protected:

    void wheelEvent(QWheelEvent *event) Q_DECL_OVERRIDE;

    void extendWindowsSize();

    void shrinkWindowsSize();

private:

    int m_originalWidth;

    int m_originalHeight;

 1 Widget::Widget(QWidget *parent) :

 2     QWidget(parent),

 3     ui(new Ui::Widget)

 4 {

 5     ...

 6     m_originalWidth=this->width();

 7     m_originalHeight=this->height();

 8 }

 9 void Widget::wheelEvent(QWheelEvent *event){

10     QWidget::wheelEvent(event);

11     QPoint scroll = event->angleDelta();

12     if(scroll.y()>0){

13         extendWindowsSize();

14     }else{

15         shrinkWindowsSize();

16     }

17 }

18 void Widget::extendWindowsSize(){

19     if(this->width()>m_originalWidth+300){

20         return;

21     }

22     this->setMaximumSize(this->width()+10,this->height()+10);

23     this->setMinimumSize(this->width()+10,this->height()+10);

24 }

25 void Widget::shrinkWindowsSize(){

26     if(this->width()<m_originalWidth-300){

27         return;

28     }

29     this->setMinimumSize(this->width()-10,this->height()-10);

30     this->setMaximumSize(this->width()-10,this->height()-10);

31 }