java - 当已位于底部并添加新项目时，将 recyclerView 保留在底部

Question

我正在创建一个聊天应用程序。当列表中当前最低的项目在屏幕上可见时，我试图使所有新消息显示在聊天应用程序的底部，但当用户向上滚动时则不会(像大多数聊天应用程序一样)。截至目前，当屏幕上仍有未使用的空间时，我已经让它发挥作用，但是当 RecyclerView 有太多 View 无法容纳时，它开始将新消息放置在屏幕之外的底部。这是我创建 RecyclerView 的方法:

private void initRecyclerView() {


    mRecyclerView = findViewById(R.id.chatRecyclerView);
    mAdapter = new ChatRecyclerViewAdapter(this, mMessages, mUsernames, mRoomID, mDatabaseReference);
    mRecyclerView.setAdapter(mAdapter);
    final LinearLayoutManager linearLayoutManager = new LinearLayoutManager(this);
    linearLayoutManager.setStackFromEnd(true);
    RecyclerView.AdapterDataObserver observer = new RecyclerView.AdapterDataObserver() {
        @Override
        public void onChanged() {
            super.onChanged();

            if(linearLayoutManager.findLastVisibleItemPosition()==(mAdapter.getItemCount()-1)){
                mRecyclerView.smoothScrollToPosition(mAdapter.getItemCount()-1);
            }
        }
    };
    mAdapter.registerAdapterDataObserver(observer);
    mRecyclerView.setLayoutManager(linearLayoutManager);

Answer 1

我最终添加了一个检查以查看 RecyclerView 是否位于底部并将该 boolean 值插入 AdapterDataObserver。我还犯了一个错误，使用观察者的 onChanged 方法而不是 onItemRangeInserted，这显然是补充 RecyclerViewAdapter 中的 notificationItemInserted 所必需的。

这是我的 RecyclerView 工作代码，如果用户位于底部，它会自动将新消息放置在底部:

 private void initRecyclerView() {
    

    //initializes and sets adapter 
    

    mRecyclerView = findViewById(R.id.chatRecyclerView);
    mAdapter = new ChatRecyclerViewAdapter(this, mMessages, mUsernames, mRoomID, mDatabaseReference);
    mRecyclerView.setAdapter(mAdapter);

    //Creates layout manager and makes it feed new RecyclerView views from the bottom


    final LinearLayoutManager linearLayoutManager = new LinearLayoutManager(this);
    linearLayoutManager.setStackFromEnd(true);

    //Makes RecyclerView scroll to bottom when notifyItemInserted is called from adapter and RecyclerView is already at bottom

    RecyclerView.AdapterDataObserver observer = new RecyclerView.AdapterDataObserver() {
        @Override
        public void onItemRangeInserted(int positionStart, int itemCount) {
            super.onItemRangeInserted(positionStart, itemCount);


            if (isAtBottom) {
                mRecyclerView.smoothScrollToPosition(mAdapter.getItemCount() - 1);
            } 
        }
    };

    //Adds logic to see if RecyclerView is at bottom or not

    mRecyclerView.addOnScrollListener(new RecyclerView.OnScrollListener() {
        @Override
        public void onScrollStateChanged(RecyclerView recyclerView, int newState) {
            super.onScrollStateChanged(recyclerView, newState);

            if (!recyclerView.canScrollVertically(1)) {
                isAtBottom = true;
            } else {
                isAtBottom = false;
            }
        }
    });

    //Assigns observer to adapter and LayoutManager to RecyclerView

    mAdapter.registerAdapterDataObserver(observer);
    mRecyclerView.setLayoutManager(linearLayoutManager);

}