Child RecyclerView setadapter doesn’t work on my Nested RecyclerView Android

I have two RecyclerViews. Parent RecyclerView works perfectly but the setadapter of the child’s in the parent adapter does not work.

I tried to make some log messages in the child adapter but they didn’t even appear so the adapter is not even starting

Below is the piece of code for better understanding.

MainFragment.java

        // Set up the RecyclerView.
        recyclerview = rootView.findViewById(R.id.recycler_view);
        adapter = new TodayAdapter(getContext());
        recyclerview.setLayoutManager(new LinearLayoutManager(getContext()));
        recyclerview.setAdapter(adapter);
        taskViewModel = new ViewModelProvider(requireActivity()).get(TaskViewModel.class);

        List<TaskList> categories = new ArrayList<>();

        TaskList completedTasks = new TaskList("Completed Tasks",null);
        TaskList incompletedTasks = new TaskList("Incompleted Tasks",null);
        List<Task> intask = new ArrayList<>();
        List<Task> comtask = new ArrayList<>();


        // Update the cached copy of the words in the adapter.
        // Get all the words from the database
        // and associate them to the adapter.
        taskViewModel.getAllTasks().observe( getActivity(), new Observer<List<Task>>() {
            @Override
            public void onChanged(List<Task> tasks) {
                for(Task task : tasks){
                    if(task.getState().equals("0")) {
                        intask.add(task);
                        incompletedTasks.setTasks(intask);
                    } else{
                        comtask.add(task);
                        completedTasks.setTasks(comtask);
                    }
                }

                categories.add(incompletedTasks);
                categories.add(completedTasks);

                adapter.setCategory(categories);
            }
        });

ParentAdapter.java

public class TodayAdapter extends RecyclerView.Adapter<TodayAdapter.TodayHolder>{
    private List<TaskList> categories;

    public TodayAdapter(List<TaskList> todayLists) {
        this.categories = todayLists;
    }

    public TodayAdapter(Context context) {
        LayoutInflater mInflater = LayoutInflater.from(context);
    }


    @NonNull
    @Override
    public TodayHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
        View itemView = LayoutInflater.from(parent.getContext()).inflate(R.layout.list_item, parent, false);
        return new TodayAdapter.TodayHolder(itemView);    }

    @SuppressLint("NotifyDataSetChanged")
    @Override
    public void onBindViewHolder(@NonNull TodayHolder holder, int position) {
        Log.d("test", Integer.toString(categories.size()));
        if(categories!=null) {
            TaskList category = categories.get(position);
            holder.title.setText(category.getTitle());

            holder.tasks.setHasFixedSize(true);
            TaskAdapter taskAdapter = new TaskAdapter();
            taskAdapter.setTasks(category.getTasks());
            holder.tasks.setAdapter(taskAdapter);
            taskAdapter.notifyDataSetChanged();
        }
    }

    @Override
    public int getItemCount() {
        if(categories!=null)
            return categories.size();
        return 0;
    }

    public class TodayHolder extends RecyclerView.ViewHolder {
        private TextView title;
        private RecyclerView tasks;
        public TodayHolder(@NonNull View itemView) {
            super(itemView);
            title=itemView.findViewById(R.id.title);
            tasks=itemView.findViewById(R.id.tasks);
        }
    }

    //Associates a list of tasks with this adapter
    public void setCategory(List<TaskList> categories) {
        this.categories = categories;
        notifyDataSetChanged();
    }
}

ChildAdapter.java

/**
 * Adapter for the RecyclerView that displays a list of words.
 */

public class TaskAdapter extends RecyclerView.Adapter<TaskAdapter.TaskHolder> {

    private List<Task> tasks; // Cached copy of words
    private OnItemClickListener listener;

    public TaskAdapter(Context context) {
        LayoutInflater mInflater = LayoutInflater.from(context);
    }

    public TaskAdapter() {
    }

    @NonNull
    @Override
    public TaskHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
        Log.d("test","rani fe task adapder");

        View itemView = LayoutInflater.from(parent.getContext()).inflate(R.layout.task_item, parent, false);
        return new TaskHolder(itemView);
    }

    @SuppressLint("ResourceAsColor")
    @Override
    public void onBindViewHolder(@NonNull TaskHolder holder, int position) {
        Log.d("test","we are in =" +position);
        if (tasks != null) {
            Task currentTask = tasks.get(position);
            holder.textViewTitle.setText(currentTask.getTitle());
            holder.textViewDescription.setText(currentTask.getDescription());

            switch (currentTask.getCategory()) {
                case "1":
                    holder.layout.setBackgroundResource(R.color.button_beige);
                    if (currentTask.getState().equals("0")){
                        holder.view.setBackgroundResource(R.drawable.uncheck_beige);
                    holder.textViewTitle.setPaintFlags(0);
                    holder.textViewDescription.setPaintFlags(0);
                    }else{
                        holder.view.setBackgroundResource(R.drawable.check_beige);
                        holder.textViewTitle.setPaintFlags(holder.textViewTitle.getPaintFlags()
                                | Paint.STRIKE_THRU_TEXT_FLAG);
                        holder.textViewDescription.setPaintFlags(holder.textViewDescription.getPaintFlags()
                                | Paint.STRIKE_THRU_TEXT_FLAG);
                    }
                    break;
                case "2":
                    holder.layout.setBackgroundResource(R.color.button_blue);

                    if(currentTask.getState().equals("0")){
                        holder.view.setBackgroundResource(R.drawable.uncheck_blue);
                        holder.textViewTitle.setPaintFlags(0);
                        holder.textViewDescription.setPaintFlags(0);
                    }
                    else{
                        holder.view.setBackgroundResource(R.drawable.check_blue);
                        holder.textViewTitle.setPaintFlags(holder.textViewTitle.getPaintFlags()
                                | Paint.STRIKE_THRU_TEXT_FLAG);
                        holder.textViewDescription.setPaintFlags(holder.textViewDescription.getPaintFlags()
                                | Paint.STRIKE_THRU_TEXT_FLAG);
                    }
                    break;
                case "3":
                    holder.layout.setBackgroundResource(R.color.button_green);
                    if(currentTask.getState().equals("0")){
                        holder.view.setBackgroundResource(R.drawable.uncheck_green);
                        holder.textViewTitle.setPaintFlags(0);
                        holder.textViewDescription.setPaintFlags(0);
                    }
                    else{
                        holder.view.setBackgroundResource(R.drawable.check_green);
                        holder.textViewTitle.setPaintFlags(holder.textViewTitle.getPaintFlags()
                                | Paint.STRIKE_THRU_TEXT_FLAG);
                        holder.textViewDescription.setPaintFlags(holder.textViewDescription.getPaintFlags()
                                | Paint.STRIKE_THRU_TEXT_FLAG);
                    }                    break;
                case "4":
                    holder.layout.setBackgroundResource(R.color.button_pink);
                    if(currentTask.getState().equals("0")){
                        holder.view.setBackgroundResource(R.drawable.uncheck_pink);
                        holder.textViewTitle.setPaintFlags(0);
                        holder.textViewDescription.setPaintFlags(0);
                    }
                    else{
                        holder.view.setBackgroundResource(R.drawable.check_pink);
                        holder.textViewTitle.setPaintFlags(holder.textViewTitle.getPaintFlags()
                                | Paint.STRIKE_THRU_TEXT_FLAG);
                        holder.textViewDescription.setPaintFlags(holder.textViewDescription.getPaintFlags()
                                | Paint.STRIKE_THRU_TEXT_FLAG);
                    }
                    break;
                case "5":
                    holder.layout.setBackgroundResource(R.color.button_purple);
                    if(currentTask.getState().equals("0")){
                        holder.view.setBackgroundResource(R.drawable.uncheck_purple);
                        holder.textViewTitle.setPaintFlags(0);
                        holder.textViewDescription.setPaintFlags(0);
                    }
                    else{
                        holder.view.setBackgroundResource(R.drawable.check_purple);
                        holder.textViewTitle.setPaintFlags(holder.textViewTitle.getPaintFlags()
                                | Paint.STRIKE_THRU_TEXT_FLAG);
                        holder.textViewDescription.setPaintFlags(holder.textViewDescription.getPaintFlags()
                                | Paint.STRIKE_THRU_TEXT_FLAG);
                    }
                    break;
                default:
                    holder.layout.setBackgroundResource(R.color.black);
                    if(currentTask.getState().equals("0")){
                        holder.view.setBackgroundResource(R.drawable.uncheck_black);
                        holder.textViewTitle.setPaintFlags(0);
                        holder.textViewDescription.setPaintFlags(0);
                    }
                    else{
                        holder.view.setBackgroundResource(R.drawable.check_black);
                        holder.textViewTitle.setPaintFlags(holder.textViewTitle.getPaintFlags()
                                | Paint.STRIKE_THRU_TEXT_FLAG);
                        holder.textViewDescription.setPaintFlags(holder.textViewDescription.getPaintFlags()
                                | Paint.STRIKE_THRU_TEXT_FLAG);
                    }
                    break;
            }
        }
    }

    //getItemCount() is called many times, and when it is first called,
    //mWords has not been updated (means initially, it's null, and we can't return null).
    @Override
    public int getItemCount() {
        if (tasks != null)
            return tasks.size();
        return 0;
    }

    //Associates a list of tasks with this adapter
    public void setTasks(List<Task> tasks) {
        this.tasks = tasks;
        notifyDataSetChanged();
        if(tasks != null){
            Log.d("test","task size =" +this.tasks.size());
        }else Log.d("test","task size =" +0);
    }

    //to get the item from the id
    public Task getTaskAt(int position) {
        return tasks.get(position);
    }

    class TaskHolder extends RecyclerView.ViewHolder {
        private TextView textViewTitle;
        private TextView textViewDescription;
        private View view;
        private LinearLayout layout;
        private RelativeLayout EditLayout;

        public TaskHolder(@NonNull View itemView) {
            super(itemView);
            textViewTitle = itemView.findViewById(R.id.text_view_title);
            textViewDescription = itemView.findViewById(R.id.text_view_description);
            layout = itemView.findViewById(R.id.color);
            view = itemView.findViewById(R.id.image);
            EditLayout = itemView.findViewById(R.id.EditLayout);

            EditLayout.setOnClickListener(v -> {
                int position = getAdapterPosition();
                listener.onItemClick(tasks.get(position));
            });

            view.setOnClickListener(v -> {
                int position = getAdapterPosition();
                listener.onDoneClick(tasks.get(position));
            });
        }
    }

    public interface OnItemClickListener {
        void onItemClick(Task task);
        void onDoneClick(Task task);
    }

    public void setOnItemClickListener(OnItemClickListener listener) {
        this.listener = listener;
    }
}

Answer

That is because you must prefetch the child items first before you create its adapter using:

layoutManager.setInitialPrefetchItemCount(parentItem, .size());

you can find a full tutorial in here

https://www.geeksforgeeks.org/how-to-create-a-nested-recyclerview-in-android/