ListAdapter with DiffUtil.ItemCallback always considers objects the same

Question

I`m trying to use ListAdapter with Room and LifeData. But i faced strange behavior of DiffUtil.ItemCallback - objects are always the same in areContentsTheSame() method. No problem with adding and removing object, but problem with changing the content.

Item class:

@Entity(tableName = "item")
data class Item(var num: Int) {

    @PrimaryKey(autoGenerate = true)
    var key: Int = 0

}

Adapter class

class LifeAdapter : ListAdapter<Item, LifeAdapter.ViewHolder>(DiffCallback()) {

    private class DiffCallback : DiffUtil.ItemCallback<Item>() {
        override fun areItemsTheSame(oldItem: Item, newItem: Item) = oldItem.key == newItem.key
        override fun areContentsTheSame(oldItem: Item, newItem: Item) = oldItem.num == newItem.num
    }

    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): ViewHolder {
        val view = LayoutInflater.from(parent.context).inflate(R.layout.item_item, parent, false)
        return ViewHolder(view)
    }

    override fun onBindViewHolder(holder: ViewHolder, pos: Int) {
        val position = holder.layoutPosition
        holder.bind(getItem(position))
    }

    class ViewHolder(view: View) : RecyclerView.ViewHolder(view) {
        fun bind(item: Item) {
            itemView.findViewById<TextView>(R.id.txt_num).text = item.num.toString()
            itemView.findViewById<TextView>(R.id.txt_key).text = item.key.toString()
        }
    }

}

Activity class:

class MainActivity : AppCompatActivity() {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        val dao = getDao(this)
        val data = dao.getAllItems()

        val adapter = LifeAdapter()
        rv.layoutManager = LinearLayoutManager(this)
        rv.adapter = adapter

        val nameObserver = Observer<List<Item>> { adapter.submitList(it) }

        data.observe(this, nameObserver)

        btn_add.setOnClickListener {
            val item = Item(Random.nextInt(0, 1000))
            runAsync { dao.insertItem(item) }
        }

        btn_change.setOnClickListener { v ->
            data.value.let {
                if (it!!.isNotEmpty()) {
                    it[0].num = 111
                    runAsync { dao.updateItem(it[0]) }
                }
            }
        }

        btn_delete.setOnClickListener { v ->
            data.value.let {
                if (it!!.isNotEmpty()) {
                    runAsync { dao.deleteItem(it[0]) }
                }
            }
        }

    }
}

Full project - https://yadi.sk/d/7tpzDhUA-udoIQ

Video - https://youtu.be/PZYeAfGzXBg

The problem is in LifeAdapter.DiffCallback class in method areContentsTheSame(). If item content (num) changing, in this method both newItem and oldItem are the same and equals to new item:

That meant the method areContentsTheSame() always return true. I checked equality by link (newItem === oldItem) and it always false as it should be. I can`t understand what is wrong. newItem and oldItem must be different when new List added via adapter.submitList() method.

Yvan Lourenço · Accepted Answer · 2020-03-18 10:07:20Z

14

Since LiveData returns the same List you have to create a new one.

Here is a shorter answer to the original answer by using toList().

recycler.observe(this, Observer{
    adapter.submitList(it.toList()) 
})

If you rather use a kotlin extension you can do something like this:

fun <T> MutableLiveData<List<T>>.add(item: T) {
    val updatedItems = this.value?.toMutableList()
    updatedItems?.add(item)
    this.value = updatedItems
}

That way you don't have to add the toList() and just use the extension.

edited Mar 18, 2020 at 10:07

answered May 17, 2019 at 12:39

Yvan Lourenço

1901 silver badge9 bronze badges

Sign up to request clarification or add additional context in comments.

5 Comments

leoneboaventura Over a year ago

I'm having this same issue as described on the question here. I tried your suggestion and it didn't work forme. Looks like the list being passed to submitList() is never the same, so using your suggestion didn't change that.

Yvan Lourenço Over a year ago

The list is always the same, the content isn't so that's why you add the .toList().

leoneboaventura Over a year ago

Actually it turned out I was updating the LiveData object manually, and then afterwards submitList was being called. As I updated the LiveData object, the UI got updated first. So a few secs later when submitList was called, the lists were already the same.

Saiprasad Prabhu Over a year ago

@leoneboaventura How did you fix it ?

Cosmin Vacaru Over a year ago

@leoneboaventura i am also facing the same problem, can you please share your solution?

Zakhar Rodionov · Accepted Answer · 2020-07-10 13:51:27Z

7

Kotlin with Data Classes

adapter.submutList(list.map { it.copy() })

answered Jul 10, 2020 at 13:51

Zakhar Rodionov

1,50317 silver badges22 bronze badges

4 Comments

Felix PK Over a year ago

Where is that list comming from? it has an unresolved reference to copy()

Zakhar Rodionov Over a year ago

@FelixPK It should be a list of data class. Data Classes has a copy function that creates the same object, but the reference in its memory is different.

msevgi Over a year ago

Thanks. i have spent a lot of time to solve, why does not update ui problem.

Daniel Wilson Over a year ago

I really don't understand wtf is going on in my code here but this has fixed it :D

illuzor · Accepted Answer · 2018-11-08 15:04:05Z

3

LiveData returns the same instances in the List.

Solution I found - create new List with copy of Items:

    val nameObserver = Observer<List<Item>> {
        val newList = mutableListOf<Item>()
        it.forEach { item -> newList.add(item.copy()) }
        adapter.submitList(newList)
    }

answered Nov 8, 2018 at 15:04

illuzor

7009 silver badges23 bronze badges

Comments

Erselan Khan · Accepted Answer · 2021-05-08 02:54:27Z

0

Replace this code:

btn_change.setOnClickListener { v ->
            data.value.let {
                if (it!!.isNotEmpty()) {
                    it[0].num = 111
                    runAsync { dao.updateItem(it[0]) }
                }
            }
        }

with this:

btn_change.setOnClickListener { v ->
            data.value.let {
                if (it!!.isNotEmpty()) {
                    runAsync { dao.updateItem(it.set(0, Item(111))) }
                }
            }
        }

answered May 8, 2021 at 2:54

Erselan Khan

8457 silver badges13 bronze badges

Collectives™ on Stack Overflow

ListAdapter with DiffUtil.ItemCallback always considers objects the same

4 Answers 4

5 Comments

4 Comments

Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

4 Answers 4

5 Comments

4 Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related