Android DiffUtil.ItemCallback areContentsTheSame with different IDs

Question

I'm using a RecyclerView to display a list of items that are retrieved from a database as Java objects. One of the fields in the object is the ID of the item in the database, so that further operations can be performed with it. My areContentsTheSame implementation compares various fields in the objects to determine if the contents of the item has changed.

The problem is that sometimes when the data source is refreshed the ID of the item changes without the visible contents changing (i.e. the same items are removed from the database and then added again, changing their ID in the process). In this case, my current areContentsTheSame implementation returns true. But when areContentsTheSame returns true, the RecyclerView views are not re-bound (onBindViewHolder is not called for the items for which areContentsTheSame returns true), so the views still hold a reference to the old object which contains the old ID thus causing an error when trying to do something with that item (such as the user tapping on it to display it).

I tried making areContentsTheSame return false when the ID changes, forcing the views to be re-bound even though no visible change has taken place, but this causes the "item changed" animation to show even though the item has not visibly changed. What I want is a way to either force the views to be re-bound without areContentsTheSame returning false or a way for areContentsTheSame to return true and trigger the re-binding without the animation being shown.

Could you show us how you use the DiffUtil inside of your Adapter? — w9jds, May 22 '18 at 19:39
@w9jds Nothing fancy, I just use `private class MyAdapter extends ListAdapter` then in the constructor I call the superclass constructor passing an anonymous subclass of `DiffUtil.ItemCallback` with `areItemsTheSame` simply returning `newItem.guid.equals(oldItem.guid)` and `areContentsTheSame` returning `newItem.equals(oldItem)`. `MyDatabaseItem.equals` compares the relevant fields. — micheal65536, May 22 '18 at 19:45

score 15 · Accepted Answer · edited Jun 20 '20 at 09:12

The best way to achieve what you want is to override the getChangePayload() method in your DiffUtil.Callback implementation.

When areItemsTheSame(int, int) returns true for two items and areContentsTheSame(int, int) returns false for them, DiffUtil calls this method to get a payload about the change.

For example, if you are using DiffUtil with RecyclerView, you can return the particular field that changed in the item and your ItemAnimator can use that information to run the correct animation.

Default implementation returns null.

So, make sure that your areContentsTheSame() method returns false when the database id changes, and then implement getChangePayload() to check for this case and return a non-null value. Maybe something like this:

@Override
public Object getChangePayload(int oldItemPosition, int newItemPosition) {
    if (onlyDatabaseIdChanged(oldItemPosition, newItemPosition)) {
        return Boolean.FALSE;
    } else {
        return null;
    }
}

It doesn't matter what object you return from this method, as long as it's not null in the case where you don't want to see the default change animation be played.

For more details on why this works, see the answer to this question: https://stackoverflow.com/a/47355363/8298909

Thanks for this. I figured that the answer would have something to do with `getChangePayload` but I wasn't too sure what the return value meant and how I could apply it to my situation. — micheal65536, May 22 '18 at 20:32

score 0 · Answer 2 · edited Sep 30 '20 at 10:40

most of the answer figure out some basic example of DiffUtil where the code look like

class MyDiffUtil(  private val oldList: List<Item>, private val newList: 
    List<Item> ) : DiffUtil.Callback() {

    override fun getOldListSize(): Int = oldList.size

    override fun getNewListSize(): Int = newList.size

    override fun areItemsTheSame(oldItemPosition: Int, newItemPosition: Int): Boolean {
        return oldList[oldItemPosition].id == newList[newItemPosition].id
    }

    override fun areContentsTheSame(oldItemPosition: Int, newItemPosition: Int): Boolean {
        return oldList[oldItemPosition].id == newList[newItemPosition].id
    }

}

from the above code will work fine when a new item added or removed an item with the list id. but when id will remain same but change some other content then adapter not update because this peace of code

    return oldList[oldItemPosition].id == newList[newItemPosition].id

here we checked id not other item so you just remove id and write this new code

class MyDiffUtil(  private val oldList: List<Item>, private val newList: 
    List<Item> ) : DiffUtil.Callback() {

    override fun getOldListSize(): Int = oldList.size

    override fun getNewListSize(): Int = newList.size

    override fun areItemsTheSame(oldItemPosition: Int, newItemPosition: Int): Boolean {
        return oldList[oldItemPosition] == newList[newItemPosition]
    }

    override fun areContentsTheSame(oldItemPosition: Int, newItemPosition: Int): Boolean {
        return oldList[oldItemPosition] == newList[newItemPosition]
    }
}

Android DiffUtil.ItemCallback areContentsTheSame with different IDs

2 Answers2

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