android - What's better? notifyDataSetChanged or notifyItemChanged in loop?

Question

So I have an activity with RecyclerView and I want to change TextView of every item in the RecyclerView by pressing button that has onClickListener() in the activity.

I'm wondering what is better in terms of performance:

1. Use notifyDataSetChanged ones.
2. Use loop with condition like int i is less than List.size() where notifyItemChanged would be called few times.

In both cases I create boolean variable in RecyclerView Adapter which is used by onBindViewHolder to know how to update item. By default it's false and after button click it becomes true so onBindViewHolder updates item in different way.

Also I would like to know if this approach is suitable at all.

Answer 1

If you are simply updating one part of the view, use the notifyItemRangeChanged()or notifyItemChanged() instead of notifiyDataSetChanged(). The difference here has to do with structural changes vs item changes. This is on the android developers RecyclerView.Adapter documentation found here.

Here is another tidbit on the differences between the two types of changes:

There are two different classes of data change events, item changes and structural changes. Item changes are when a single item has its data updated but no positional changes have occurred. Structural changes are when items are inserted, removed or moved within the data set.

This is taken from that aforementioned page,

If you are writing an adapter it will always be more efficient to use the more specific change events if you can. Rely on notifyDataSetChanged() as a last resort.

So just to clarify use notifyDataSetChanged() as a last resort, and instead ask yourself if you can preform one of these methods instead, and if you can use it instead:

notifyItemChanged(int)
notifyItemInserted(int)
notifyItemRemoved(int)
notifyItemRangeChanged(int, int)
notifyItemRangeInserted(int, int)
notifyItemRangeRemoved(int, int)

which makes sense because notifyDataSetChanged() will pretty much try to redraw everything based on the data and make no previous assumptions on it, while the other methods will just look for changes. That means the adapter has to do a lot more work that is not necessary. This is what notifyDataSetChanged() does:

This event does not specify what about the data set has changed, forcing any observers to assume that all existing items and structure may no longer be valid. LayoutManagers will be forced to fully rebind and relayout all visible views.

This also makes sense to use the incremental or range approach, because you are changing the text, you need to go get each new text and when you do that you should tell the adapter you changed it. Now, if you do a button click and get all new text values, and create a new list or something then call heavy notifyDataSetChanged().