#include "widget.h"

Widget::Widget():
    BigButton("放大",this),
    LittleButton("縮小",this),
    LiftButton("向左",this),
    RightButton("向右",this),
    UpButton("向上",this),
    DownButton("向下",this),
    Paint(10,10,810,810),
    Alloffset(0,0),
    label("100%",this)
{
    ratio=1.0;             //初始化圖片縮放比例
    action= Widget::None;
    pixW =985;            //設置圖片尺寸為985*740
    pixH=740;

    pix =new QPixmap;
    pix->load(":/pic/img.jpg");

    BigButton.setGeometry(822,10,60,25);
    connect(&BigButton,SIGNAL(clicked()),this,SLOT(onBigClicked()));

    LittleButton.setGeometry(822,40,60,25);
    connect(&LittleButton,SIGNAL(clicked()),this,SLOT(onLittleClicked()));

    LiftButton.setGeometry(822,70,60,25);
    connect(&LiftButton,SIGNAL(clicked()),this,SLOT(OnLiftClicked()));
    RightButton.setGeometry(822,100,60,25);
    connect(&RightButton,SIGNAL(clicked()),this,SLOT(OnRightClicked()));
    UpButton.setGeometry(822,130,60,25);
    connect(&UpButton,SIGNAL(clicked()),this,SLOT(onUpClicked()));
    DownButton.setGeometry(822,160,60,25);
    connect(&DownButton,SIGNAL(clicked()),this,SLOT(onDownClicked()));
    label.move(840,200);
    resize(890,850);

}

bool Widget::event(QEvent * event)
{
    static bool press=false;
    static QPoint PreDot;

    if(event->type()== QEvent::MouseButtonPress )
    {
           QMouseEvent *mouse=dynamic_cast<QMouseEvent* >(event);

           //判斷鼠標是否是左鍵按下,且鼠標位置是否在繪畫區域
           if(mouse->button()==Qt::LeftButton &&Paint.contains(mouse->pos()))
           {
               press=true;
               QApplication::setOverrideCursor(Qt::OpenHandCursor); //設置鼠標樣式

               PreDot =mouse->pos();
           }

    }
else if(event->type()== QEvent::MouseButtonRelease)
{
    QMouseEvent *mouse=dynamic_cast<QMouseEvent* >(event);

    //判斷鼠標是否是左鍵釋放,且之前是在繪畫區域
    if(mouse->button()==Qt::LeftButton && press )
    {
        QApplication::setOverrideCursor(Qt::ArrowCursor); //改回鼠標樣式
        press=false;
    }
}

 if(event->type()==QEvent::MouseMove)              //移動圖片
 {
      if(press)
     {
        QMouseEvent *mouse=dynamic_cast<QMouseEvent* >(event);

        offset.setX(mouse->x() - PreDot.x());
        offset.setY(mouse->y() - PreDot.y());
        PreDot =mouse->pos();
        action = Widget::Move;
        this->update();
     }
 }
return QWidget::event(event);
}

void Widget::wheelEvent(QWheelEvent* event)     //鼠標滑輪事件
{
 if (event->delta()>0) {      //上滑,縮小

    action=Widget::Shrink;
    this->update();

 } else {                    //下滑,放大
     action=Widget::Amplification;
     this->update();
 }

 event->accept();
}



void Widget::paintEvent(QPaintEvent *event)
{
  QPainter painter(this);
  int NowW=ratio *pixW;
  int NowH=ratio *pixH;

if(action==Widget::Amplification)           //縮小
{
      ratio-=0.1*ratio;
    if(ratio<0.018)
      ratio =0.01;

    
    QString str;
    str.sprintf("%.0f%",ratio*100);
    label.setText(str) ;
}
else  if(action==Widget::Shrink)           //放大
{

     ratio+=0.1*ratio;
   if(ratio>4.5)
     ratio =5.000;

    
    QString str;
    str.sprintf("%.0f%",ratio*100);
    label.setText(str);
}
if(action==Widget::Amplification || action==Widget::Shrink)      //更新圖片
{
  NowW =ratio *pixW;
  NowH =ratio *pixH;
  pix->load(":/pic/img.jpg");                 //重新裝載,因為之前的圖片已經被縮放過
  *pix=pix->scaled(NowW, NowH,Qt::KeepAspectRatio);
  action=Widget::None;

}

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

    int offsety=Alloffset.y()+offset.y();
    Alloffset.setY(offsety);
    action=Widget::None;
}

if(abs(Alloffset.x())>=(Paint.width()/2 + NowW/2 -10))    //限制X偏移值
{
    if(Alloffset.x()>0)
        Alloffset.setX(Paint.width()/2 + NowW/2 -10);
    else
     Alloffset.setX(-Paint.width()/2 + -NowW/2 +10);
}
if(abs(Alloffset.y())>=(Paint.height()/2 + NowH/2 -10))    //限制Y偏移值
{
    if(Alloffset.y()>0)
        Alloffset.setY(Paint.height()/2 + NowH/2 -10);
    else
     Alloffset.setY(-Paint.height()/2 + -NowH/2 +10);

}

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

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

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

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

int w=(NowW - sx)>Paint.width()? Paint.width() : (NowW - sx);
if(w>(Paint.width()-x))
    w =Paint.width()-x;

int h=(NowH - sy)>Paint.height()? Paint.height() : (NowH - sy);
if(h>(Paint.height()-y))
    h =Paint.height()-y;

  painter.drawRect(Paint.x()-1,Paint.y()-1,Paint.width()+1,Paint.height()+1); //畫框
  painter.drawTiledPixmap(x+Paint.x(),y+Paint.y(),w,h,*pix,sx,sy);             //繪畫圖形
}

void  Widget::onLittleClicked()
{
  action=Widget::Amplification;
  this->update();
}

void  Widget::onBigClicked()
{
  action=Widget::Shrink;
  this->update();
}
void Widget::onUpClicked()
{
  action=Widget::Move;
  offset.setX(0);
  offset.setY(-20);

  this->update();
}
void Widget::onDownClicked()
{
  action=Widget::Move;
  offset.setX(0);
  offset.setY(20);
  this->update();
}
void Widget::OnLiftClicked()
{
  action=Widget::Move;
  offset.setX(-20);
  offset.setY(0);

  this->update();
}
void Widget::OnRightClicked()
{
  action=Widget::Move;
  offset.setX(20) ;
  offset.setY(0) ;

  this->update();
}

文章來源：https://www.cnblogs.com/lifexy/p/9057046.html

20.QT-Qpixmap實現圖片鼠標縮放,鼠標拖動示例(詳解)