QGraphicsView 的使用（滾輪縮放）

1. 繼承QGraphicsView;

class MyGraphicsView : public QGraphicsView

2. 重寫構造函數

MyGraphicsView(QGraphicsScene *scene, QWidget *parent = 0);

3.設置 scene 尺寸

scene->setSceneRect(0, 0, 1280, 720);

4. 重寫paintEvent 函數，在其中設置世界座標變換和縮放

    QPainter painter(viewport());
    painter.setWorldTransform(this->viewportTransform(), true);
    painter.scale(m_zoomLevel, m_zoomLevel);

5. 重寫 wheelEvent 函數，

void QGraphicsView::wheelEvent(QWheelEvent* event)
{
    if (QApplication::keyboardModifiers() == Qt::ControlModifier) {
        qreal rate = event->delta() / 1200.00;
        m_zoomLevel += rate;
        updateZoomLevel(m_zoomLevel);
    }
    QGraphicsView::wheelEvent(event);
}

void MyGraphicsView::updateZoomLevel(qreal zoom)
{
    if (zoom >= 0.1 && zoom <= 9.99)
        m_zoomLevel = zoom;
    else if (zoom < 0.1)
        m_zoomLevel = 0.1;
    else
        m_zoomLevel = 9.99;

    int x = (width() - width() * zoom) * 0.5;
    int y = (height() - height() * zoom) * 0.5;
    scene()->setSceneRect(x, y, width() * zoom - 4, height() * zoom - 4);
    viewport()->update();
}

QGraphicsView 的重繪，用viewport()->update(); 直接調update是調用view 的刷新，而不是scene的刷新。