代码如下，需要注意的是MotionEvent的ACTION_DOWN, ACTION_UP 是这么理解的:
ACTION_DOWN - A pressed gesture has started, the motion contains the initial starting location.
ACTION_UP - A pressed gesture has finished, the motion contains the final release location as well as any intermediate points since the last down or move event.
ACTION_MOVE - A change has happened during a press gesture (between {@link #ACTION_DOWN} and {@link #ACTION_UP}). The motion contains the most recent point, as well as any intermediate points since the last down or move event. -
简而言之，ACTION_DOWN, ACTION_UP 类似于java script里面键盘事件OnKeyDown, OnKeyUp 或鼠标事件OnMouseDown, OnMouseUp, 而不是说手指往上划拉或往下划拉了一下。