RecyclerView倒序无限滚动

Question

我正在创建一个聊天应用程序，尝试以相反的方式实现RecyclerView的无限滚动，从底部开始向上滚动，当到达顶部时，加载更多。

当用户打开聊天界面时，应用程序默认获取最后20条消息并滚动到底部。我希望在用户向上滚动时从服务器获取更多消息。

以下是我正在测试的通用无限滚动代码：

protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        layoutManager = new LinearLayoutManager(this);
        recyclerView =  (RecyclerView)findViewById(R.id.recyclerview);
        recyclerView.setLayoutManager(layoutManager);
        recyclerView.setHasFixedSize(true);


        adapter = new RecyclerViewAdapter<String, ItemViewHolder>(String.class,R.layout.item,ItemViewHolder.class,list) {
            @Override
            protected void populateViewHolder(ItemViewHolder viewHolder, String model, int position) {
                if(model != null){
                    viewHolder.textView.setText(model);
                }
            }
        };
        recyclerView.setAdapter(adapter);
        recyclerView.setOnScrollListener(new EndlessReverseRecyclerViewScrollListener(layoutManager) {
            @Override
            public void onLoadMore(int page, int totalItemsCount, RecyclerView view) {
                loadMore();
            }
        });
        loadInit();
    }

    private void loadInit(){
        for(int x = 0; x < 20;x++){
            list.add(list.size()+": "+getRandomString(8));
            adapter.notifyDataSetChanged();
        }
        layoutManager.scrollToPosition(list.size() -1);
    }

    private void loadMore(){
        for(int x = 0; x < 20;x++){
            list.add(list.size()+": "+getRandomString(8));
            adapter.notifyDataSetChanged();
        }
    }

无限滚动类

public EndlessReverseRecyclerViewScrollListener(LinearLayoutManager layoutManager) {
        this.layoutManager = layoutManager;
    }

    // This happens many times a second during a scroll, so be wary of the code you place here.
    // We are given a few useful parameters to help us work out if we need to load some more data,
    // but first we check if we are waiting for the previous load to finish.
    @Override
    public void onScrolled(RecyclerView view, int dx, int dy) {
        int lastVisibleItemPosition = 0;
        int totalItemCount = layoutManager.getItemCount();



        if (layoutManager instanceof StaggeredGridLayoutManager) {
            int[] lastVisibleItemPositions = ((StaggeredGridLayoutManager) layoutManager).findLastVisibleItemPositions(null);
            // get maximum element within the list
            lastVisibleItemPosition = getLastVisibleItem(lastVisibleItemPositions);
        } else if (layoutManager instanceof GridLayoutManager) {
            lastVisibleItemPosition = ((GridLayoutManager) layoutManager).findLastVisibleItemPosition();
        } else if (layoutManager instanceof LinearLayoutManager) {
            lastVisibleItemPosition = ((LinearLayoutManager) layoutManager).findLastVisibleItemPosition();
        }

        Log.d(TAG," *** last visible: "+lastVisibleItemPosition);
        Log.d(TAG," *** total: "+totalItemCount);
        // If the total item count is zero and the previous isn't, assume the
        // list is invalidated and should be reset back to initial state
        if (totalItemCount < previousTotalItemCount) {
            this.currentPage = this.startingPageIndex;
            this.previousTotalItemCount = totalItemCount;
            if (totalItemCount == 0) {
                this.loading = true;
            }
        }
        // If it’s still loading, we check to see if the dataset count has
        // changed, if so we conclude it has finished loading and update the current page
        // number and total item count.
        if (loading && (totalItemCount > previousTotalItemCount)) {
            loading = false;
            previousTotalItemCount = totalItemCount;
        }

        // If it isn’t currently loading, we check to see if we have breached
        // the visibleThreshold and need to reload more data.
        // If we do need to reload some more data, we execute onLoadMore to fetch the data.
        // threshold should reflect how many total columns there are too
        if (!loading && (lastVisibleItemPosition + visibleThreshold) > totalItemCount) {
            currentPage++;
            onLoadMore(currentPage, totalItemCount, view);
            loading = true;
        }
    }

    // Call this method whenever performing new searches
    public void resetState() {
        this.currentPage = this.startingPageIndex;
        this.previousTotalItemCount = 0;
        this.loading = true;
    }

    // Defines the process for actually loading more data based on page
    public abstract void onLoadMore(int page, int totalItemsCount, RecyclerView view);

这个代码段可以完美地从顶部向底部滚动。请问如何使其从底部到顶部滚动？谢谢。

附注：ReverseLayout不是我要找的解决方案，因为它只会颠倒我的项目顺序，而这不是我想要实现的目标。

- spongyboss

Answer 1

reverseLayout属性可以实现RecyclerView的LayoutManager的反向布局。

Kotlin:

myRecyclerView.layoutManager = LinearLayoutManager(
    context = this,
    orientation = LinearLayoutManager.VERTICAL,
    reverseLayout = true
).apply { stackFromEnd = true }

XML：

<androidx.recyclerview.widget.RecyclerView
    ...
    app:layoutManager="androidx.recyclerview.widget.LinearLayoutManager"
    app:reverseLayout="true"
    app:stackFromEnd="true"
    android:orientation="vertical />

如果您不想让您的项目从底部堆叠，可以将stackFromEnd设置为false。

- Rainmaker

---

不要忘记在设置Recycler的布局之前直接将其设置为LinearLayoutManager，而不是Recycler的布局。 - Rainmaker

Answer 2

最近几天我一直面临着同样的问题，这是我想到的解决方法。 可以监听上/下两个方向的无限滚动。

注意：我没有针对"GridLayoutManager"进行测试。

public abstract class EndlessRecyclerViewScrollListener extends RecyclerView.OnScrollListener {
    // Sets the starting page index
    private static final int startingPageIndex = 0;
    // The minimum amount of items to have below your current scroll position
    // before loading more.
    private int visibleThreshold = 5;
    // The current offset index of data you have loaded
    private int currentPage = 0;
    // The total number of items in the dataset after the last load
    private int previousTotalItemCount = 0;
    // True if we are still waiting for the last set of data to load.
    private boolean loading = true;
    private RecyclerView.LayoutManager mLayoutManager;
    private LoadOnScrollDirection mDirection;

    public EndlessRecyclerViewScrollListener(LinearLayoutManager layoutManager, LoadOnScrollDirection direction) {
        this.mLayoutManager = layoutManager;
        mDirection = direction;
    }

    public EndlessRecyclerViewScrollListener(GridLayoutManager layoutManager, LoadOnScrollDirection direction) {
        this.mLayoutManager = layoutManager;
        visibleThreshold = visibleThreshold * layoutManager.getSpanCount();
        mDirection = direction;
    }

    public EndlessRecyclerViewScrollListener(StaggeredGridLayoutManager layoutManager, LoadOnScrollDirection direction) {
        this.mLayoutManager = layoutManager;
        visibleThreshold = visibleThreshold * layoutManager.getSpanCount();
        mDirection = direction;
    }

    // This happens many times a second during a scroll, so be wary of the code you place here.
    // We are given a few useful parameters to help us work out if we need to load some more data,
    // but first we check if we are waiting for the previous load to finish.
    @Override
    public void onScrolled(@NonNull RecyclerView view, int dx, int dy) {
        int lastVisibleItemPosition = 0;
        int firstVisibleItemPosition = 0;
        int totalItemCount = mLayoutManager.getItemCount();

        if (mLayoutManager instanceof StaggeredGridLayoutManager) {
            int[] lastVisibleItemPositions =
                    ((StaggeredGridLayoutManager) mLayoutManager).findLastVisibleItemPositions(null);
            int[] firstVisibleItemPositions =
                    ((StaggeredGridLayoutManager) mLayoutManager).findFirstVisibleItemPositions(null);
            // get maximum element within the list
            lastVisibleItemPosition = getLastVisibleItem(lastVisibleItemPositions);
            firstVisibleItemPosition = getFirstVisibleItem(firstVisibleItemPositions);
        } else if (mLayoutManager instanceof LinearLayoutManager) {
            lastVisibleItemPosition =
                    ((LinearLayoutManager) mLayoutManager).findLastVisibleItemPosition();
            firstVisibleItemPosition =
                    ((LinearLayoutManager) mLayoutManager).findFirstVisibleItemPosition();
        } else if (mLayoutManager instanceof GridLayoutManager) {
            lastVisibleItemPosition = ((GridLayoutManager) mLayoutManager).findLastVisibleItemPosition();
            firstVisibleItemPosition = ((GridLayoutManager) mLayoutManager).findFirstVisibleItemPosition();
        }

        switch (mDirection) {
            case BOTTOM:
                // If the total item count is zero and the previous isn't, assume the
                // list is invalidated and should be reset back to initial state
                if (totalItemCount < previousTotalItemCount) {
                    this.currentPage = startingPageIndex;
                    this.previousTotalItemCount = totalItemCount;
                    if (totalItemCount == 0) {
                        this.loading = true;
                    }
                }
                // If it’s still loading, we check to see if the dataset count has
                // changed, if so we conclude it has finished loading and update the current page
                // number and total item count.
                if (loading && (totalItemCount > previousTotalItemCount)) {
                    loading = false;
                    previousTotalItemCount = totalItemCount;
                }

                // If it isn’t currently loading, we check to see if we have breached
                // the visibleThreshold and need to reload more data.
                // If we do need to reload some more data, we execute onLoadMore to fetch the data.
                // threshold should reflect how many total columns there are too
                if (!loading && (lastVisibleItemPosition + visibleThreshold) > totalItemCount) {
                    currentPage++;
                    onLoadMore(currentPage, totalItemCount);
                    loading = true;
                }
                break;
            case TOP:
                // If the total item count is zero and the previous isn't, assume the
                // list is invalidated and should be reset back to initial state
                if (totalItemCount < previousTotalItemCount) {
                    this.currentPage = startingPageIndex;
                    this.previousTotalItemCount = totalItemCount;
                    if (totalItemCount == 0) {
                        this.loading = true;
                    }
                }
                // If it’s still loading, we check to see if the dataset count has
                // changed, if so we conclude it has finished loading and update the current page
                // number and total item count.
                if (loading && (totalItemCount > previousTotalItemCount)) {
                    loading = false;
                    previousTotalItemCount = totalItemCount;
                }

                // If it isn’t currently loading, we check to see if we have breached
                // the visibleThreshold and need to reload more data.
                // If we do need to reload some more data, we execute onLoadMore to fetch the data.
                // threshold should reflect how many total columns there are too
                if (!loading && firstVisibleItemPosition < visibleThreshold) {
                    currentPage++;
                    onLoadMore(currentPage, totalItemCount);
                    loading = true;
                }
                break;
        }
    }

    private int getLastVisibleItem(int[] lastVisibleItemPositions) {
        int maxSize = 0;
        for (int i = 0; i < lastVisibleItemPositions.length; i++) {
            if (i == 0) {
                maxSize = lastVisibleItemPositions[i];
            } else if (lastVisibleItemPositions[i] > maxSize) {
                maxSize = lastVisibleItemPositions[i];
            }
        }
        return maxSize;
    }

    private int getFirstVisibleItem(int[] firstVisibleItemPositions) {
        int maxSize = 0;
        for (int i = 0; i < firstVisibleItemPositions.length; i++) {
            if (i == 0) {
                maxSize = firstVisibleItemPositions[i];
            } else if (firstVisibleItemPositions[i] > maxSize) {
                maxSize = firstVisibleItemPositions[i];
            }
        }
        return maxSize;
    }

    // Defines the process for actually loading more data based on page
    public abstract void onLoadMore(int page, int totalItemsCount);

    public enum LoadOnScrollDirection {
        TOP, BOTTOM
    }
}

实现

LinearLayoutManager layoutManager = new LinearLayoutManager(this, LinearLayoutManager.VERTICAL, false);
        layoutManager.setStackFromEnd(true);

        mBinding.rvChats.setLayoutManager(layoutManager);
        mBinding.rvChats.addOnScrollListener(new EndlessRecyclerViewScrollListener(layoutManager, EndlessRecyclerViewScrollListener.LoadOnScrollDirection.TOP) {
            @Override
            public void onLoadMore(int page, int totalItemsCount) {
                mViewModel.fetchMessages(page);
            }
        });

- Pasan Randula Eramusugoda

Answer 3

对于新手面临问题的情况，我的解决方案与被接受的答案相同，但附带我的代码片段。

 LinearLayoutManager linearLayoutManager = new LinearLayoutManager(this);
 linearLayoutManager.setReverseLayout(true);
 chatRecyclerView.setLayoutManager(linearLayoutManager);

为了实现无限和流畅的滚动，我使用了安卓提供的pagination库。

- Abhishek Garg