Qt学习笔记（1）---QT5利用事件过滤器实现在控件上绘图

首先，感谢 作者：一去丶二三里 博客地址：http://blog.csdn.net/liang19890820  

Qt中要在控件中绘制图像，需要重写paintevent函数，本文通过EventFilter(事件过滤器)来实现在控件上的图形绘制。

.h文件

#ifndef WIDGET_H
#define WIDGET_H

#include 
#include 

namespace Ui {
class Widget;
}

class Widget : public QWidget
{
    Q_OBJECT

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

public slots:
    bool eventFilter(QObject *, QEvent *);

private:
    Ui::Widget *ui;
    bool mouseIsdown;
    QPointF startPos;
    QPointF endPos;
};

#endif // WIDGET_H

.cpp文件

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

Widget::Widget(QWidget *parent) :
    QWidget(parent),
    ui(new Ui::Widget)
{
    ui->setupUi(this);
    mouseIsdown = false;
    ui->label->installEventFilter(this);

}

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


bool Widget::eventFilter(QObject *watched, QEvent *event)
{
    if(watched == ui->label )
    {
        if(event->type() == QEvent::MouseButtonPress)
        {
            QMouseEvent *mouseEvent=(QMouseEvent *)event;

            if(mouseEvent->buttons()&Qt::LeftButton)
            {
                mouseIsdown = true;
                startPos = mouseEvent->pos();
            }

        }
        if(event->type() == QEvent::MouseMove)
        {
            QMouseEvent *mouseEvent=(QMouseEvent *)event;

            if(mouseIsdown)
            {
                endPos = mouseEvent->pos();
                ui->label->update();
            }
        }

        if(event->type() == QEvent::MouseButtonRelease)
        {
            QMouseEvent *mouseEvent=(QMouseEvent *)event;

            endPos = mouseEvent->pos();
            mouseIsdown = false;

        }
        if(event->type() == QEvent::Paint)
        {

            QPainter painter(ui->label);

            QPen pen = painter.pen();

            pen.setWidth(3);
            pen.setColor(QColor(173, 255, 47));
            //反走样
            painter.setRenderHint(QPainter::Antialiasing, true);
            painter.setPen(pen);
            // 绘制直线
            painter.drawLine(startPos, endPos);

        }
    }

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

eventFilter(QObject *watched, QEvent *event)中

watched为事件过滤器的对象即本文中的label , event 为事件类型。

其中需要注意的有两点：

1.在构造函数中添加过滤器installEventFilter(this)如果一个控件添加了多个过滤器，后添加会被选择执行。

2.返回父类的eventfliterreturn QWidget::eventFilter(watched,event);