1

In my RecyclerView, I used ListAdapter combined with DiffUtil.ItemCallback.

class CustomDiffCallback : DiffUtil.ItemCallback<TypeInterface>() {
    override fun areItemsTheSame(p0: TypeInterface, p1: TypeInterface): Boolean {
        return ((p0 is Type1 && p1 is Type1) && p0.id == p1.id)
                || (p0 is Type2 && p1 is Type2)
                || (p0 is Type3 && p1 is Type3)

    }

    override fun areContentsTheSame(p0: TypeInterface, p1: TypeInterface): Boolean {
        return ((p0 is Type1 && p1 is Type1) && p0 == p1)
                || (p0 is Type2 && p1 is Type2)
                || (p0 is Type3 && p1 is Type3)
    }
}

The RecyclerView items have 3 different types all inheriting from same interface.

Strange thing is, when I inserted an item into the second last position of the list, instead of showing the last item, it duplicates the last item twice.

Ex) oldlist = A B C D, newlist = A B C X D

the list appears as A B C D D

I used debugger to investigate into what is going on in the computation process, and found that the computation was performed in the following order:

areContentsTheSame(oldlist[4], newlist[5])
areContentsTheSame(oldlist[0], newlist[0])
areContentsTheSame(oldlist[1], newlist[0])
areContentsTheSame(oldlist[2], newlist[2])
areContentsTheSame(oldlist[3], newlist[3])

Leaving out newlist[4], which is the newly inserted element.

Is this natural blind spot embedded in DiffUtil.ItemCallback or am I using the ItemCallback in a wrong way?

Improve this question
4
  • There's no id or equality comparison between Type2 and Type3 items? What is the meaning of this Commented Sep 16 at 9:50
  • Yes the Type2 and Type3 does nkt have id. They are considered unique on their own. It is legacy code from company. Commented Sep 16 at 14:32
  • Are your IDs completely unique? Because it seems that newlist[0] matches both oldlist[0] and oldlist[1]. Or you're mutating the lists instead of serving new instances (diffutil hates that) Commented Sep 16 at 17:09
  • Type1 id is unique. For lists, I checked with debugger if the two lists are the same. They are not :( they are having different hash values Commented Sep 17 at 13:14
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