qt事件过滤器

1、新建Qt对话框项目

2、注册按钮，建立事件过滤器：

    ui->setupUi(this);

    lastPoint=QPoint(-1,-1);

    ui->pushButton->installEventFilter(this);

    ui->pushButton_2->installEventFilter(this);

    ui->pushButton_3->installEventFilter(this);

    m_press=false;

3、添加处理事件：

bool GragMoveDialog::eventFilter(QObject *target, QEvent *event)
{
     if(target==ui->pushButton||target==ui->pushButton_2||target==ui->pushButton_3)
     {
         if(event->type()==QEvent::MouseButtonPress)    //���갴��
         {
             m_press=true;
             return false;
         }
         else if(event->type()==QEvent::MouseMove&&m_press)    //�����϶���m_press�����갴�±�־
         {
             QMouseEvent *mevent=static_cast<QMouseEvent*> (event);
             newPoint=mevent->globalPos();
             if(lastPoint.x()==-1)
                 lastPoint=newPoint;
             QPushButton *mtarget=static_cast<QPushButton*> (target);
             QRect rect(mtarget->geometry().x()+newPoint.x()-lastPoint.x(),mtarget->geometry().y()+newPoint.y()-lastPoint.y(),mtarget->geometry().width(),mtarget->geometry().height());
             mtarget->setGeometry(rect);
             lastPoint=newPoint;
             update();
             return true;
         }
         else if(event->type()==QEvent::MouseButtonRelease)  //�����ͷ�
         {
             lastPoint=QPoint(-1,-1);
             m_press=false;
             return false;
         }
         else{
             return false;
         }
     }
     else return GragMoveDialog::eventFilter(target,event);
}
4、寻找直线及重绘

QPainterPath  *findLines(QPoint a,QPoint b)
{
    QPainterPath *path=new QPainterPath();
    path->moveTo(a.x(),a.y());
    path->lineTo(a.x(),b.y());
    path->lineTo(b.x(),b.y());
    return path;
}
void GragMoveDialog::paintEvent(QPaintEvent*)
{
    QPainterPath path;
    path=*findLines(ui->pushButton->geometry().topLeft(),ui->pushButton_2->geometry().topLeft());
    QPainter painter(this);
    QPen pen;
    pen.setColor(QColor(255,0,255));
    pen.setWidth(2);
    painter.setPen(pen);
    painter.drawPath(path);
}

 实现的功能有：3个按钮可以拖拽，其中两个之间有连线，按钮移动连线也跟着移动。