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::OnMouseMove(UINT nFlags, CPoint point)
nFlags里有按键的信息，看MSDN中关于nFlags的说明
nFlags
Indicates whether various virtual keys are down. This parameter can be any combination of the following values:
MK_CONTROL   Set if the CTRL key is down.
MK_LBUTTON   Set if the left mouse button is down.
MK_MBUTTON   Set if the middle mouse button is down.
MK_RBUTTON   Set if the right mouse button is down.
MK_SHIFT   Set if the SHIFT key is down.