QGraphicsView Scene 鼠标移动缩放功能

项目场景：

创建一个QGraphicsView和QGraphicsScene，实现中键移动缩放

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移动代码实现

实现方法：移动鼠标时改变滚动条的值

void GraphicsView::mousePressEvent(QMouseEvent* event)
{
    if (event->button() == Qt::MiddleButton)
    {
        midButtonPressed = event->pos();
    }
    else
    {
        QGraphicsView::mousePressEvent(event);
    }
}

void GraphicsView::mouseMoveEvent(QMouseEvent* event)
{
    if (event->buttons() & Qt::MiddleButton)
    {
        auto offset = event->pos() - midButtonPressed;
        // scrollContentsBy(offset.x(), offset.y());	//错误代码
        verticalScrollBar()->setValue(verticalScrollBar()->value() - offset.y());
        horizontalScrollBar()->setValue(horizontalScrollBar()->value() - offset.x());
        midButtonPressed = event->pos();
    }
    else
    {
        QGraphicsView::mouseMoveEvent(event);
    }
}

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缩放代码实现：

实现方法：通过QGraphicsView::scale()进行缩放

void GraphicsView::wheelEvent(QWheelEvent* event)
{
    if (event->angleDelta().y() > 0)    //forward
    {
        m_scale = qMax(m_scale, 1.0);   //reset to 1 if m_scale < 1
        m_scale += 0.05;

        if (transform().m11() > 2)
            m_scale = 1;
    }
    else if (event->angleDelta().y() < 0)   //backward
    {
        m_scale = qMin(m_scale, 1.0);   //reset to 1 if m_scale > 1
        m_scale -= 0.05;

        if (transform().m11() < 0.3)
            m_scale = 1;
    }

    scale(m_scale, m_scale);
}

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错误提示：

1. QGraphicsView提供了scrollContentsBy(int dx, int dy) 的方法可以移动内容，但是在调用该方法移动并进行缩放会发现移动重置，查阅文档如下，应当修改为移动滚动条的值来移动

Calling this function in order to scroll programmatically is an error,
use the scroll bars instead (e.g. by calling QScrollBar::setValue() directly).

2.QGraphicsView::scale每次缩放的基础值都是1，大于1放大，小于1缩小。故每次缩放时先重置为1再计算缩放比例。当然也可以通过setTransform来直接修改矩阵中缩放值来修改缩放比