如何在recyclerview的每个项目上添加layout_params？

Question

I have a List <> of items, these items are part of a model class. Whenever an item is added to the List <> , I would like to add layout params like margin_left or margin_right for only the added item.

{....} 
    @Override
    public void onBindViewHolder(@NonNull ViewHolder holder, int position)
    {   
        Comment com = mList.get(position);
    }
    @Override
    public int getItemCount() {
        return mList.size();
    }

Answer 1

It can be done by using RecyclerView.Adapter 's getItemViewType(int position) method. It gets adapter position and returns int viewType which can be used later in onCreateViewHolder(@NonNull ViewGroup parent, int viewType) to inflate layout according to viewType . You can learn more about it here . Bellow how I could achieve setting LayoutParams for every item individually:

An interface to mark an object as object with LayoutParams :

interface Parameterized {

    RecyclerView.LayoutParams  getParams();
}

The model class:

public class Comment implements Parameterized {

    private String content;
    private RecyclerView.LayoutParams params;

    public Comment(String content) {
        this.content = content;
    }

    public String getContent() {
        return content;
    }

    public void setContent(String content) {
        this.content = content;
    }

    @Override
    public RecyclerView.LayoutParams getParams() {
        return params;
    }

    public void setParams(RecyclerView.LayoutParams params) {
        this.params = params;
    }
}

Using setParams() method, we are able to set RecyclerView.LayoutParams to a comment object, that can be used later by an adapter later on.

The adapter:

public class CommentRecyclerAdapter extends RecyclerView.Adapter<CommentRecyclerAdapter.MyViewHolder> {

    private List<Comment> comments = new ArrayList<>();

    @NonNull
    @Override
    public MyViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
        View v = LayoutInflater.from(parent.getContext()).inflate(R.layout.item_name, parent, false);
        v.setLayoutParams(comments.get(viewType).getParams());
        return new MyViewHolder(v);
    }

    @Override
    public int getItemViewType(int position) {
        return position;
    }

    @Override
    public void onBindViewHolder(@NonNull MyViewHolder holder, int position) {
        Comment comment = comments.get(position);
        holder.tvComment.setText(comment.getContent());
    }

    @Override
    public int getItemCount() {
        return comments.size();
    }

    public void updateComments(List<Comment> comments) {
        this.comments = comments;
        notifyDataSetChanged();
    }

    public void addComment(Comment comment) {
        comments.add(comment);
        notifyItemInserted(comments.size());
    }

    class MyViewHolder extends RecyclerView.ViewHolder {
        TextView tvComment;

        MyViewHolder(@NonNull View itemView) {
            super(itemView);

            tvComment = itemView.findViewById(R.id.tv_comment);
        }
    }
}

Usage in an activity / a fragment as an example:

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main2);

    recyclerView = findViewById(R.id.rv_comments);
    recyclerView.setLayoutManager(new LinearLayoutManager(this));

    adapter = new CommentRecyclerAdapter();
    recyclerView.setAdapter(adapter);

    adapter.updateComments(getGeneratedComments());

    Comment comment = new Comment("Added comment");
    RecyclerView.LayoutParams params = new RecyclerView.LayoutParams(
            ViewGroup.LayoutParams.MATCH_PARENT,
            ViewGroup.LayoutParams.WRAP_CONTENT
    );
    params.rightMargin = 30;
    params.leftMargin = 30;
    params.topMargin = 50;
    comment.setParams(params);

    adapter.addComment(comment);
}

private List<Comment> getGeneratedComments() {
    List<Comment> comments = new ArrayList<>();

    for (int i = 1; i < 7; i++) {
        Comment comment = new Comment("Comment " + i);
        RecyclerView.LayoutParams params = new RecyclerView.LayoutParams(
                ViewGroup.LayoutParams.MATCH_PARENT,
                ViewGroup.LayoutParams.WRAP_CONTENT
        );

        params.topMargin = 20 * i;
        comment.setParams(params);
        comments.add(comment);
    }

    return comments;
}

Adapter's updateComments(List<Comment> comments) method updates all the list, and addComment(Comment comment) adds an item with params at the end of the list.

Here haw does this look like: