直接上代码

widget.h
#ifndef WIDGET_H
#define WIDGET_H

#include <QLabel>
#include <QPixmap>
#include <QWidget>
#include <QResizeEvent>
#include <QMouseEvent>

class Widget : public QWidget
{
    Q_OBJECT
private:
    int x,y;
    bool isPress;
    QPixmap pixDst;
    QPixmap scalePix;
    QPixmap pImgScale;
    QLabel* labBG;
    QLabel* labScale;
public:
    Widget(QWidget *parent = nullptr);
    ~Widget();
protected:
    virtual void resizeEvent(QResizeEvent*);
    virtual void mousePressEvent(QMouseEvent*);
    virtual void mouseReleaseEvent(QMouseEvent*);
    virtual void mouseMoveEvent(QMouseEvent*);
};
#endif // WIDGET_H

widget.cpp
#include "widget.h"

constexpr int iw=240;
constexpr int ih=240;
constexpr int iep=5;
constexpr int iwCopy = 80;
constexpr int ihCopy = 80;

Widget::Widget(QWidget *parent)
    : QWidget(parent),x(0),y(0)
{
    isPress=false;
    setFixedSize(960,720);
    labBG=new QLabel(this);
    labScale=new QLabel(this);
    //显示图片
    QPixmap pixSrc;
    pixSrc.load("./bg.jpg");
    pixDst=pixSrc.scaled(this->width(),this->height(),Qt::IgnoreAspectRatio);
    labBG->setPixmap(pixDst);
}

Widget::~Widget()
{
}

void Widget::resizeEvent(QResizeEvent *event)
{
    labScale->resize(iw, ih);
    labBG->setGeometry(0,0,this->width(),this->height());
    QWidget::resizeEvent(event);
}

void Widget::mousePressEvent(QMouseEvent *event)
{
    isPress=true;
    QWidget::mousePressEvent(event);
}

void Widget::mouseReleaseEvent(QMouseEvent *event)
{
    isPress=false;
    labScale->hide();
    QWidget::mouseReleaseEvent(event);
}

void Widget::mouseMoveEvent(QMouseEvent *event)
{
    if(isPress){
        if(labScale->isVisible()){}
        else{
            labScale->show();
        }

        x = event->pos().x();
        if ((x + iw) > width()){
            x = x - iw;
            x -= iep;
        }
        else{
            x += iep;
        }

        y = event->pos().y();
        if ((y + ih) > height()){
            y = y - ih;
            y -= iep;
        }
        else{
            y += iep;
        }
        labScale->move(x, y);

        x = event->pos().x() - iwCopy / 2;
        y = event->pos().y() - ihCopy / 2;
        x = (x < 0) ? 0 : x;
        y = (y < 0) ? 0 : y;

        scalePix = pixDst.copy(x, y, iwCopy, ihCopy);
        pImgScale = scalePix.scaled(iw, ih, Qt::IgnoreAspectRatio);
        labScale->setPixmap(pImgScale);
        labScale->show();
    }
    else{
        labScale->hide();
    }
    QWidget::mouseMoveEvent(event);
}