# 2024.04.16

梳理按下鼠标左键并移动来绘制自由曲线的过程

* 按下鼠标左键，产生一个QMouseEvent类的指针，记作event
* QMouseEvent类继承自QEvent类，QEvent类有一个枚举值属性Type，用来表示事件的类型
* event->Type的值就是 MouseButtonPress = 2
* QMouseEvent类有一个protected的MouseButton枚举值属性b
  * 通过button()方法可以调用b的值
* event的b的值就是LeftButton = 0x00000001
* 根据event的Type的值，Qt自动判断要调用mousePressEvent()函数，并将event作为变量传递给mousePressEvent()

```cpp
void MainWindow::mousePressEvent(QMouseEvent* event)
{    
    if (event->button() == Qt::LeftButton)
        lastPoint = event->pos();
}
```

* 通过pos()方法，得到按下鼠标左键时鼠标的位置，将其存储到lastPoint中
* 在按下鼠标左键的情况下，移动鼠标，那么在下一瞬间，又产生一个新的QMouseEvent类的指针，仍记作event，这时event->Type的值就是 MouseMove = 5

```cpp
void MainWindow::mouseMoveEvent(QMouseEvent* event)
{
    if (event->buttons() & Qt::LeftButton)
    {
        endPoint = event->pos();
        update();
    }
}
```

* 通过pos()方法，获得此时鼠标的位置，将其存储到endPoint中
* 调用update()函数，进而调用paintEvent()函数

```cpp
void MainWindow::paintEvent(QPaintEvent*)
{
    QPainter pp(&pix);
    pp.drawLine(lastPoint, endPoint);
    lastPoint = endPoint;
    QPainter painter(this);
    painter.drawPixmap(0, 0, pix);
}
```

* 因为pix是个全局变量，所以在其上画的每一条线段都可以保留下来
* 将lastPoint的值更新为endPoint的值，然后开始循环往复
* 最后的最后，移动完最后一下，松开鼠标，调用

```cpp
void MainWindow::mouseReleaseEvent(QMouseEvent* event)
{
    if (event->button() == Qt::LeftButton) //鼠标左键释放    
    {
        endPoint = event->pos();
        update();
    }
}
```

* 最后lastPoint和endPoint值都是最后松开鼠标的地方


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