OnScrollListener接口官方定義是：Interface definition for a callback to be invoked when the list or grid has been scrolled.翻譯過來大意是指：當列表或者網格（AbsListView抽象類的實現子類，一般應該是指ListView類和GridView類及其子類）產生了滾動行爲時，將會回調該接口。該接口是AbsListView類內部的一個靜態類。所以，一般的控件比如button，textView等並不能添加該接口監聽。
滾動行爲有三種狀態：
1. SCROLL_STATE_FLING ： The user had previously been scrolling using touch and had performed a fling.可理解爲手指觸摸屏幕滑動了一下，鬆開手後由於慣性view仍會滑動一段距離，這一過程即位Fling（猛衝）。對應的整形常量值爲2。
2. SCROLL_STATE_IDLE ： The view is not scrolling.指view不再滾動。對應的整形常量值爲0。
3. SCROLL_STATE_TOUCH_SCROLL ：The user is scrolling using touch, and their finger is still on the screen.指按着屏幕滑動時的狀態。對應的整形常量值爲1。
該接口有兩個抽象方法分別爲：onScrollStateChanged(),和onScroll（）
其中：void onScrollStateChanged(AbsListView view, int scrollState)會在滾動狀態發生改變時回調，比如從滑動狀態到停止下來時就會回調該方法。它的兩個參數：view表示添加了該接口監聽的組件，scrollState表示當前的滾動狀態。
void onScroll(AbsListView view, int firstVisibleItem, int visibleItemCount, int totalItemCount)
- Callback method to be invoked when the list or grid has been scrolled. This will be called after the scroll has completed
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- 參數：
view- The view whose scroll state is being reportedfirstVisibleItem- the index of the first visible cell (ignore if visibleItemCount == 0)指view中可看到的第一項所對應的索引值。visibleItemCount- the number of visible cells指整個view中可看到的條目的數量。totalItemCount- the number of items in the list adaptor。指整個view中所包含的總的條目的數量。
