Qt 通过重写QGraphicItem实现绘制、拖动、缩放、旋转矩形

本例程通过重写了一个类，继承自QGraphicItem，来实现了在qgraphicsScene上绘制、拖动、缩放、旋转矩形。
效果如下：

其实要实现绘制、拖动、缩放矩形都不难，难的是在旋转之后还要支持缩放。
我的思路是：
1.实现绘制矩形：只要定义一个全局变量QRectF m_oldRect，在外面矩形大小传进来，然后在paint函数里面绘制这个矩形就行
2.实现拖动矩形：重写mousePressEvent，mouseMoveEvent，mouseReleaseEvent，记录鼠标按下的起始点和移动时候的点，并用moveBy()函数来移动矩形即可
3.实现缩放：在矩形内部靠近4条边的地方定义4个矩形，当鼠标按下的时候在这4个矩形方框内，则将矩形往4个方向拉伸
4.实现旋转：
我给之前定义的矩形全部配一个QPolygonF，因为我只能通过绘制多边形的方式来画出旋转之后的矩形。
矩形正上方的圆圈我是通过三角函数+直线方程来计算，让其始终绘制在矩形左右两个顶点的中垂线上方。
当鼠标落在圆形内部的时候可以控制矩形旋转。
在矩形旋转之后，再进行拉伸的时候，我是通过的计算鼠标的点离对面的边的距离来重新计算矩形的大小，然后计算对应的QPolygonF的大小。

主要代码如下：
mygraphicrectitem.h

#ifndef MYGRAPHICRECTITEM_H
#define MYGRAPHICRECTITEM_H
#include <QObject>
#include <QWidget>
#include <QMouseEvent>
#include <QGraphicsScene>
#include <QGraphicsRectItem>
#include <QGraphicsSceneMouseEvent>
#include <QRect>
#include <QPainter>
#include <QPolygon>
#include <QList>
enum STATE_FLAG{
    DEFAULT_FLAG=0,
    MOV_LEFT_LINE,//标记当前为用户按下矩形的左边界区域
    MOV_TOP_LINE,//标记当前为用户按下矩形的上边界区域
    MOV_RIGHT_LINE,//标记当前为用户按下矩形的右边界区域
    MOV_BOTTOM_LINE,//标记当前为用户按下矩形的下边界区域
    MOV_RIGHTBOTTOM_RECT,//标记当前为用户按下矩形的右下角
    MOV_RECT,//标记当前为鼠标拖动图片移动状态
    ROTATE//标记当前为旋转状态
};
class myGraphicRectItem:public QObject,public QGraphicsItem
{
    Q_OBJECT
public:
    myGraphicRectItem(QGraphicsItem *parent = nullptr);
    //myGraphicRectItem(QRectF m_OriginRect = QRectF(0,0,100,100));
    QRectF  boundingRect() const;
    ~myGraphicRectItem();
    void setRectSize(QRectF mrect,bool bResetRotateCenter = true);
    void paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget);
    void mousePressEvent(QGraphicsSceneMouseEvent *event);
    void mouseMoveEvent(QGraphicsSceneMouseEvent *event);
    void mouseReleaseEvent(QGraphicsSceneMouseEvent *event);
    void SetRotate(qreal RotateAngle,QPointF ptCenter=QPointF(-999,-999));
    QPointF getRotatePoint(QPointF ptCenter, QPointF ptIn, qreal angle);//获取旋转后的点
    QList<QPointF> getRotatePoints(QPointF ptCenter,QList<QPointF> ptIns,qreal angle);//获取多个旋转后的点
    QPolygonF getRotatePolygonFromRect(QPointF ptCenter,QRectF rectIn,qreal angle);//将矩形旋转之后返回多边形
    QRectF getCrtPosRectToSceen();
    QRectF m_SmallRotateRect;//矩形顶部用来表示旋转的标记的矩形
    QPolygonF m_SmallRotatePolygon;//矩形顶部用来表示旋转的标记的矩形旋转后形成的多边形
    QPointF getSmallRotateRectCenter(QPointF ptA,QPointF ptB);//获取旋转时候矩形正上方的旋转标记矩形
    QRectF  getSmallRotateRect(QPointF ptA,QPointF ptB);
    bool    m_bRotate;
    qreal   m_RotateAngle;
    QPointF m_RotateCenter;

private:
    QRectF  m_oldRect;
    QPolygonF m_oldRectPolygon;
    QRectF  m_RotateAreaRect;
    bool    m_bResize;
    QPolygonF m_insicedPolygon;
    QRectF  m_insicedRectf;
    QPolygonF m_leftPolygon;
    QRectF  m_leftRectf;
    QPolygonF m_topPolygon;
    QRectF  m_topRectf;
    QPolygonF m_rightPolygon;
    QRectF  m_rightRectf;
    QPolygonF m_bottomPolygon;
    QRectF  m_bottomRectf;
//    QPolygonF m_rbPolygon;
//    QRectF  m_rbRectf;
    QPointF m_startPos;
    STATE_FLAG m_StateFlag;
    QPointF *pPointFofSmallRotateRect;
protected:

};
#endif // MYGRAPHICRECTITEM_H

mygraphicrectitem.cpp

#include "mygraphicrectitem.h"
#include <QtMath>
#include <QDebug>

myGraphicRectItem::myGraphicRectItem(QGraphicsItem *parent):
    m_bResize(false),
    m_oldRect(0,0,100,100),
    m_bRotate(false),
    m_RotateAngle(0),
    m_StateFlag(DEFAULT_FLAG)
{
    //setParent(parent);
    setRectSize(m_oldRect);
    setToolTip("Click and drag me!");  //提示
    setCursor(Qt::ArrowCursor);   //改变光标形状,手的形状
    setFlag(QGraphicsItem::ItemIsMovable);
    //    setAcceptDrops(true);
    pPointFofSmallRotateRect = new QPointF[4];
    SetRotate(0);
    setFlag(QGraphicsItem::ItemIsSelectable);//
}

QRectF myGraphicRectItem::boundingRect() const
{
    //return m_oldRectPolygon.boundingRect();
    QRectF boundingRectF = m_oldRectPolygon.boundingRect();
    return QRectF(boundingRectF.x()-40,boundingRectF.y()-40,boundingRectF.width()+80,boundingRectF.height()+80);
}

myGraphicRectItem::~myGraphicRectItem()
{
    delete []pPointFofSmallRotateRect;
    pPointFofSmallRotateRect = nullptr;
}

void myGraphicRectItem::setRectSize(QRectF mrect, bool bResetRotateCenter)
{
    m_oldRect = mrect;
    if(bResetRotateCenter)
    {
        m_RotateCenter.setX(m_oldRect.x()+m_oldRect.width()/2);
        m_RotateCenter.setY(m_oldRect.y()+m_oldRect.height()/2);
    }
    m_oldRectPolygon = getRotatePolygonFromRect(m_RotateCenter,m_oldRect,m_RotateAngle);

    m_insicedRectf = QRectF(m_oldRect.x()+8,m_oldRect.y()+8,m_oldRect.width()-16,m_oldRect.height()-16);
    m_insicedPolygon =getRotatePolygonFromRect(m_RotateCenter,m_insicedRectf,m_RotateAngle);

    m_leftRectf = QRectF(m_oldRect.x(),m_oldRect.y(),8,m_oldRect.height()-8);
    m_leftPolygon = getRotatePolygonFromRect(m_RotateCenter,m_leftRectf,m_RotateAngle);

    m_topRectf = QRectF(m_oldRect.x()+8,m_oldRect.y(),m_oldRect.width()-8,8);
    m_topPolygon = getRotatePolygonFromRect(m_RotateCenter,m_topRectf,m_RotateAngle);

    m_rightRectf = QRectF(m_oldRect.right()-8,m_oldRect.y()+8,8,m_oldRect.height()-16);
    m_rightPolygon = getRotatePolygonFromRect(m_RotateCenter,m_rightRectf,m_RotateAngle);

    m_bottomRectf = QRectF(m_oldRect.x(),m_oldRect.bottom()-8,m_oldRect.width()-8,8);
    m_bottomPolygon = getRotatePolygonFromRect(m_RotateCenter,m_bottomRectf,m_RotateAngle);

//    m_rbRectf = QRectF(m_oldRect.right()-8,m_oldRect.bottom()-8,8,8);
//    m_rbPolygon = getRotatePolygonFromRect(m_RotateCenter,m_rbRectf,m_RotateAngle);

    m_SmallRotateRect = getSmallRotateRect(mrect.topLeft(),mrect.topRight());//矩形正上方的旋转标记矩形
    m_SmallRotatePolygon = getRotatePolygonFromRect(m_RotateCenter,m_SmallRotateRect,m_RotateAngle);
}

void myGraphicRectItem::paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget)
{
    QPen mPen = QPen(Qt::yellow);
    painter->setPen(mPen);
    //绘制旋转后的矩形
    painter->drawPolygon(m_oldRectPolygon);
    //绘制旋转圆形
    mPen.setWidth(2);
    mPen.setColor(Qt::green);
    painter->setPen(mPen);
    QPointF pf = getSmallRotateRectCenter(m_oldRectPolygon[0],m_oldRectPolygon[1]);
    QRectF rect = QRectF(pf.x()-10,pf.y()-10,20,20);
    painter->drawEllipse(rect);//绘制圆形
    painter->drawPoint(pf);//绘制点
}

void myGraphicRectItem::mousePressEvent(QGraphicsSceneMouseEvent *event)
{
    if(event->button()== Qt::LeftButton)
    {
        m_startPos = event->pos();//鼠标左击时，获取当前鼠标在图片中的坐标，
        if(m_SmallRotatePolygon.containsPoint(m_startPos,Qt::WindingFill))//旋转矩形
        {
            setCursor(Qt::PointingHandCursor);
            m_StateFlag = ROTATE;
        }
        else if(m_insicedPolygon.containsPoint(m_startPos,Qt::WindingFill))//在矩形内框区域时按下鼠标，则可拖动图片
        {
            setCursor(Qt::ClosedHandCursor);   //改变光标形状,手的形状
            m_StateFlag = MOV_RECT;//标记当前为鼠标拖动图片移动状态
        }
        else if(m_leftPolygon.containsPoint(m_startPos,Qt::WindingFill))
        {
            setCursor(Qt::SizeHorCursor);
            m_StateFlag = MOV_LEFT_LINE;//标记当前为用户按下矩形的左边界区域
        }
        else if(m_rightPolygon.containsPoint(m_startPos,Qt::WindingFill))
        {
            setCursor(Qt::SizeHorCursor);
            m_StateFlag = MOV_RIGHT_LINE;//标记当前为用户按下矩形的右边界区域
        }
        else if(m_topPolygon.containsPoint(m_startPos,Qt::WindingFill))
        {
            setCursor(Qt::SizeVerCursor);
            m_StateFlag = MOV_TOP_LINE;//标记当前为用户按下矩形的上边界区域
        }
        else if(m_bottomPolygon.containsPoint(m_startPos,Qt::WindingFill))
        {
            setCursor(Qt::SizeVerCursor);
            m_StateFlag = MOV_BOTTOM_LINE;//标记当前为用户按下矩形的下边界区域
        }
//        else if(m_rbPolygon.containsPoint(m_startPos,Qt::WindingFill))
//        {
//            setCursor(Qt::SizeFDiagCursor);
//            m_StateFlag = MOV_RIGHTBOTTOM_RECT;//标记当前为用户按下矩形的右下角
//        }
        else
        {
            m_StateFlag = DEFAULT_FLAG;
        }
    }
    else
    {
        QGraphicsItem::mousePressEvent(event);
    }
}

void myGraphicRectItem::mouseMoveEvent(QGraphicsSceneMouseEvent *event)
{
    if(m_StateFlag == ROTATE)
    {
       int nRotateAngle = atan2((event->pos().x()-m_RotateCenter.x()),(event->pos().y()-m_RotateCenter.y()))*180/M_PI;
       SetRotate(180-nRotateAngle);
       setRectSize(m_oldRect);
       //qDebug()<<nRotateAngle;
    }
    else if(m_StateFlag == MOV_RECT)
    {
        QPointF point = (event->pos() - m_startPos);
        moveBy(point.x(), point.y());
        setRectSize(m_oldRect);
        scene()->update();
    }
    else if(m_StateFlag == MOV_LEFT_LINE)
    {
        QPointF pf = QPointF((m_oldRectPolygon.at(1).x()+m_oldRectPolygon.at(2).x())/2,((m_oldRectPolygon.at(1).y()+m_oldRectPolygon.at(2).y())/2));
        //计算到右侧边中点的距离
        qreal dis = sqrt((event->pos().x()-pf.x())*(event->pos().x()-pf.x()) +(event->pos().y()-pf.y())*(event->pos().y()-pf.y()));
        qreal dis2LT = sqrt((event->pos().x()-m_oldRectPolygon.at(0).x())*(event->pos().x()-m_oldRectPolygon.at(0).x()) +(event->pos().y()-m_oldRectPolygon.at(0).y())*(event->pos().y()-m_oldRectPolygon.at(0).y()));
        qreal dis2RT = sqrt((event->pos().x()-m_oldRectPolygon.at(1).x())*(event->pos().x()-m_oldRectPolygon.at(1).x()) +(event->pos().y()-m_oldRectPolygon.at(1).y())*(event->pos().y()-m_oldRectPolygon.at(1).y()));
        if(dis<16||dis2LT>dis2RT)
        {
            return;
        }
        else
        {
            QRectF newRect(m_oldRect);
            newRect.setLeft(m_oldRect.right()-dis);
            newRect.setRight(m_oldRect.right());
            setRectSize(newRect,false);
            m_RotateCenter=QPointF((m_oldRectPolygon.at(0).x()+m_oldRectPolygon.at(2).x())/2,(m_oldRectPolygon.at(0).y()+m_oldRectPolygon.at(2).y())/2);
            m_oldRect.moveCenter(m_RotateCenter);
            setRectSize(m_oldRect);
            scene()->update();//必须要用scene()->update()，不能用update();否则会出现重影
        }
    }
    else if(m_StateFlag == MOV_TOP_LINE)
    {
        //底边中点
        QPointF pf = QPointF((m_oldRectPolygon.at(2).x()+m_oldRectPolygon.at(3).x())/2,((m_oldRectPolygon.at(2).y()+m_oldRectPolygon.at(3).y())/2));
        //计算到底边中点的距离
        qreal dis = sqrt((event->pos().x()-pf.x())*(event->pos().x()-pf.x()) +(event->pos().y()-pf.y())*(event->pos().y()-pf.y()));
        qreal dis2LT = sqrt((event->pos().x()-m_oldRectPolygon.at(0).x())*(event->pos().x()-m_oldRectPolygon.at(0).x()) +(event->pos().y()-m_oldRectPolygon.at(0).y())*(event->pos().y()-m_oldRectPolygon.at(0).y()));
        qreal dis2LB = sqrt((event->pos().x()-m_oldRectPolygon.at(3).x())*(event->pos().x()-m_oldRectPolygon.at(3).x()) +(event->pos().y()-m_oldRectPolygon.at(3).y())*(event->pos().y()-m_oldRectPolygon.at(3).y()));
        if(dis<16||dis2LT>dis2LB)
        {
            return;
        }
        else
        {
            QRectF newRect(m_oldRect);
            newRect.setTop(m_oldRect.bottom()-dis);
            newRect.setBottom(m_oldRect.bottom());
            setRectSize(newRect,false);
            m_RotateCenter=QPointF((m_oldRectPolygon.at(0).x()+m_oldRectPolygon.at(2).x())/2,(m_oldRectPolygon.at(0).y()+m_oldRectPolygon.at(2).y())/2);
            m_oldRect.moveCenter(m_RotateCenter);
            setRectSize(m_oldRect);
            scene()->update();//必须要用scene()->update()，不能用update();否则会出现重影
        }
    }
    else if(m_StateFlag == MOV_RIGHT_LINE)
    {
        QPointF pf = QPointF((m_oldRectPolygon.at(0).x()+m_oldRectPolygon.at(3).x())/2,((m_oldRectPolygon.at(0).y()+m_oldRectPolygon.at(3).y())/2));
        //计算到左侧边中点的距离
        qreal dis = sqrt((event->pos().x()-pf.x())*(event->pos().x()-pf.x()) +(event->pos().y()-pf.y())*(event->pos().y()-pf.y()));
        qreal dis2LT = sqrt((event->pos().x()-m_oldRectPolygon.at(0).x())*(event->pos().x()-m_oldRectPolygon.at(0).x()) +(event->pos().y()-m_oldRectPolygon.at(0).y())*(event->pos().y()-m_oldRectPolygon.at(0).y()));
        qreal dis2RT = sqrt((event->pos().x()-m_oldRectPolygon.at(1).x())*(event->pos().x()-m_oldRectPolygon.at(1).x()) +(event->pos().y()-m_oldRectPolygon.at(1).y())*(event->pos().y()-m_oldRectPolygon.at(1).y()));
        if(dis<16||dis2LT<dis2RT)
        {
            return;
        }
        else
        {
            QRectF newRect(m_oldRect);
            newRect.setLeft(m_oldRect.left());
            newRect.setRight(m_oldRect.left()+dis);
            setRectSize(newRect,false);
            m_RotateCenter=QPointF((m_oldRectPolygon.at(0).x()+m_oldRectPolygon.at(2).x())/2,(m_oldRectPolygon.at(0).y()+m_oldRectPolygon.at(2).y())/2);
            m_oldRect.moveCenter(m_RotateCenter);
            setRectSize(m_oldRect);
            scene()->update();//必须要用scene()->update()，不能用update();否则会出现重影
        }
    }
    else if(m_StateFlag == MOV_BOTTOM_LINE)
    {
        //顶边中点
        QPointF pf = QPointF((m_oldRectPolygon.at(0).x()+m_oldRectPolygon.at(1).x())/2,((m_oldRectPolygon.at(0).y()+m_oldRectPolygon.at(1).y())/2));
        //计算到底边中点的距离
        qreal dis = sqrt((event->pos().x()-pf.x())*(event->pos().x()-pf.x()) +(event->pos().y()-pf.y())*(event->pos().y()-pf.y()));
        qreal dis2LT = sqrt((event->pos().x()-m_oldRectPolygon.at(0).x())*(event->pos().x()-m_oldRectPolygon.at(0).x()) +(event->pos().y()-m_oldRectPolygon.at(0).y())*(event->pos().y()-m_oldRectPolygon.at(0).y()));
        qreal dis2LB = sqrt((event->pos().x()-m_oldRectPolygon.at(3).x())*(event->pos().x()-m_oldRectPolygon.at(3).x()) +(event->pos().y()-m_oldRectPolygon.at(3).y())*(event->pos().y()-m_oldRectPolygon.at(3).y()));
        if(dis<16||dis2LT<dis2LB)
        {
            return;
        }
        else
        {
            QRectF newRect(m_oldRect);
            newRect.setTop(m_oldRect.top());
            newRect.setBottom(m_oldRect.top()+dis);
            setRectSize(newRect,false);
            m_RotateCenter=QPointF((m_oldRectPolygon.at(0).x()+m_oldRectPolygon.at(2).x())/2,(m_oldRectPolygon.at(0).y()+m_oldRectPolygon.at(2).y())/2);
            m_oldRect.moveCenter(m_RotateCenter);
            setRectSize(m_oldRect);
            scene()->update();//必须要用scene()->update()，不能用update();否则会出现重影
        }
    }
}

void myGraphicRectItem::mouseReleaseEvent(QGraphicsSceneMouseEvent *event)
{
    setCursor(Qt::ArrowCursor);
    if(m_StateFlag == MOV_RECT)
    {
        m_StateFlag = DEFAULT_FLAG;
    }
    else {
        QGraphicsItem::mouseReleaseEvent(event);
    }
}

void myGraphicRectItem::SetRotate(qreal RotateAngle, QPointF ptCenter)
{
    m_bRotate = true;
    if(ptCenter.x()==-999 && ptCenter.y()==-999)
    {
        m_RotateCenter = QPointF(m_oldRect.x()+m_oldRect.width()/2,m_oldRect.y()+m_oldRect.height()/2);
    }
    else
    {
        m_RotateCenter = ptCenter;
    }
    m_RotateAngle = RotateAngle;
    this->update();
}

QPointF myGraphicRectItem::getRotatePoint(QPointF ptCenter, QPointF ptIn, qreal angle)
{
    double dx = ptCenter.x();
    double dy = ptCenter.y();
    double x = ptIn.x();
    double y = ptIn.y();
    double xx,yy;
    xx = (x-dx)*cos(angle*M_PI/180)-(y-dy)*sin(angle*M_PI/180)+dx;
    yy = (x-dx)*sin(angle*M_PI/180)+(y-dy)*cos(angle*M_PI/180)+dy;

    return QPointF(xx,yy);
}

QList<QPointF> myGraphicRectItem::getRotatePoints(QPointF ptCenter, QList<QPointF> ptIns, qreal angle)
{
    QList<QPointF> lstPt;
    for(int i = 0;i<ptIns.count();i++)
    {
        lstPt.append(getRotatePoint(ptCenter,ptIns.at(i),angle));
    }
    return lstPt;
}

QPolygonF myGraphicRectItem::getRotatePolygonFromRect(QPointF ptCenter, QRectF rectIn, qreal angle)
{
    QVector<QPointF> vpt;
    QPointF pf = getRotatePoint(ptCenter,rectIn.topLeft(),angle);
    vpt.append(pf);
    pf = getRotatePoint(ptCenter,rectIn.topRight(),angle);
    vpt.append(pf);
    pf = getRotatePoint(ptCenter,rectIn.bottomRight(),angle);
    vpt.append(pf);
    pf = getRotatePoint(ptCenter,rectIn.bottomLeft(),angle);
    vpt.append(pf);
    pf = getRotatePoint(ptCenter,rectIn.topLeft(),angle);
    vpt.append(pf);
    return QPolygonF(vpt);
}

QRectF myGraphicRectItem::getCrtPosRectToSceen()
{
    QRectF retRect = QRectF(m_oldRect.x()+pos().x(),m_oldRect.y()+pos().y(),m_oldRect.width(),m_oldRect.height());
    return retRect;
}
QRectF myGraphicRectItem::getSmallRotateRect(QPointF ptA,QPointF ptB)
{
    QPointF pt = getSmallRotateRectCenter(ptA,ptB);
    return QRectF(pt.x()-10,pt.y()-10,20,20);
}
QPointF myGraphicRectItem::getSmallRotateRectCenter(QPointF ptA,QPointF ptB)
{
    QPointF ptCenter = QPointF((ptA.x()+ptB.x())/2,(ptA.y()+ptB.y())/2);//A,B点的中点C
    //中垂线方程式为 y=x*k + b;
    qreal x,y;//旋转图标矩形的中心
    if(abs(ptB.y()-ptA.y())<0.1)
    {
        if(ptA.x()<ptB.x())//矩形左上角在上方
        {
            x = ptCenter.x();
            y = ptCenter.y()-20;
        }
        else//矩形左上角在下方
        {
            x = ptCenter.x();
            y = ptCenter.y()+20;
        }
    }
    else if(ptB.y()>ptA.y())//顺时针旋转0-180
    {
        qreal k = (ptA.x()-ptB.x())/(ptB.y()-ptA.y());//中垂线斜率
        qreal b = (ptA.y()+ptB.y())/2-k*(ptA.x()+ptB.x())/2;
        //求AB线中垂线上离AB中点20个像素的点C的坐标
        x = 20*cos(atan(k))+ptCenter.x();
        y = k*x+b;
    }
    else if(ptB.y()<ptA.y())//顺时针旋转180-360
    {
        qreal k = (ptA.x()-ptB.x())/(ptB.y()-ptA.y());//中垂线斜率
        qreal b = (ptA.y()+ptB.y())/2-k*(ptA.x()+ptB.x())/2;
        //求AB线中垂线上离AB中点20个像素的点C的坐标
        x = -20*cos(atan(k))+ptCenter.x();
        y = k*x+b;
    }
    return QPointF(x,y);
}