Android DiffUtil in item-specific situations

Question

I usually use diffutil with Recyclerview when needed. Right now I have situation where the items I get from backend look like this:

data class CarouselItem(var url: String, var pictureUrl: String, var visible: String)

All 3 fields can be the same for 2 items. I am thinking what is the best way to create diffUtil here. IMHO I should make some wrapper that will add a field by which I can differentiate 2 items (for example ID field). Also in this case in my diffUtil, override function areContentsTheSame would have to compare all 3 fields of item like this:

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

I think this takes processor power almost the same as when I don't use DiffUtil since it compares the whole item to the whole item (no benefit of comparing only 1 field).

Is this the right way to do it and is it overkill in this case? I would like to know what are my best option as an android developer when I have situations like this (items like this) in the future? (I like to follow clean code concepts and do things the best way if possible)

I wonder why you have the same picture id multiple times in the carousel? — Eugen Martynov, Jan 05 '22 at 15:08
I don't understand the question, sorry. Is it about performance? — Eugen Martynov, Jan 05 '22 at 15:09
@EugenMartynov pictureId is the url of the picture. The name is strange but the backend side put it that way. I changed it to pictureUrl to remove confusion in the question. Question is is this the best way to do it as I said (with wrapper) and is it beneficial to do it at all in situations like this, since we compare whole items — Kratos, Jan 05 '22 at 15:14
Unless you have thousands of items in your List at once, you are overconcerned with the performance of your equality checks. Comparing three Strings is trivial. Your `areContentsTheSame` function is overly complicated though. Since you're using a data class, you can directly compare the two objects instead of comparing their individual properties. Also, it is simpler to use `ListAdapter` with `DiffUtil.ItemCallback` than to work with `DiffUtil.Callback`. — Tenfour04, Jan 05 '22 at 15:14
@Tenfour04 thanks for your comments. Yes, I agree compare is better way (I bet my money I would see it before committing and pushing the code :P). Sorry, adapter is created before by other devs so I should not change it to ListAdapter. Yes, they can change, smart guess! Ok will change all var to val if you say so. But would like to say that one of our main adapters has diffutil list - mutable properties combination and never faced a problem in production (huge user numbers) regarding this. — Kratos, Jan 05 '22 at 15:39
@Tenfour04 In the end, I would like to say that I'm concerned with the performance of the equality check only because it looked to me that I lose benefits of DiffUtil if I must compare the whole item with the item. Doesn't it then (by performance) work almost the same as notifyDataSetChanged() function (since I use whole items all the time - even in getChangePayload function)? — Kratos, Jan 05 '22 at 15:39

score 2 · Accepted Answer · answered Jan 05 '22 at 16:18

Since you're using a data class, areContentsTheSame() can be simplified to simply do an equality check:

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

This kind of comparison is typically trivial. The point of checking all the items against each other is to avoid the nuclear option of notifyDataSetChanged(), which causes a complete do-over of layout of all the views in every row. View layout refreshes are very, very expensive in comparison to property comparisons. (Think about how it's having to compute the size in pixels of every character of text it's going to display in two dimensions and add it all up, etc. whereas comparing two Strings is a very fast comparison of bytes.)

Even if the equality checks were not trivial in comparison to view layout changes, remember that DiffUtil does the equality checks on a background thread, but view layout refreshes must be done on the main thread, so using DiffUtil prevents the UI from feeling janky.

Now if your two lists have nothing in common, DiffUtil will report that and all the views will have to be laid out anyway just like with notifyDataSetChanged(). DiffUtil might not be a good choice if the entire contents of the list are completely replaced with each change. This might be the case if you're doing some sort of paging with your RecyclerView.

The reason you should use val instead of var for items used for DiffUtil is to prevent misuse. var would work fine as long as you don't use it for items in a list and then expect to be able to compare old and new lists with DiffUtil. areContentsTheSame() is pointless if you've mutated items in the old list. It might result in the views for a row not getting updated when they should.

As for your question about unique IDs, you can generate them if you need to, but the best way to do that is situation dependent. (How your data is stored, how and when it changes, what is expected to remain constant, etc.) You should have at least one property in your item class that can be used for areItemsTheSame()'s comparison. You can omit that and just compare the two items the same way as you would with areContentsTheSame(), but you lose the benefits of appropriate animations of items that have had their contents changed while still representing the same thing.

Android DiffUtil in item-specific situations

1 Answers1