QGraphicsView的paintEvent雙緩存繪畫

MyGraphicsView.h

 1 #ifndef MYGRAPHICSVIEW_H
 2 #define MYGRAPHICSVIEW_H
 3 
 4 #include <QGraphicsView>
 5 #include <QMouseEvent>
 6 #include <QPaintEvent>
 7 #include <QKeyEvent>
 8 #include <QPainter>
 9 #include <QPixmap>
10 #include <QDebug>
11 
12 class MyGraphicsView : public QGraphicsView
13 {
14     Q_OBJECT
15 
16 public:
17     MyGraphicsView(QWidget *parent);
18     ~MyGraphicsView();
19 
20 protected:
21     void mousePressEvent(QMouseEvent *event);
22     void mouseDoubleClickEvent(QMouseEvent *event);
23     void mouseMoveEvent(QMouseEvent *event);
24     void mouseReleaseEvent(QMouseEvent *event);
25     void paintEvent(QPaintEvent *event);
26     void keyPressEvent(QKeyEvent *event);
27 
28 private:
29     QPixmap pix;
30     QPoint lasetPoint;
31     QPoint endPoint;
32     QPixmap tempPix;
33     bool isDrawing;
34     bool isDoubleClick;
35 };
36 
37 #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)
{
    /// 直接在窗體上繪圖，前1次畫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上繪圖，前1次前2次前3次...畫的都保存住了(滑鼠每移動一點就會觸發paintEvent函數而畫一次)，所以在pix上會呈現好多矩形
//     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);

    /// 雙緩衝繪圖，原理是在拖動過程中先把原來的圖形複製到tempPix裡面併在tempPix裡面畫，我們此時看到的就是在tempPix里的圖形。只在滑鼠釋放的時候才在pix繪一次。
    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);
}

三種繪製方法效果圖：