onBindViewHolder() is never called on view at position even though RecyclerView.findViewHolderForAdapterPosition() returns null at that position

Question

I have a list with 13 items (although items may be added or removed), positions 0-12. When the fragment containing the RecyclerView is first shown, only positions 0 through 7 are visible to the user (position 7 being only half visible). In my adapter I Log every time a view holder is binded/bound (idk if grammar applies here) and record its position.

Adapter

@Override
public void onBindViewHolder(final ViewHolder holder, final int position) {
    Log.d(TAG, "onBindViewHolder() position: " + position);
    ...
}

From my Log I see that positions 0-7 are bound:

I have a selectAll() method that gets each ViewHolder by adapter position. If the returned holder is NOT null I use the returned holder to update the view to show it's selected. If the returned holder IS null I call selectOnBind() a method that flags the view at that position update to show it's selected when it's binded rather than in real time since it's not currently shown:

public void selectAll() {
    for (int i = 0; i < numberOfItemsInList; i++) {
        MyAdapter.ViewHolder holder = (MyAdapter.ViewHolder)
                mRecyclerView.findViewHolderForAdapterPosition(i);

        Log.d(TAG, "holder at position " + i + " is " + holder);

        if (holder != null) {
            select(holder);
        } else {
            selectOnBind(i);
        }
    }
}

In this method I Log the holder along with its position:

So up to this point everything seems normal. We have positions 0-7 showing, and according to the Log these are the positions bound. When I hit selectAll() without changing the visible views (scrolling) I see that positions 0-7 are defined and 8-12 are null. So far so good.

Here's where it gets interesting. If after calling selectAll() I scroll further down the list positions 8 and 9 do not show they are selected.

When checking the Log I see that it's because they are never bound even though they were reported to be null:

Even more confusing is that this does not happen every time. If I first launch the app and test this it may work. But it seems to happen without fail afterwards. I'm guessing it has something to do with the views being recycled, but even so wouldn't they have to be bound?

EDIT (6-29-16)
After an AndroidStudio update I cannot seem to reproduce the bug. It works as I expected it to, binding the null views. If this problem should resurface, I will return to this post.

Answer 1

This is happening because:

• The views are not added to the recyclerview (getChildAt will not work and will return null for that position)
• They are cached also (onBind will not be called)

Calling recyclerView.setItemViewCacheSize(0) will fix this "problem".

Because the default value is 2 (private static final int DEFAULT_CACHE_SIZE = 2; in RecyclerView.Recycler), you'll always get 2 views that will not call onBind but that aren't added to the recycler

Answer 2

In your case views for positions 8 and 9 are not being recycled, they are being detached from the window and will be attached again. And for these detached view onBindViewHolder is not called, only onViewAttachedToWindow is called. If you override these function in your adapter, you can see what I am talking.

@Override
    public void onViewRecycled(ViewHolder vh){
        Log.wtf(TAG,"onViewRecycled "+vh);
    }

    @Override
    public void onViewDetachedFromWindow(ViewHolder viewHolder){
        Log.wtf(TAG,"onViewDetachedFromWindow "+viewHolder);
    }

Now in order to solve your problem you need to keep track of the views which were supposed to recycled but get detached and then do your section process on

@Override
    public void onViewAttachedToWindow(ViewHolder viewHolder){
        Log.wtf(TAG,"onViewAttachedToWindow "+viewHolder);
    }

Answer 3

The answers by Pedro Oliveira and Zartha are great for understanding the problem, but I don't see any solutions I'm happy with.

I believe you have 2 good options depending on what you're doing:

Option 1

If you want onBindViewHolder() to get called for an off-screen view regardless if it's cached/detached or not, then you can do:

RecyclerView.ViewHolder view_holder = recycler_view.findViewHolderForAdapterPosition( some_position );

if ( view_holder != null )
{
    //manipulate the attached view
}
else //view is either non-existant or detached waiting to be reattached
    notifyItemChanged( some_position );

The idea is that if the view is cached/detached, then notifyItemChanged() will tell the adapter that view is invalid, which will result in onBindViewHolder() getting called.

Option 2

If you only want to execute a partial change (and not everything inside onBindViewHolder()), then inside of onBindViewHolder( ViewHolder view_holder, int position ), you need to store the position in the view_holder, and execute the change you want in onViewAttachedToWindow( ViewHolder view_holder ).

I recommend option 1 for simplicity unless your onBindViewHolder() is doing something intensive like messing with Bitmaps.

Answer 4

I think playing with view is not a good idea in recyclerview. The approach I always use to follow to just introduce a flag to the model using for RecyclerView. Let assume your model is like -

class MyModel{
    String name;
    int age;
}

If you are tracking the view is selected or not then introduce one boolean to the model. Now it will look like -

class MyModel{
    String name;
    int age;
    boolean isSelected;
}

Now your check box will be selected/un-selected on the basis of the new flag isSelected (in onBindViewHolder() ). On every selection on view will change the value of corresponding model selected value to true, and on unselected change it to false. In your case just run a loop to change all model's isSelected value to true and then call notifyDataSetChanged().

For Example, let assume your list is

ArrayList<MyModel> recyclerList;
private void selectAll(){
    for(MyModel myModel:recyclerList)
        myModel.isSelected = true;
    notifyDataSetChanged();
}

My suggestion, while using recyclerView or ListView to less try to play with views.

So in your case -

@Override
public void onBindViewHolder(final ViewHolder holder, final int position) {
   holder.clickableView.setTag(position);
   holder.selectableView.setTag(position);
   holder.checkedView.setChecked(recyclerList.get(position).isSelected);
    Log.d(TAG, "onBindViewHolder() position: " + position);
    ...
}

@Override
public void onClick(View view){
    int position = (int)view.getTag();
    recyclerList.get(position).isSelected = !recyclerList.get(position).isSelected;
}

@Override
public void onCheckedChanged(CompoundButton buttonView, boolean isChecked) {
      int position = (int)buttonView.getTag();
      recyclerList.get(position).isSelected = isChecked;
}

Hope it will help you, Please let me know if you need any further explanation :)

Answer 5

So I think you question is answered below by @Pedro Oliveira. The main sense of RecycleView, that he using special algorithms for caching ViewHolder in any time. So next onBindViewHolder(...) may not work, for ex. if view is static, or something else.

And about your question you think to use RecycleView for dynamic changed Views. DON'T DO IT! Because RecycleView invalidates views and has caching system, so you will have a lot of problems.

Use LinkedListView for this task!

Answer 6

When you have large number of items in the list you have passed to recyclerview adapter you will not encounter the issue of onBindViewHolder() not executing while scrolling.

But if the list has less items(I have checked on list size 5) you may encounter this issue.

Better solution is to check list size.

Please find sample code below.

private void setupAdapter(){
    if (list.size() <= 10){
        recycler.setItemViewCacheSize(0);
     }
     recycler.setAdapter(adapter);
     recycler.setLayoutManager(linearLayoutManager);
}