Qt图片在窗口上下左右移动

#ifndef KEYEVENT_H
#define KEYEVENT_H

#include <QWidget>

QT_BEGIN_NAMESPACE
namespace Ui { class keyEvent; }
QT_END_NAMESPACE

class keyEvent : public QWidget
{
    Q_OBJECT

public:
    keyEvent(QWidget *parent = nullptr);
    ~keyEvent();
    void drawPix();         //自定义成员函数用来在pixmap上绘制网格线和图片
protected:

    void keyPressEvent(QKeyEvent *event) override;      //重写键盘按下事件
    void paintEvent(QPaintEvent *event) override;       //重写绘图事件

private:
    Ui::keyEvent *ui;
    QPixmap *m_pix;
    QImage m_image;
    int m_startX;   //图标顶点的位置
    int m_startY;
    int m_width;    //界面的宽度和高度
    int m_height;
    int m_step;     //图片移动的步长

};
#endif // KEYEVENT_H

keyEvent::keyEvent(QWidget *parent)
    : QWidget(parent)
    , ui(new Ui::keyEvent)
{
    ui->setupUi(this);
    setWindowTitle("键盘事件");
    setAutoFillBackground(true);
    setFixedSize(512,256);
    m_width=size().width();
    m_height=size().height();

    m_pix=new QPixmap(m_width,m_height);
    m_pix->fill(Qt::white);

    m_image.load("../car.png"); //当前工作目录默认Debug目录下，car.png在上一级目录下
    m_startX=100;
    m_startY=100;
    m_step=20;
    drawPix();
}

void keyEvent::drawPix()
{
    m_pix->fill(Qt::white);
    QPainter painter(this);
    QPen pen(Qt::DotLine); //虚线

    painter.begin(m_pix);   //m_pix为绘图设备
    painter.setPen(pen);

    //按照步长画纵向的网格虚线
    for(int i=m_step;i<m_width;i+=m_step){
        painter.drawLine(QPoint(i,0),QPoint(i,m_height));

    }
    //按照步长画横向的网格虚线
    for(int j=m_step;j<m_height;j+=m_step){
        painter.drawLine(QPoint(0,j),QPoint(m_width,j));
    }
    painter.end();

    //画图片
    painter.begin(m_pix);
    painter.drawImage(QPoint(m_startX,m_startY),m_image);
    painter.end();
}

void keyEvent::keyPressEvent(QKeyEvent *event)
{
    //按下ctrl键移动一个像素
    if(event->modifiers()==Qt::ControlModifier){
        if(event->key()==Qt::Key_Left){
            m_startX=(m_startX-1)<0?m_startX:m_startX-1;
        }else if(event->key()==Qt::Key_Right){
            m_startX=(m_startX+1+m_image.width())>m_width?m_startX:m_startX+1;
        }else if(event->key()==Qt::Key_Up){
            m_startY=(m_startY-1)<0?m_startY:m_startY-1;
        }else if(event->key()==Qt::Key_Down){
            m_startY=(m_startY+1+m_image.height())>m_height?m_startY:m_startY+1;
        }
    }else{
        //调整图片左上角的位置到网格顶点
        m_startX=m_startX+m_startX%m_step;
        m_startY=m_startY+m_startY%m_step;
        if(event->key()==Qt::Key_Left){
            m_startX=(m_startX-m_step)<0?m_startX:m_startX-m_step;
        }else if(event->key()==Qt::Key_Right){
            m_startX=(m_startX+m_step+m_image.width())>m_width?m_startX:m_startX+m_step;
        }else if(event->key()==Qt::Key_Up){
            m_startY=(m_startY-m_step)<0?m_startY:m_startY-m_step;
        }else if(event->key()==Qt::Key_Down){
            m_startY=(m_startY+m_step+m_image.height())>m_height?m_startY:m_startY+m_step;
        }
    }
    drawPix();  //刷新图片的位置
    update();//触发窗口重绘 ,执行paintEvent();

}

void keyEvent::paintEvent(QPaintEvent *event)
{
    //定义Qpainter对象，设置窗口为绘图设备，在窗口绘制pixmap
    QPainter painter;
    painter.begin(this);//绘图设备为窗口
    painter.drawPixmap(QPoint(0,0),*m_pix);
    painter.end();


}