RecyclerView 判断滑到底/顶部的方法

判断RecyclerView到达底部的几种方法

项目中的案例

mRvChat.addOnScrollListener(new RecyclerView.OnScrollListener() {
	private int minLeftItemCount=10;//剩余多少条时开始加载更多

	@Override
	public void onScrolled(RecyclerView recyclerView, int dx, int dy) {
		if (recyclerView.getLayoutManager() instanceof LinearLayoutManager) {
			LinearLayoutManager layoutManager = (LinearLayoutManager) recyclerView.getLayoutManager();

			int itemCount = layoutManager.getItemCount();
			int lastPosition = layoutManager.findLastCompletelyVisibleItemPosition();
			XLog.tag("bqt3").i("【总数】" + itemCount + "【位置】" + lastPosition);

			if (lastPosition == layoutManager.getItemCount() - 1) {//容错处理，保证滑到最后一条时一定可以加载更多
				this.onLoadMore();
			} else {
				if (itemCount > minLeftItemCount) {
					if (lastPosition == itemCount - minLeftItemCount) {
						//一定要意识到，onScrolled方法并不是一直被回调的，估计最多一秒钟几十次
						//所以当此条件满足时，可能并没有回调onScrolled方法，也就不会调用onLoadMore方法
						//所以一定要想办法弥补这隐藏的bug，最简单的方式就是当滑到最后一条时一定可以加载更多
						this.onLoadMore();
					}
				} else {//（第一次进入时）如果总数特别少，直接加载更多
					this.onLoadMore();
				}
			}
		}
	}

	private void onLoadMore() {
		if (canLoadMore) {
			canLoadMore = false;
			ChatReqHelper.requestGroupHis(groupId, ((ChatModel) mGroupChats.get(mGroupChats.size() - 1)).getMsgId());
			XLog.tag("bqt3").i("【加载更多数据】");
		}
	}
});

利用 lastVisibleItemPosition 判断

简单判断

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

当屏幕中最后一个子项 lastVisibleItemPosition 等于所有子项个数 totalItemCount - 1，那么RecyclerView就到达了底部。

但是，如果 totalItemCount 等于1，并且这个item的高度比屏幕还要高时，我们可以发现这个item没完全显示出来就已经被判断为拉到底部。

利用 computeVerticalScrollRange 等判断

这种方法原理其实很简单，而且也是View自带的方法

public static boolean isSlideToBottom(RecyclerView recyclerView) {
	return recyclerView != null
			&& recyclerView.computeVerticalScrollExtent()//当前屏幕显示区域的高度
			+ recyclerView.computeVerticalScrollOffset()//当前屏幕之前滑过的距离
			>= recyclerView.computeVerticalScrollRange();//整个View控件的高度
}

这种方法经过测试，暂时还没发现有bug，而且它用的是View自带的方法，所以个人觉得比较靠谱。

利用 canScrollVertically(direction) 判断

这种方法更简单，就通过简单的调用方法，就可以得到你想要的结果。

这种方法与第二种方法其实是同一种方法，我们看看 canScrollVertically 的源码：

/**
 * Check if this view can be scrolled vertically in a certain direction.
 *
 * @param direction Negative to check scrolling up, positive to check scrolling down.
 * @return true if this view can be scrolled in the specified direction, false otherwise.
 */
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;
	}
}

canScrollVertically(1)的值表示是否能向上滚动，true表示能滚动，false表示已经滚动到底部

canScrollVertically(-1)的值表示是否能向下滚动，true表示能滚动，false表示已经滚动到顶部

利用 LinearLayoutManager 来判断

这种方法其实是比较呆板的，就是利用LinearLayoutManager的几个方法算出已经滑过的子项的距离、屏幕的高度、RecyclerView的总高度，最后将高度作比较。

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 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;
}

算出所有子项的总高度

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;
}

其他

public static int getHeaderHeight(RecyclerView recyclerView) {
	int headerCildHeight = 0;
	
	LinearLayoutManager layoutManager = (LinearLayoutManager) recyclerView.getLayoutManager();
	int firstHeaderPos = layoutManager.findFirstCompletelyVisibleItemPosition();
	View headerCild = layoutManager.findViewByPosition(firstHeaderPos);
	
	if (headerCild != null) {
		RecyclerView.LayoutParams params = (RecyclerView.LayoutParams) headerCild.getLayoutParams();
		headerCildHeight = headerCild.getHeight() + params.topMargin + params.bottomMargin;
	}
	return headerCildHeight;
}

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;
}

虽然这种方法看上去比较呆板的同时考虑不很周全，但这种方法可以对RecylerView的LinearLayoutManager有深一步的理解！

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