QT uses drag and drop picture to control and mouse to move picture

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Update time ：2021-04-15

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New knowledge that I haven't done before ,qt Provided the interface , In fact, it is just to read the local file path .

Reference resources ：
QT-Qpixmap Achieve picture mouse zoom , Mouse drag example

The effect is given below ：

Here's the code ：
The header file ：

#ifndef WIDGET_H
#define WIDGET_H

#include <QWidget>
#include <QDragEnterEvent>
#include <QDropEvent>
#include <QUrl>
#include <QList>
#include <QMimeData>
#include <QPainter>
#include <QDebug>
#include <QImage>
#include <QRect>


namespace Ui {
    
class Widget;
}

class Widget : public QWidget
{
    
    Q_OBJECT

public:
    explicit Widget(QWidget *parent = 0);
    ~Widget();


    // Mouse or keyboard operation 
    enum  Type {
    
            None          = 0,
            Amplification ,
            Shrink,
            Lift,
            Right,
            Up,
            Down,
            Move
        };

protected:
    // Drag and drop window 
    bool eventFilter(QObject *watched, QEvent *event);

    void paintEvent(QPaintEvent *e);

    bool event(QEvent * event);

private slots:
    void on_pushButton_clicked();

private:
    Ui::Widget *ui;


    // Picture length and width 
    int picHeight,picWidth;


    QImage pic;         // picture 
    QRect Paint;         // Painting area 

    QPoint Alloffset;           // Total offset 
    QPoint offset;              // One time image offset value 

    int action;

};

#endif // WIDGET_H

Implementation file ：

#include "widget.h"
#include "ui_widget.h"

Widget::Widget(QWidget *parent) :
    QWidget(parent),
    ui(new Ui::Widget)
{
    
    ui->setupUi(this);
    // Install event filter 
    ui->widget->installEventFilter(this);
    ui->widget->setAcceptDrops(true);
    this->setMouseTracking(true);


    Alloffset.setX(0);
    Alloffset.setY(0);

    action = Widget::None;


    // Set the painting area -*- The coordinates of the upper left corner and the length and width of the label 
    // QPoint labelPos=ui->label->mapFromGlobal(QCursor::pos());// Global coordinates 
    Paint.setRect(9,9,388,582);

}

Widget::~Widget()
{
    
    delete ui;
}

bool Widget::eventFilter(QObject *watched, QEvent *event)
{
    
    if (watched == ui->widget)
    {
    
        if (event->type() == QEvent::DragEnter)
        {
    
            // [[2]]:  When dragging and dropping, the mouse enters widget when , widget Accept drag and drop actions 
            QDragEnterEvent *dee = dynamic_cast<QDragEnterEvent *>(event);
            dee->acceptProposedAction();
            return true;
        } else if (event->type() == QEvent::Drop)
        {
    
            // [[3]]:  When the release operation occurs ,  Get drag and drop data 
            QDropEvent *de = dynamic_cast<QDropEvent *>(event);
            QList<QUrl> urls = de->mimeData()->urls();

            if (urls.isEmpty())
            {
    
                return true;
            }
            QString path = urls.first().toLocalFile();

            // [[4]]:  stay widget Show drag and drop pictures on 
            QImage image(path); // QImage Yes I/O Optimized , QPixmap Optimize the display .
            pic=image;
            if (!image.isNull())
            {
    
                // image = image.scaled(ui->label->size(),
                // Qt::KeepAspectRatio,
                // Qt::SmoothTransformation);
                picHeight = image.height();
                picWidth =image.width();
// qDebug()<<"pic wh:"<<picWidth<<picHeight;
                this->update();
                // ui->textEdit->append("pic h:"+QString::number(picHeight)+"w:"+QString::number(picWidth));
            }

            return true;
        }
    }

    return QWidget::eventFilter(watched, event);
}

void Widget::paintEvent(QPaintEvent *e)
{
    
    QPainter painter(this);

    if(action==Widget::Move)                    // Move 
    {
    
        int offsetx=Alloffset.x()+offset.x();
        Alloffset.setX(offsetx);

        int offsety=Alloffset.y()+offset.y();
        Alloffset.setY(offsety);

        qDebug()<<"alloffset:"<<offsetx<<offsety;
        action=Widget::None;
    }

    // Limit X Offset value 
    if(abs(Alloffset.x())>=(Paint.width()/2 + picWidth/2 - ui->widget->x()))
    {
    
        if(Alloffset.x()>0)
            Alloffset.setX(Paint.width()/2 + picWidth/2 - ui->widget->x());
        else
            Alloffset.setX(-Paint.width()/2 + -picWidth/2 + ui->widget->x());
    }
    // Limit Y Offset value 
    if(abs(Alloffset.y())>=(Paint.height()/2 + picHeight/2 - ui->widget->y()))
    {
    
        if(Alloffset.y()>0)
            Alloffset.setY(Paint.height()/2 + picHeight/2 -ui->widget->y());
        else
            Alloffset.setY(-Paint.height()/2 + -picHeight/2 +ui->widget->y());

    }

    int x = Paint.width()/2 + Alloffset.x() -picWidth/2;
    if(x<0)
        x=0;

    int y = Paint.height()/2 + Alloffset.y() -picHeight/2;
    if(y<0)
        y=0;



    qDebug()<<"pot:"<<x+Paint.x()<<y+Paint.y();


    QPoint pot;
    pot.setX(ui->widget->x());
    pot.setY(ui->widget->y());

// pot.setX(x+Paint.x());
// pot.setY(y+Paint.y());
    // Left boundary 
    if(x+Paint.x() <= ui->widget->x())
    {
    
        pot.setX(ui->widget->x());
    }else
    {
    
        pot.setX(x+Paint.x());
    }
    // Upper boundary 
    if(y+Paint.y() <= ui->widget->y())
    {
    
        pot.setY(ui->widget->y());
    }else
    {
    
        pot.setY(y+Paint.y());
    }


    // Right border 
    if(x+Paint.x()+picWidth > ui->widget->width())
    {
    
        pot.setX(ui->widget->width()-picWidth);
    }else
    {
    
        pot.setX(x+Paint.x());
    }

    // Lower boundary 
    if(y+Paint.y()+picHeight > ui->widget->height())
    {
    
        pot.setY(ui->widget->height()-picHeight);
    }else
    {
    
        pot.setY(y+Paint.y());
    }



    if(!pic.isNull())
    {
    
        painter.drawTiledPixmap(pot.x(),pot.y(),picWidth,picHeight,QPixmap::fromImage(pic),0,0);             // Draw a picture 

        painter.setPen(QPen(Qt::red,4));// Set brush form 

        painter.setBrush(QColor(255,0,0));
        painter.drawEllipse(pot.x()+picWidth/2,pot.y()+picHeight/2,20,20);             // Draw a picture 
        painter.setPen(QPen(Qt::blue,5));// Set brush form 
        QString text=QString::number(pot.x()+picWidth/2)+","+QString::number(pot.y()+picHeight/2);
        painter.drawText(pot.x()+picWidth/2+30,pot.y()+picHeight/2+30,text );             // Draw a picture 

        // painter.drawPixmap(0,0,picWidth,picHeight,QPixmap::fromImage(pic));
    }


}

bool Widget::event(QEvent *event)
{
    
    // Whether the mouse is pressed down in the area 
    static bool bPress=false;
    static QPoint pressPot;              // Coordinates when the mouse is pressed down 

    if(event->type() == QEvent::MouseButtonPress )
    {
    
        QMouseEvent *mouse = dynamic_cast<QMouseEvent* >(event);
        if(mouse->button()==Qt::LeftButton && Paint.contains(mouse->pos()))
        {
    
            bPress = true;
            pressPot = mouse->pos();
// qDebug()<<"press pot:"<<mouse->x()<<","<<mouse->y()<<bPress;
        }

    }
    else if(event->type() == QEvent::MouseButtonRelease )
    {
    
        QMouseEvent *mouse = dynamic_cast<QMouseEvent* >(event);
        if(mouse->button()==Qt::LeftButton)
        {
    
            bPress=false;
// qDebug()<<"release pot:"<<mouse->x()<<","<<mouse->y();
        }
    }

    // Move 
    if(event->type() == QEvent::MouseMove)
    {
    
        QMouseEvent *mouse = dynamic_cast<QMouseEvent* >(event);
        // Some are pressed down 
        if(bPress)
        {
    
            // Get image offset 
            offset.setX(mouse->x() - pressPot.x());
            offset.setY(mouse->y() - pressPot.y());
            pressPot = mouse->pos();

            action = Widget::Move;
            this->update();
// qDebug()<<"move pot:"<<mouse->x()<<","<<mouse->y()<<bPress;

        }
    }


    return QWidget::event(event);
}


// Analog keyboard 
void Widget::on_pushButton_clicked()
{
    
    action=Widget::Move;
    offset.setX(-20) ;
    offset.setY(-20) ;
    this->update();
}