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.

Answer 1

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)
    }

Answer 2

Kotlin with Data Classes

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

Answer 3

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.