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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

  1. mRvChat.addOnScrollListener(new RecyclerView.OnScrollListener() {
  2. 	private int minLeftItemCount=10;//剩余多少条时开始加载更多
  4. 	@Override
  5. 	public void onScrolled(RecyclerView recyclerView, int dx, int dy) {
  6. 		if (recyclerView.getLayoutManager() instanceof LinearLayoutManager) {
  7. 			LinearLayoutManager layoutManager = (LinearLayoutManager) recyclerView.getLayoutManager();
  9. 			int itemCount = layoutManager.getItemCount();
  10. 			int lastPosition = layoutManager.findLastCompletelyVisibleItemPosition();
  11. 			XLog.tag("bqt3").i("【总数】" + itemCount + "【位置】" + lastPosition);
  13. 			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
  14. 				this.onLoadMore();
  15. 			} else {
  16. 				if (itemCount > minLeftItemCount) {
  17. 					if (lastPosition == itemCount - minLeftItemCount) {
  18. 						//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
  19. 						//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
  20. 						//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
  21. 						this.onLoadMore();
  22. 					}
  23. 				} else {//（第一次进入时）如果总数特别少，直接加载更多
  24. 					this.onLoadMore();
  25. 				}
  26. 			}
  27. 		}
  28. 	}
  30. 	private void onLoadMore() {
  31. 		if (canLoadMore) {
  32. 			canLoadMore = false;
  33. 			ChatReqHelper.requestGroupHis(groupId, ((ChatModel) mGroupChats.get(mGroupChats.size() - 1)).getMsgId());
  34. 			XLog.tag("bqt3").i("【加载更多数据】");
  35. 		}
  36. 	}
  37. });

利用 lastVisibleItemPosition 判断

简单判断
  1. public static boolean isVisBottom(RecyclerView recyclerView){  
  2.   LinearLayoutManager layoutManager = (LinearLayoutManager) recyclerView.getLayoutManager();  
  3.   int lastVisibleItemPosition = layoutManager.findLastVisibleItemPosition(); //屏幕中最后一个可见子项的position
  4.   int visibleItemCount = layoutManager.getChildCount(); //当前屏幕所看到的子项个数
  5.   int totalItemCount = layoutManager.getItemCount(); //当前RecyclerView的所有子项个数
  6.   int state = recyclerView.getScrollState(); //RecyclerView的滑动状态
  7.   if(visibleItemCount > 0 && lastVisibleItemPosition == totalItemCount - 1 && state == recyclerView.SCROLL_STATE_IDLE){   
  8.      return true; 
  9.   }else {   
  10.      return false;  
  11.   }
  12. }
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
  1. public static boolean isSlideToBottom(RecyclerView recyclerView) {
  2. 	return recyclerView != null
  3. 			&& recyclerView.computeVerticalScrollExtent()//当前屏幕显示区域的高度
  4. 			+ recyclerView.computeVerticalScrollOffset()//当前屏幕之前滑过的距离
  5. 			>= recyclerView.computeVerticalScrollRange();//整个View控件的高度
  6. }
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：
  1. /**
  2.  * Check if this view can be scrolled vertically in a certain direction.
  3.  *
  4.  * @param direction Negative to check scrolling up, positive to check scrolling down.
  5.  * @return true if this view can be scrolled in the specified direction, false otherwise.
  6.  */
  7. public boolean canScrollVertically(int direction) {
  8. 	final int offset = computeVerticalScrollOffset();
  9. 	final int range = computeVerticalScrollRange() - computeVerticalScrollExtent();
  10. 	if (range == 0) return false;
  11. 	if (direction < 0) {
  12. 	    return offset > 0;
  13. 	} else {
  14. 	    return offset < range - 1;
  15. 	}
  16. }
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
  1. public static int getItemHeight(RecyclerView recyclerView) {
  2. 	int itemHeight = 0;
  3. 	View child = null;
  4. 	LinearLayoutManager layoutManager = (LinearLayoutManager) recyclerView.getLayoutManager();
  5. 	int firstPos = layoutManager.findFirstCompletelyVisibleItemPosition();
  6. 	int lastPos = layoutManager.findLastCompletelyVisibleItemPosition();
  7. 	child = layoutManager.findViewByPosition(lastPos);
  8. 	if (child != null) {
  9. 		RecyclerView.LayoutParams params = (RecyclerView.LayoutParams) child.getLayoutParams();
  10. 		itemHeight = child.getHeight() + params.topMargin + params.bottomMargin;
  11. 	}
  12. 	return itemHeight;
  13. }
算出滑过的子项的总距离
  1. public static int getLinearTotalHeight(RecyclerView recyclerView) {
  2. 	int totalHeight = 0;
  3. 	LinearLayoutManager layoutManager = (LinearLayoutManager) recyclerView.getLayoutManager();
  4. 	View child = layoutManager.findViewByPosition(layoutManager.findFirstVisibleItemPosition());
  5. 	int headerCildHeight = getHeaderHeight(recyclerView);
  6. 	if (child != null) {
  7. 		int itemHeight = getItemHeight(recyclerView);
  8. 		int childCount = layoutManager.getItemCount();
  9. 		totalHeight = headerCildHeight + (childCount - 1) * itemHeight;
  10. 	}
  11. 	return totalHeight;
  12. }
算出所有子项的总高度
  1. public static boolean isLinearBottom(RecyclerView recyclerView) {
  2. 	boolean isBottom = true;
  3. 	int scrollY = getLinearScrollY(recyclerView);
  4. 	int totalHeight = getLinearTotalHeight(recyclerView);
  5. 	int height = recyclerView.getHeight();
  6. 	//    Log.e("height","scrollY  " + scrollY + "  totalHeight  " +  totalHeight + "  recyclerHeight  " + height);  
  7. 	if (scrollY + height < totalHeight) {
  8. 		isBottom = false;
  9. 	}
  10. 	return isBottom;
  11. }
其他
  1. public static int getHeaderHeight(RecyclerView recyclerView) {
  2. 	int headerCildHeight = 0;
  3. 	
  4. 	LinearLayoutManager layoutManager = (LinearLayoutManager) recyclerView.getLayoutManager();
  5. 	int firstHeaderPos = layoutManager.findFirstCompletelyVisibleItemPosition();
  6. 	View headerCild = layoutManager.findViewByPosition(firstHeaderPos);
  7. 	
  8. 	if (headerCild != null) {
  9. 		RecyclerView.LayoutParams params = (RecyclerView.LayoutParams) headerCild.getLayoutParams();
  10. 		headerCildHeight = headerCild.getHeight() + params.topMargin + params.bottomMargin;
  11. 	}
  12. 	return headerCildHeight;
  13. }
  15. public static int getLinearScrollY(RecyclerView recyclerView) {
  16. 	int scrollY = 0;
  17. 	LinearLayoutManager layoutManager = (LinearLayoutManager) recyclerView.getLayoutManager();
  18. 	int headerCildHeight = getHeaderHeight(recyclerView);
  19. 	int firstPos = layoutManager.findFirstVisibleItemPosition();
  20. 	View child = layoutManager.findViewByPosition(firstPos);
  21. 	int itemHeight = getItemHeight(recyclerView);
  22. 	if (child != null) {
  23. 		int firstItemBottom = layoutManager.getDecoratedBottom(child);
  24. 		scrollY = headerCildHeight + itemHeight * firstPos - firstItemBottom;
  25. 		if (scrollY < 0) {
  26. 			scrollY = 0;
  27. 		}
  28. 	}
  29. 	
  30. 	return scrollY;
  31. }

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！




文章来自：http://www.cnblogs.com/baiqiantao/p/6958811.html
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