listView滚动事件
实现接口：
android.widget.AbsListView.OnScrollListener
Interface definition for a callback to be invoked when the list or grid has been scrolled.
实现接口的抽象方法
public void onScrollStateChanged(AbsListView view, int scrollState)
public void onScroll(AbsListView view, int firstVisibleItem, int visibleItemCount, int totalItemCount)
回调方法onScroll说明：
Callback method to be invoked when the list or grid has been scrolled. This will be called after the scroll has completed
Specified by: onScroll(...) in OnScrollListener
Parameters:
view The view whose scroll state is being reported
firstVisibleItem the index of the first visible cell (ignore if visibleItemCount == 0)
visibleItemCount the number of visible cells
totalItemCount the number of items in the list adaptor