()->update();
        }
    }

    QGraphicsView::mouseMoveEvent(event);
}

void MyGraphicsView::mouseReleaseEvent(QMouseEvent *event)
{
    if (event->button() == Qt::LeftButton)
    {
        if (!isDoubleClick)
        {
            endPoint = event->pos();
            isDrawing = false;
            this->viewport()->update();
        }
    }

    QGraphicsView::mouseReleaseEvent(event);
}


void MyGraphicsView::paintEvent(QPaintEvent *event)
{
    /// 直接在窗体上绘图，前1次画的矩形是不能保存住的
//     QPainter painter(this->viewport());
//     int x, y, w, h;
//     x = lasetPoint.x();
//     y = lasetPoint.y();
//     w = endPoint.x() - x;
//     h = endPoint.y() - y;
//     painter.drawRect(x, y, w, h);

    /// 直接在pix上绘图，前1次前2次前3次...画的都保存住了(鼠标每移动一点就会触发paintEvent函数而画一次)，所以在pix上会呈现好多矩形
//     int x, y, w, h;
//     x = lasetPoint.x();
//     y = lasetPoint.y();
//     w = endPoint.x() - x;
//     h = endPoint.y() - y;
//     QPainter pp(&pix);
//     pp.drawRect(x, y, w, h);
//     QPainter painter(this->viewport());
//     painter.drawPixmap(0, 0, pix);

    /// 双缓冲绘图，原理是在拖动过程中先把原来的图形复制到tempPix里面并在tempPix里面画，我们此时看到的就是在tempPix里的图形。只在鼠标释放的时候才在pix绘一次。
    int x, y, w, h;
    x = lasetPoint.x();
    y = lasetPoint.y();
    w = endPoint.x() - x;
    h = endPoint.y() - y;

    QPainter painter(this->viewport());
    if (isDrawing)
    {
        tempPix = pix;
        QPainter pp(&tempPix);
        pp.drawRect(x, y, w, h);
        painter.drawPixmap(0, 0, tempPix);
    }
    else
    {
        QPainter pp(&pix);
        pp.drawRect(x, y, w, h);
        painter.drawPixmap(0, 0, pix);
    }

    QGraphicsView::paintEvent(event);
}

void MyGraphicsView::keyPressEvent(QKeyEvent *event)
{
    QGraphicsView::keyPressEvent(event);
}

三种绘制方法效果图：