public abstract boolean onScroll ( e1,  e2, float distanceX, float distanceY)

Since: 

Notified when a scroll occurs with the initial on down  and the current move . The distance in x and y is also supplied for convenience.

Parameters

e1	The first down motion event that started the scrolling.
e2	The move motion event that triggered the current onScroll.
distanceX	The distance along the X axis that has been scrolled since the last call to onScroll. This is NOT the distance between e1 and e2.
distanceY	The distance along the Y axis that has been scrolled since the last call to onScroll. This is NOT the distance between e1 and e2.

Returns

• true if the event is consumed, else false

e1 初次触控地图的event1  down事件

e2 每次触发onScroll函数得到的的event2   最后一次move事件

distance是上一次的event2 减去 当前event2得到的结果 //注意到顺序 

lastEvent2 - event2 = distance

验证：

if (lastE2 != null){
    Log.i(TAG, "distanceX:" + distanceX + " = " + (lastE2.getX()-e2.getX()));
    Log.i(TAG, "distanceY:" + distanceY + " = " + (lastE2.getY()-e2.getY()));
}
lastE2 = MotionEvent.obtain(e2);