public abstract static class OnScrollListener {
/**
* Callback method to be invoked when RecyclerView's scroll state changes.
*
* @param recyclerView The RecyclerView whose scroll state has changed.
* @param newState The updated scroll state. One of {@link #SCROLL_STATE_IDLE},
* {@link #SCROLL_STATE_DRAGGING} or {@link #SCROLL_STATE_SETTLING}.
*/
public void onScrollStateChanged(RecyclerView recyclerView, int newState){}
/**
* Callback method to be invoked when the RecyclerView has been scrolled. This will be
* called after the scroll has completed.
* <p>
* This callback will also be called if visible item range changes after a layout
* calculation. In that case, dx and dy will be 0.
*
* @param recyclerView The RecyclerView which scrolled.
* @param dx The amount of horizontal scroll.
* @param dy The amount of vertical scroll.
*/
public void onScrolled(RecyclerView recyclerView, int dx, int dy){}
}
可见，OnScrollListenerll类是一个什么类？“abstract”，即抽象类。我们知道，子类是必须实现父类的抽象方法。其中，onScrollStateChanged 、onScrolled不是抽象方法，有自己的方法体，无需子类必须实现。
onScrolled方法3个参数：1、recyclerView 是要被监听的列表；2、x坐标滑动值；3、y坐标滑动值。
onScrollStateChanged方法2个参数：1、recyclerView 是要被监听的列表；2、newState是滑动状态。
滑动状态值如下：（抬起、滑动中）
/**
* The RecyclerView is not currently scrolling.当前未滚动。
*/
public static final int SCROLL_STATE_IDLE = 0;
/**
* The RecyclerView is currently being dragged by outside input such as user touch input.
* 目前正在被外部输入拖拽，比如用户触摸输入。
*/
public static final int SCROLL_STATE_DRAGGING = 1;
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