android - RecyclerView corrupts view using notifyItemMoved()

Question

I'm having a problem with using the notifyItemMoved() method. It seems to be incorrectly displaying unmoved views.

My list has 4 element in it. What I want to do is animate a swap between item 1 and item 3. Items 1 and 3 swap correctly, but item 2 displays what was at item 3!

So the list starts off looking something like this:

Item 0
Item 1
Item 2
Item 3

And ends like this:

Item 0
Item 3
Item 3 <-- What the heck has this changed for?
Item 1

My adapter is backed by a List mProductList. I call the following code:

public void sortBackingListUsingSortingList(List<ProductWrapper> newProductItems) {
    Log.e("", "Before:");
    for(ProductWrapper wrapper : mProductItems) wrapper.log();
    for(int i = 0; i < newProductItems.size(); i++) {
        ProductWrapper currentItem   = mProductItems.get(i);
        ProductWrapper correctItem   = newProductItems.get(i);

        if(!currentItem.equals(correctItem)) {
            // Item in wrong place
            int indexOfCorrectItem = getIndexOfItemInList(mProductItems, correctItem);
            Collections.swap(mProductItems, i, indexOfCorrectItem);
            notifyItemMoved(i, indexOfCorrectItem);
            Log.e("", "notifyItemMoved(" + i + ", " + indexOfCorrectItem+")");
            Log.e("", "After:");
            for(ProductWrapper wrapper : mProductItems) wrapper.log();
        }
    }
}

I've also added logging to onBindViewHolder to check if my view logic is being called:

@Override
public void onBindViewHolder(HolderBasic holder, int position) {
    Log.e("", "onBindViewHolder(holder, " + position + ")");
    holder.fill(mProductItems.get(position));
}

My logs look like this:

09-02 14:39:17.853: Before:
09-02 14:39:17.853: Item 0
09-02 14:39:17.853: Item 1
09-02 14:39:17.853: Item 2
09-02 14:39:17.853: Item 3

09-02 14:39:17.854: notifyItemMoved(1, 3)

09-02 14:39:17.854: After:
09-02 14:39:17.854: Item 0
09-02 14:39:17.854: Item 3
09-02 14:39:17.854: Item 2
09-02 14:39:17.854: Item 1

09-02 14:39:17.867: onBindViewHolder(holder, 1)
09-02 14:39:17.874: onBindViewHolder(holder, 3)

As you can see, no reason for Item 2 to have change it's display at all - and yet, it does. Anybody know why?

EDIT

I can get around above by looping through the entire adapter and calling notifyItemChanged() on every item. Inefficient and not a good solution, but is invisible to the user.

Answer 1

Thank you to @david.mihola for leading me to what I'm doing wrong.

This took so long to figure out as the symptom didn't make the problem obvious!

I was doing this:

Collections.swap(mProductItems, i, indexOfCorrectItem);
notifyItemMoved(i, indexOfCorrectItem)

But, I obviously didn't think through what notifyItemMoved() was actually doing. It is only notifying the adapter that item i has moved to indexOfCorrectItem it isn't telling the adapter that indexOfCorrectItem has also moved to i.

Under the covers it was doing the following:

1. Move item 1 to 3
2. Move what was at 2 to 1 to fill the gap
3. Move what was at 3 to 2 to fill the gap
4. notifyItemChanged(1);
5. notifyItemChanged(3);

The above of course leaves item 3 moved down to item 2 without a refreshed view! It was steps 4 and 5 which were hiding the problem by making item1 and item3 display correctly and leaving item2 incorrect!

As soon as I realised this I tried the following code:

notifyItemMoved(indexOfCorrectItem, i);
notifyItemMoved(i, indexOfCorrectItem);

This left the list in the correct order, but it short circuited the animation.

So, instead, I dumped swapping altogether:

mProductItems.remove(indexOfCorrectItem);
mProductItems.add(i, correctItem);
notifyItemMoved(indexOfCorrectItem, i);