Firebase ui - 在 android 中为 FirebaseRecyclerView 设置标题视图

Question

我需要在RecyclerView 中添加一个标题视图，并且我使用的适配器来自 Firebase ui。

对于普通的RecyclerView 适配器，我只需处理不同的视图类型并处理一些计数/位置偏移。

但是在FirebaseAdapter 中采用相同的方法似乎是一种不好的做法，在这种情况下最好的方法是什么。

这是我能得到的最接近的。

       mAdapter = new FirebaseRecyclerAdapter<Comment, RecyclerView.ViewHolder>(Comment.class,
            R.layout.item_comment, CommentHolder.class, ref) {

        final static int TYPE_HEADER = 0;
        final static int TYPE_ITEM = 1;

        @Override
        protected void populateViewHolder(RecyclerView.ViewHolder viewHolder, Comment comment, int position) {
            //cast to comment holder
            final CommentHolder commentViewHolder = (CommentHolder) viewHolder;

            //initialize view
            commentViewHolder.setComment(comment.getComment());
            commentViewHolder.setTime(comment.getTimestamp());

            //get user details and set view
            DatabaseReference ref = FirebaseDatabase.getInstance().getReference(FireContract.PATH_USER_PROFILE)
                    .child(comment.getUserKey());
            ref.addListenerForSingleValueEvent(new ValueEventListener() {
                @Override
                public void onDataChange(DataSnapshot dataSnapshot) {
                    User user = dataSnapshot.getValue(User.class);
                    if(user != null) {
                        commentViewHolder.setName(user.getName());
                        commentViewHolder.setUsername("@" + user.getUsername());
                    }
                }

                @Override
                public void onCancelled(DatabaseError databaseError) {

                }
            });
        }

        //return header view holder is type is header else return super from firebase ui
        @Override
        public RecyclerView.ViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
            if(viewType == TYPE_HEADER){
                View view = LayoutInflater.from(getActivity()).inflate(R.layout.post_details,
                        parent, false);

                return new PostViewHolder(view);
            }
            else
                return super.onCreateViewHolder(parent, viewType);
        }

        //initialize post details as header if position is 0 else call super with position less
        //by one to cater for the header offset
        @Override
        public void onBindViewHolder(RecyclerView.ViewHolder viewHolder, int position) {
            if(position == 0)
                initializePostDetails((PostViewHolder) viewHolder);
            else
                super.onBindViewHolder(viewHolder, position - 1);
        }

        //return view type header if position is 0 else call super from firebase ui
        @Override
        public int getItemViewType(int position) {
            if(position == 0)
                return TYPE_HEADER;

            return super.getItemViewType(position - 1);
        }

        //set count equal to 1 plus actual list items, 1 is added for header view
        @Override
        public int getItemCount() {
            return super.getItemCount() + 1;
        }

    };

但这会产生cannot resolve constructor 问题，但即使在解决此问题后也可能无法正常工作。

Answer 1

所以，这是我能够将适配器升级到的内容，它确实对我有用，但不确定它是否仍然是实现此目标的最佳方法。如果有人可以验证这一点，或者可以提出更好更简单的解决方案，那就太好了

mAdapter = new FirebaseRecyclerAdapter<Comment, RecyclerView.ViewHolder>(Comment.class,
            R.layout.item_comment, RecyclerView.ViewHolder.class, ref) {

        final static int TYPE_HEADER = 0;
        final static int TYPE_ITEM = 1;

        @Override
        protected void populateViewHolder(RecyclerView.ViewHolder viewHolder, Comment comment, int position) {
            Log.d(TAG, "running populateViewHolder");
        }

        //return header view holder is type is header else return super from firebase ui
        @Override
        public RecyclerView.ViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
            if(viewType == TYPE_HEADER){
                View view = LayoutInflater.from(getActivity()).inflate(R.layout.post_details,
                        parent, false);

                return new PostViewHolder(view);
            }
            //if type is not header its item view, so return commentHolder instance
            else{
                View view = LayoutInflater.from(getActivity()).inflate(R.layout.item_comment,
                        parent, false);

                return new CommentHolder(view);
            }

        }

        //initialize post details as header if position is 0 else call super with position less
        //by one to cater for the header offset
        @Override
        public void onBindViewHolder(RecyclerView.ViewHolder viewHolder, int position) {
            //if holder is post view holder, initialize header
            if(viewHolder instanceof PostViewHolder)
                initializePostDetails((PostViewHolder) viewHolder);

            else{
                //cast to comment holder
                final CommentHolder commentViewHolder = (CommentHolder) viewHolder;
                Comment comment = getItem(position);

                //initialize view
                commentViewHolder.setComment(comment.getComment());
                commentViewHolder.setTime(comment.getTimestamp());

                //get user details and set view
                DatabaseReference ref = FirebaseDatabase.getInstance().getReference(FireContract.PATH_USER_PROFILE)
                        .child(comment.getUserKey());
                ref.addListenerForSingleValueEvent(new ValueEventListener() {
                    @Override
                    public void onDataChange(DataSnapshot dataSnapshot) {
                        User user = dataSnapshot.getValue(User.class);
                        if(user != null) {
                            commentViewHolder.setName(user.getName());
                            commentViewHolder.setUsername("@" + user.getUsername());
                        }
                    }

                    @Override
                    public void onCancelled(DatabaseError databaseError) {

                    }
                });
            }

        }

        //return view type header if position is 0 else call super from firebase ui
        @Override
        public int getItemViewType(int position) {
            if(position == 0)
                return TYPE_HEADER;

            return TYPE_ITEM;
        }

        //set count equal to 1 plus actual list items, 1 is added for header view
        @Override
        public int getItemCount() {
            return super.getItemCount() + 1;
        }

        //return comment item, decrement 1 from position to avoid out of rand exception as it was
        //incremented for adding header
        @Override
        public Comment getItem(int position) {
            return super.getItem(position - 1);
        }
    };

Answer 2

更新（2016 年 12 月 6 日）：如果您需要检查一个空的 FirebaseRecyclerView，这有点麻烦，但到目前为止，这是我知道的唯一方法。 所以总结一下，你需要处理两种情况。

1. 当列表中有项目时 -- 您可以在 Recycler 视图中添加项目装饰器以在列表中有项目时制作标题。 （正如我在原帖中所说，或as seen here）。如果您不需要担心空的回收站视图，这就是您需要做的。

2. 当列表为空时 - 项目装饰器无法显示，因此您可以添加一个在 View.VISIBLE（对于空列表）和 View.GONE（对于填充列表）之间切换的视图。 （更多信息请查看this link）

   View emptyView = findViewById(R.id.empty_view);
   dataBaseReference.addListenerValueEvent(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
             if (dataSnapshot.hasChildren()) {
                   emptyView.setVisibility(View.GONE);
                } else {
                   emptyView.setVisibility(View.VISIBLE);
                }
   
        @Override
        public void onCancelled(DatabaseError databaseError) {
   
        }
   

   });

所以一旦列表中有东西，我们就拿走那个空视图，并显示项目装饰器，它会随着回收器视图的滚动而移动。

**原始帖子**

我最近遇到了同样的困境。我使用了一个正常的 RecyclerViewAdapter 的解决方案，它工作得很好，here，它只是将一个 ItemDecorator 添加到 recyclerView 本身，如下所示：

recyclerView.addItemDecoration(new HeaderDecoration(this,
                           recyclerView,  R.layout.test_header));

从这里你可以制作一个自定义的物品装饰器。如果你需要它水平或任何东西，你会弄乱那里的值（大多数引用顶部的东西需要引用左侧）：

public class HeaderDecoration extends RecyclerView.ItemDecoration {

private View mLayout;

public HeaderDecoration(final Context context, RecyclerView parent, @LayoutRes int resId) {
    // inflate and measure the layout
    mLayout = LayoutInflater.from(context).inflate(resId, parent, false);
    mLayout.measure(View.MeasureSpec.makeMeasureSpec(0, View.MeasureSpec.UNSPECIFIED),
            View.MeasureSpec.makeMeasureSpec(0, View.MeasureSpec.UNSPECIFIED));
}


@Override
public void onDraw(Canvas c, RecyclerView parent, RecyclerView.State state) {
    super.onDraw(c, parent, state);
    // layout basically just gets drawn on the reserved space on top of the first view
    mLayout.layout(parent.getLeft(), 0, parent.getRight(), mLayout.getMeasuredHeight());
    for (int i = 0; i < parent.getChildCount(); i++) {
        View view = parent.getChildAt(i);
        if (parent.getChildAdapterPosition(view) == 0) {
            c.save();
            final int height = mLayout.getMeasuredHeight();
            final int top = view.getTop() - height;
            c.translate(0, top);
            mLayout.draw(c);
            c.restore();
            break;
        }
    }
}

@Override
public void getItemOffsets(Rect outRect, View view, RecyclerView parent, RecyclerView.State state) {
    if (parent.getChildAdapterPosition(view) == 0) {
        outRect.set(0, mLayout.getMeasuredHeight(), 0, 0);
    } else {
        outRect.setEmpty();
    }
}

}

到目前为止，它的问题是，对于一个空列表，没有项目装饰器。我正在查看空视图以获取回收者视图，当我使用 Firebase 时我会更新它。