RecyclerView 判断滑到底部 顶部 预加载 更多 分页 MD

Markdown版本笔记	我的GitHub首页	我的博客	我的微信	我的邮箱
MyAndroidBlogs	baiqiantao	baiqiantao	bqt20094	[email protected]

RecyclerView 判断滑到底部 顶部 预加载 更多 分页 MD

---

目录

目录
项目中的案例【预加载】
项目中的另一个案例
利用 lastVisibleItemPosition 判断【不靠谱】
利用 computeVerticalScrollRange 等判断【不靠谱】
利用 canScrollVertically(direction) 判断【比较靠谱】
利用 LinearLayoutManager 来判断【垃圾】

判断RecyclerView到达底部的几种方法

项目中的案例【预加载】

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】");
        }
    }
});

项目中的另一个案例

rv.addOnScrollListener(new RecyclerView.OnScrollListener() {
    @Override
    public void onScrolled(RecyclerView recyclerView, int dx, int dy) {
        super.onScrolled(recyclerView, dx, dy);
        StaggeredGridLayoutManager layoutManager = (StaggeredGridLayoutManager) rv.getLayoutManager();
        int[] positions = layoutManager.findLastVisibleItemPositions(null);
        if (positions != null && positions.length > 0) {
            for (int position : positions) {
                if (position >= mDatas.size() - layoutManager.getSpanCount() || !rv.canScrollVertically(1)) {//滑到底部了
                    if (canLoadMore) {
                        XLog.tag("bqt_red").i("加载更多 "+position+" "+rv.canScrollVertically(1));
                        canLoadMore = false;
                        doRequest();
                    }
                    break;
                }
            }
        }
    }
});

利用 lastVisibleItemPosition 判断【不靠谱】

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【不靠谱】

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【比较靠谱】

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【垃圾】

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;
}

• 其他逻辑

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！

2017.06.23