十四、QGraphicsView的paintEvent雙緩存繪畫（既自由繪畫）

RectView.h

#ifndef RECTVIEW_H
#define RECTVIEW_H

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

class RectView : public QGraphicsView
{
    Q_OBJECT
public:
    RectView(QWidget *parent);
    ~RectView();

    // QWidget interface
protected:
    void mousePressEvent(QMouseEvent *event) Q_DECL_OVERRIDE;
    void mouseReleaseEvent(QMouseEvent *event) Q_DECL_OVERRIDE;
    void mouseDoubleClickEvent(QMouseEvent *event) Q_DECL_OVERRIDE;
    void mouseMoveEvent(QMouseEvent *event) Q_DECL_OVERRIDE;
    void keyPressEvent(QKeyEvent *event) Q_DECL_OVERRIDE;
    void paintEvent(QPaintEvent *event) Q_DECL_OVERRIDE;
    void resizeEvent(QResizeEvent *event) Q_DECL_OVERRIDE;

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

#endif // RECTVIEW_H

RectView.cpp

#include "rectview.h"

RectView::RectView(QWidget *parent) : QGraphicsView(parent)
{
    isDrawing = false;
    isDoubleClick = false;
}

RectView::~RectView()
{

}

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

    QGraphicsView::mousePressEvent(event);
}

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

    QGraphicsView::mouseReleaseEvent(event);
}

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

    QGraphicsView::mouseDoubleClickEvent(event);
}

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

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

/**
 * @brief RectView::paintEvent
 *     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裏面並在tempPix裏面畫，
 * 我們此時看到的就是在tempPix裏的圖形。只在鼠標釋放的時候纔在pix繪一次。
 */
void RectView::paintEvent(QPaintEvent *event)
{
    QPainter painter(this->viewport());
    if (isDrawing)
    {
        tempPix = pix;
        QPainter pp(&tempPix);
        pp.drawLine(lasetPoint, endPoint);
        painter.drawPixmap(0, 0, tempPix);
    }
    else
    {
        QPainter pp(&pix);
        pp.drawLine(lasetPoint, endPoint);
        painter.drawPixmap(0, 0, pix);
    }

    QGraphicsView::paintEvent(event);
}

//這裏纔是真正讀取控件寬高的地方
void RectView::resizeEvent(QResizeEvent *event)
{
    pix = QPixmap(width(), height());
    pix.fill(Qt::white);
}