QMouseEvent中兩類座標系統，一類是視窗座標，一類是顯示器座標。

1、QPoint QMouseEvent::pos() 

      這個只是返回相對這個widget(過載了QMouseEvent的widget)的位置。

       const Returns the position of the mouse cursor, relative to the widget that received the event. If you move the widget as a result of the mouse event, use the global position returned by globalPos() to avoid a shaking motion. 

2、QPoint QMouseEvent::globalPos() 

     視窗座標，這個是返回滑鼠的全域性座標

     const Returns the global position of the mouse cursor at the time of the event. This is important on asynchronous window systems like X11. Whenever you move your widgets around in response to mouse events, globalPos() may differ a lot from the current pointer position QCursor::pos(), and from QWidget::mapToGlobal(pos()).

3、QPoint QCursor::pos() [static] 

      返回相對顯示器的全域性座標

      Returns the position of the cursor (hot spot) of the primary screen in global screen coordinates. You can call QWidget::mapFromGlobal() to translate it to widget coordinates. Note: The position is queried from the windowing system. If mouse events are generated via other means (e.g., via QWindowSystemInterface in a unit test), those fake mouse moves will not be reflected in the returned value. Note: On platforms where there is no windowing system or cursors are not available, the returned position is based on the mouse move events generated via QWindowSystemInterface.

4.1  QPoint QWidget::mapToGlobal(const QPoint & pos)  const 

       將視窗座標轉換成顯示器座標

       Translates the widget coordinate pos to global screen coordinates. For example, mapToGlobal(QPoint(0,0)) would give the global coordinates of the top-left pixel of the widget. See also mapFromGlobal(), mapTo(), and mapToParent().

4.2   QPoint QWidget::mapFromGlobal(const QPoint & pos) const 

       將顯示器座標轉換成視窗座標

       Translates the global screen coordinate pos to widget coordinates.

5.1 QPoint QWidget::mapToParent(const QPoint & pos) const

       將視窗座標獲得的pos轉換成父類widget的座標

Translates the widget coordinate pos to a coordinate in the parent widget.

5.2 QPoint QWidget::mapFromParent(const QPoint & pos) const 

       將父類視窗座標轉換成當前視窗座標

Translates the parent widget coordinate pos to widget coordinates. Same as mapFromGlobal() if the widget has no parent.

6.3 QPoint QWidget::mapTo(const QWidget * parent, const QPoint & pos) const 

       將當前視窗座標轉換成指定parent座標。

Translates the widget coordinate pos to the coordinate system of parent. The parent must not be 0 and must be a parent of the calling widget. See also mapFrom(), mapToParent(), mapToGlobal(), and underMouse().

7、QWidget::pos() : QPoint

這個屬性獲得的是當前目前控制元件在父視窗中的位置，

This property holds the position of the widget within its parent widget.

If the widget is a window, the position is that of the widget on the desktop, including its frame.

When changing the position, the widget, if visible, receives a move event (moveEvent()) immediately. If the widget is not currently visible, it is guaranteed to receive an event before it is shown.

By default, this property contains a position that refers to the origin.

Warning: Calling move() or setGeometry() inside moveEvent() can lead to infinite recursion.

See the Window Geometry documentation for an overview of geometry issues with windows.

8、const QPointF & QMouseEvent::screenPos() const

Returns the position of the mouse cursor as a QPointF, relative to the screen that received the event.

和QPoint QMouseEvent::globalPos() 值相同，但是型別更高精度的QPointF

This function was introduced in Qt 5.0.

總結一：經過試驗，QMouseEvent::globalPos()  和 QCursor::pos()效果一樣，但是Qt幫助文件說不一樣，可是我獲得值確實相同的。

QCursor::pos() == QMouseEvent::globalPos() 都是全域性座標；

總結二：將button：posBtn直接轉換成全域性座標。

QMouseEvent::globalPos() ==  ui.posBtn->mapToGlobal(ui.posBtn->pos());

總結三：將全域性座標（滑鼠當前座標，QCursor::pos()）直接轉換成當前

當前視窗相對座標 ==  ui.posBtn->mapFromGlobal(QCursor::pos());