ViewDragHelper

回調順序：

shouldInterceptTouchEvent：

DOWN:
	getOrderedChildIndex(findTopChildUnder)
	->onEdgeTouched

MOVE:
	getOrderedChildIndex(findTopChildUnder)
	->getViewHorizontalDragRange & 
	  getViewVerticalDragRange(checkTouchSlop)(MOVE中可能不止一次)
	->clampViewPositionHorizontal&
	  clampViewPositionVertical
	->onEdgeDragStarted
	->tryCaptureView
	->onViewCaptured
	->onViewDragStateChanged

** processTouchEvent: **

DOWN:
	getOrderedChildIndex(findTopChildUnder)
	->tryCaptureView
	->onViewCaptured
	->onViewDragStateChanged
	->onEdgeTouched
MOVE:
	->STATE==DRAGGING:dragTo
	->STATE!=DRAGGING:
		onEdgeDragStarted
		->getOrderedChildIndex(findTopChildUnder)
		->getViewHorizontalDragRange&
		  getViewVerticalDragRange(checkTouchSlop)
		->tryCaptureView
		->onViewCaptured
		->onViewDragStateChanged

[參考]

**Android ViewDragHelper徹底解析 自定義ViewGroup神器**

http://blog.csdn.net/lmj623565791/article/details/46858663

    **自定義控件輔助神器ViewDragHelper**

    http://www.jianshu.com/p/e4d1f88ca922