RecyclerView 上拉加载更多及滚动到底部的判断

1. 根据 item 判断是否到达底部
这种方法最常见，一般都是像下面这样实现：

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public static boolean isVisBottom(RecyclerView recyclerView){    LinearLayoutManager layoutManager = (LinearLayoutManager) recyclerView.getLayoutManager();    //屏幕中最后一个可见子项的position   int lastVisibleItemPosition = layoutManager.findLastVisibleItemPosition();    //当前屏幕所看到的子项个数   int visibleItemCount = layoutManager.getChildCount();    //当前RecyclerView的所有子项个数   int totalItemCount = layoutManager.getItemCount();    //RecyclerView的滑动状态   int state = recyclerView.getScrollState();    if(visibleItemCount > 0 && lastVisibleItemPosition == totalItemCount - 1 && state == recyclerView.SCROLL_STATE_IDLE){        return true;   }else {        return false;    } }

用这种方式判断是否滚动到底部时，只要最后一个 item 显示出一点，就会触发加载更多，用户此时看不到在 FooterView 处的 加载更多 字样（与拖拽展示出加载更多的需求不符）；另外，当 RecyclerView 的 item 过少不足填满整个 RecyclerView 时，也会触发 加载更多 ；因此，这种方式不符合我们的要求。

2. 使用 canScrollVertically(int direction) 判断是否到达底部

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1 2	RecyclerView.canScrollVertically(1)的值表示是否能向上滚动，false表示已经滚动到底部 RecyclerView.canScrollVertically(-1)的值表示是否能向下滚动，false表示已经滚动到顶部

这种方法看似简单，其实同样存在一些陷阱。当 RecyclerView 的 item 过少不足填满整个 RecyclerView 时，无论上拉还是下拉都会触发加载更多；另外，direction 不只可取1和-1，只需保证正负就能达到一样的效果。

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// View#canScrollVertically(int direction) 源码 public boolean canScrollVertically(int direction) {     final int offset = computeVerticalScrollOffset();     final int range = computeVerticalScrollRange() - computeVerticalScrollExtent();     if (range == 0) return false;     if (direction < 0) {         return offset > 0;     } else {         return offset < range - 1;     } }

3. 通过 LinearLayoutManager 进行一系列的计算
这种方法极不推荐使用，过程很复杂，不过对于理解 View 的布局有很大的帮助。这种方法共分为四步，下面将网上的方法抄录如下（请读者自行验证是否也存在方法1,2同样的问题）：

算出一个子项的高度

public static int getItemHeight(RecyclerView recyclerView) {
int itemHeight = 0;
View child = null;
LinearLayoutManager layoutManager = (LinearLayoutManager) recyclerView.getLayoutManager();
int firstPos = layoutManager.findFirstCompletelyVisibleItemPosition();
int lastPos = layoutManager.findLastCompletelyVisibleItemPosition();
child = layoutManager.findViewByPosition(lastPos);
if (child != null) {
RecyclerView.LayoutParams params = (RecyclerView.LayoutParams) child.getLayoutParams();
itemHeight = child.getHeight() + params.topMargin + params.bottomMargin;
}
return itemHeight;
}

算出滑过的子项的总距离

public static int getLinearScrollY(RecyclerView recyclerView) {
int scrollY = 0;
LinearLayoutManager layoutManager = (LinearLayoutManager) recyclerView.getLayoutManager();
int headerCildHeight = getHeaderHeight(recyclerView);
int firstPos = layoutManager.findFirstVisibleItemPosition();
View child = layoutManager.findViewByPosition(firstPos);
int itemHeight = getItemHeight(recyclerView);
if (child != null) {
int firstItemBottom = layoutManager.getDecoratedBottom(child);
scrollY = headerCildHeight + itemHeight * firstPos - firstItemBottom;
if(scrollY < 0){
scrollY = 0;
}
}
return scrollY;
}

算出所有子项的总高度

public static int getLinearTotalHeight(RecyclerView recyclerView) { int totalHeight = 0;
LinearLayoutManager layoutManager = (LinearLayoutManager) recyclerView.getLayoutManager();
View child = layoutManager.findViewByPosition(layoutManager.findFirstVisibleItemPosition());
int headerCildHeight = getHeaderHeight(recyclerView);
if (child != null) {
int itemHeight = getItemHeight(recyclerView);
int childCount = layoutManager.getItemCount();
totalHeight = headerCildHeight + (childCount - 1) * itemHeight;
}
return totalHeight;
}

高度作比较

public static boolean isLinearBottom(RecyclerView recyclerView) {
boolean isBottom = true;
int scrollY = getLinearScrollY(recyclerView);
int totalHeight = getLinearTotalHeight(recyclerView);
int height = recyclerView.getHeight();
// Log.e(“height”,”scrollY ” + scrollY + ” totalHeight ” + totalHeight + ” recyclerHeight ” + height);
if (scrollY + height < totalHeight) {
isBottom = false;
}
return isBottom;
}

判断 RecyclerView 拖拽

这一步比较简单，直接监听滚动即可。

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RecyclerView.addOnScrollListener(new OnScrollListener() {         @Override         public void onScrollStateChanged(RecyclerView recyclerView, int newState) {             super.onScrollStateChanged(recyclerView, newState);             if (newState == SCROLL_STATE_DRAGGING) {                 // 拖拽状态，实际使用中还需要判断 加载更多 是否已显示             }         }           @Override         public void onScrolled(RecyclerView recyclerView, int dx, int dy) {             super.onScrolled(recyclerView, dx, dy);         }     });

推荐方法

该方法在 View#canScrollVertically(int direction) 的基础上，针对上拉拖拽且有可能 items 没有填充满整个 RecyclerView 这个场景做了优化，代码如下：

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RecyclerView.addOnScrollListener(new OnScrollListener() {         @Override         public void onScrollStateChanged(RecyclerView recyclerView, int newState) {             super.onScrollStateChanged(recyclerView, newState);             if (newState == SCROLL_STATE_DRAGGING && 没有触发加载更多) {                 if (RecyclerView.computeVerticalScrollOffset() > 0) {// 有滚动距离，说明可以加载更多，解决了 items 不能充满 RecyclerView                 的问题及滑动方向问题                     boolean isBottom = false ;                     isBottom = RecyclerView.computeVerticalScrollExtent()                         + RecyclerView.computeVerticalScrollOffset()                         == RecyclerView.computeVerticalScrollRange() ;                     // 也可以使用 方法2                     // isBottom = !RecyclerView.canScrollVertically(1) ;                     if (isBottom) {                         // 说明滚动到底部,触发加载更多                         ...                     }                 }             }         }           @Override         public void onScrolled(RecyclerView recyclerView, int dx, int dy) {             super.onScrolled(recyclerView, dx, dy);         }     });