*onScrollStateChanged(RecyclerView recyclerView, int newState)：滑动状态的回调。
那么，我们的着重点就在这个两个方法上了。
对于向上加载更多，我们需要有如下判断
--是否是向上滑动
--是否滑动到底部
--当前是否正在加载数据
--当前状态是否是滑动停止的状态
实现比较复杂，定义一个类LoadDataScrollController，继承类RecyclerView.OnScrollListener，
因为onScrollStateChanged实在状态改变时的回调，无法时时的获取显示的条目以及位置，所以我们在onScrolled中获取相应位置，
@Override
public void onScrolled(RecyclerView recyclerView, int dx, int dy) {
/**
* 获取布局参数
*/
RecyclerView.LayoutManager layoutManager = recyclerView.getLayoutManager();
//如果为null，第一次运行，确定布局类型
if (mLayoutManagerType == null) {
if (layoutManager instanceof LinearLayoutManager) {
mLayoutManagerType = LayoutManagerType.LINEAR_LAYOUT;
} else if (layoutManager instanceof GridLayoutManager) {
mLayoutManagerType = LayoutManagerType.GRID_LAYOUT;
} else if (layoutManager instanceof StaggeredGridLayoutManager) {
mLayoutManagerType = LayoutManagerType.STAGGERED_GRID_LAYOUT;
} else {
throw new RuntimeException("LayoutManager should be LinearLayoutManager,GridLayoutManager,StaggeredGridLayoutManager");
}
}
//对于不太能够的布局参数，不同的方法获取到当前显示的最后一个条目数
switch (mLayoutManagerType) {
case LINEAR_LAYOUT:
mLastVisibleItemPosition = ((LinearLayoutManager) layoutManager).findLastVisibleItemPosition();
break;
case GRID_LAYOUT:
mLastVisibleItemPosition = ((GridLayoutManager) layoutManager).findLastVisibleItemPosition();
break;
case STAGGERED_GRID_LAYOUT:
StaggeredGridLayoutManager staggeredGridLayoutManager = (StaggeredGridLayoutManager) layoutManager;
if (mLastPostions == null) {
mLastPostions = new int[staggeredGridLayoutManager.getSpanCount()];
}
staggeredGridLayoutManager.findLastVisibleItemPositions(mLastPostions);
mLastVisibleItemPosition = findMax(mLastPostions);
break;
default:
break;
}
}
