QT achieve adaptive image under display, zoom (centering the position of the mouse to zoom), drag, and a reset

System configuration: ThinkPad T570, Windows10, QT5.12.2 ( Qt Creater 4.8.2)
I have come to realize the above functions in the Graphics View by qt control.
My source download link: https://download.csdn.net/download/weixin_43935474/11119305
code to achieve the following steps:
1. Create a qt Interface Engineering, a Graphics View will drag the control to the main screen, as shown below:

2 Add the following code in the mainwindow.h:
contains its own writing class in the header

#include "imagewidget.h"

Add in the class:

private:
    ImageWidget     *m_Image;
public:
    void            recvShowPicSignal(QImage image);//接收并显示图片的函数

3. Add recvShowPicSignal function entity mainwindow.cpp as follows:

void MainWindow::recvShowPicSignal(QImage image)
{
    QPixmap ConvertPixmap=QPixmap::fromImage(image);//The QPixmap class is an off-screen image representation that can be used as a paint device
    QGraphicsScene  *qgraphicsScene = new QGraphicsScene;//要用QGraphicsView就必须要有QGraphicsScene搭配着用
    m_Image = new ImageWidget(&ConvertPixmap);//实例化类ImageWidget的对象m_Image，该类继承自QGraphicsItem，是自己写的类
    int nwith = ui->ImageGraphic->width();//获取界面控件Graphics View的宽度
    int nheight = ui->ImageGraphic->height();//获取界面控件Graphics View的高度
    m_Image->setQGraphicsViewWH(nwith,nheight);//将界面控件Graphics View的width和height传进类m_Image中
    qgraphicsScene->addItem(m_Image);//将QGraphicsItem类对象放进QGraphicsScene中
    ui->ImageGraphic->setSceneRect(QRectF(-(nwith/2),-(nheight/2),nwith,nheight));//使视窗的大小固定在原始大小，不会随图片的放大而放大（默认状态下图片放大的时候视窗两边会自动出现滚动条，并且视窗内的视野会变大），防止图片放大后重新缩小的时候视窗太大而不方便观察图片
    ui->ImageGraphic->setScene(qgraphicsScene);//Sets the current scene to scene. If scene is already being viewed, this function does nothing.
    ui->ImageGraphic->setFocus();//将界面的焦点设置到当前Graphics View控件
}

4. Create a c ++ class ImageWidget, header imagewidget.h follows:

#ifndef IMAGEWIDGET_H
#define IMAGEWIDGET_H

#include <QWidget>
#include <QtGui>
#include <QPixmap>
#include <QPainter>
#include <QRectF>
#include <QMouseEvent>
#include <QPointF>
#include <QDragEnterEvent>
#include <QGraphicsSceneWheelEvent>
#include <QGraphicsItem>

enum Enum_ZoomState{
    NO_STATE,
    RESET,
    ZOOM_IN,
    ZOOM_OUT
};
// class ImageWidget :public QObject, QGraphicsItem
class ImageWidget :public QGraphicsItem
{
    //Q_OBJECT
public:
    ImageWidget(QPixmap *pixmap);
    QRectF  boundingRect() const;
    void    paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget);
    void    wheelEvent(QGraphicsSceneWheelEvent *event);
    void    ResetItemPos();
    void    mousePressEvent(QGraphicsSceneMouseEvent *event);
    void    mouseMoveEvent(QGraphicsSceneMouseEvent *event);
    void    mouseReleaseEvent(QGraphicsSceneMouseEvent *event);
    qreal   getScaleValue() const;
    void    setQGraphicsViewWH(int nwidth,int nheight);
private:
    qreal       m_scaleValue;
    qreal       m_scaleDafault;
    QPixmap     m_pix;
    int         m_zoomState;
    bool        m_isMove;
    QPointF     m_startPos;
};
#endif // IMAGEWIDGET_H

5.ImageWidget source file imagewidget.cpp reads as follows:

#include "imagewidget.h"
#include <QDebug>
#include <QGraphicsSceneMouseEvent>
#include <QPointF>
#include <QGraphicsSceneDragDropEvent>
#include <QDrag>
#include <math.h>

ImageWidget::ImageWidget(QPixmap *pixmap)
{
    m_pix = *pixmap;
    setAcceptDrops(true);//If enabled is true, this item will accept hover events; otherwise, it will ignore them. By default, items do not accept hover events.
    m_scaleValue = 0;
    m_scaleDafault = 0;
    m_isMove = false;
}

QRectF ImageWidget::boundingRect() const
{
    return QRectF(-m_pix.width() / 2, -m_pix.height() / 2,
                  m_pix.width(), m_pix.height());
}

void ImageWidget::paint(QPainter *painter, const QStyleOptionGraphicsItem *,
                    QWidget *)
{
    painter->drawPixmap(-m_pix.width() / 2, -m_pix.height() / 2, m_pix);
}

void ImageWidget::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，
        m_isMove = true;//标记鼠标左键被按下
    }
    else if(event->button() == Qt::RightButton)
    {
        ResetItemPos();//右击鼠标重置大小
    }

}

void ImageWidget::mouseMoveEvent(QGraphicsSceneMouseEvent *event)
{
    if(m_isMove)
    {
        QPointF point = (event->pos() - m_startPos)*m_scaleValue;
        moveBy(point.x(), point.y());
    }
}

void ImageWidget::mouseReleaseEvent(QGraphicsSceneMouseEvent *)
{
    m_isMove = false;//标记鼠标左键已经抬起
}


void ImageWidget::wheelEvent(QGraphicsSceneWheelEvent *event)//鼠标滚轮事件
{
    if((event->delta() > 0)&&(m_scaleValue >= 50))//最大放大到原始图像的50倍
    {
        return;
    }
    else if((event->delta() < 0)&&(m_scaleValue <= m_scaleDafault))//图像缩小到自适应大小之后就不继续缩小
    {
        ResetItemPos();//重置图片大小和位置，使之自适应控件窗口大小
    }
    else
    {
        qreal qrealOriginScale = m_scaleValue;
        if(event->delta() > 0)//鼠标滚轮向前滚动
        {
            m_scaleValue*=1.1;//每次放大10%
        }
        else
        {
            m_scaleValue*=0.9;//每次缩小10%
        }
        setScale(m_scaleValue);
        if(event->delta() > 0)
        {
            moveBy(-event->pos().x()*qrealOriginScale*0.1, -event->pos().y()*qrealOriginScale*0.1);//使图片缩放的效果看起来像是以鼠标所在点为中心进行缩放的
        }
        else
        {
            moveBy(event->pos().x()*qrealOriginScale*0.1, event->pos().y()*qrealOriginScale*0.1);//使图片缩放的效果看起来像是以鼠标所在点为中心进行缩放的
        }
    }
}

void ImageWidget::setQGraphicsViewWH(int nwidth, int nheight)//将主界面的控件QGraphicsView的width和height传进本类中，并根据图像的长宽和控件的长宽的比例来使图片缩放到适合控件的大小
{
    int nImgWidth = m_pix.width();
    int nImgHeight = m_pix.height();
    qreal temp1 = nwidth*1.0/nImgWidth;
    qreal temp2 = nheight*1.0/nImgHeight;
    if(temp1>temp2)
    {
        m_scaleDafault = temp2;
    }
    else
    {
        m_scaleDafault = temp1;
    }
    setScale(m_scaleDafault);
    m_scaleValue = m_scaleDafault;
}

void ImageWidget::ResetItemPos()//重置图片位置
{
    m_scaleValue = m_scaleDafault;//缩放比例回到一开始的自适应比例
    setScale(m_scaleDafault);//缩放到一开始的自适应大小
    setPos(0,0);
}

qreal ImageWidget::getScaleValue() const
{
    return m_scaleValue;
}

6. The image function introduced here will not go into the final display as follows: