Android RecylerView ListAdapter (Kotlin)

December 22, 2019

Why use ListAdapter

It only update items which has changed (with animation shown).

NOTE: Beware of some ListAdapter Caveats.

Fragment

class AlbumListFragment : Fragment() {
    private val viewModel by viewModels<AlbumListViewModel>()

    private lateinit var adapter: LocalAdapter

    override fun onCreateView(
        inflater: LayoutInflater, container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View? {
        return inflater.inflate(R.layout.albumlist, container, false)
    }

    override fun onActivityCreated(savedInstanceState: Bundle?) {
        super.onActivityCreated(savedInstanceState)

        initList()
    }

    private fun initList() {
        adapter = LocalAdapter(viewModel)
        list.adapter = adapter

        val items = mutableListOf<ViewItem>()
        // TODO: load items
        adapter.submitList(items)
    }

    sealed class ViewItem(open val id: String, val resource: Int) {
        data class AlbumItem(override val id: String, val title: String, val childCount: Int) : ViewItem(id, R.layout.albumlist_item)
    }

    class LocalAdapter(val viewModel: AlbumListViewModel) : ListAdapter<ViewItem, LocalAdapter.ViewHolder>(DiffCallback()) {
        override fun getItemViewType(position: Int): Int {
            return getItem(position).resource
        }

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

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

        private class DiffCallback: DiffUtil.ItemCallback<ViewItem>() {
            override fun areItemsTheSame(oldItem: ViewItem, newItem: ViewItem): Boolean {
                if (oldItem.resource != newItem.resource) return false
                // check if id is the same
                return oldItem.id == newItem.id
            }

            @SuppressLint("DiffUtilEquals")
            override fun areContentsTheSame(oldItem: ViewItem, newItem: ViewItem): Boolean {
                // check if content is the same
                // equals using data class
                return oldItem == newItem
            }
        }

        inner class ViewHolder(override val containerView: View) : RecyclerView.ViewHolder(containerView),
            LayoutContainer {

            // why update ui here? easier to access view without need to holder.titleTextView
            fun bind(item: ViewItem) {
                when(item) {
                    is ViewItem.AlbumItem -> {
                        titleTextView.text = item.title
                        detailTextView.text = "${item.childCount} items"
                    }
                }
            }
        }

    }
}

NOTE: Refer Handle RecyclerView Click Event With LiveData.

NOTE: Refer Kotlin Android Extentions ViewBinding for RecyclerView ViewHolder (LayoutContainer).

R.layout.albumlist

<?xml version="1.0" encoding="utf-8"?>
<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    tools:context=".view.AlbumListFragment">

    <androidx.recyclerview.widget.RecyclerView
        android:id="@+id/list"
        app:layoutManager="androidx.recyclerview.widget.LinearLayoutManager"
        android:layout_width="match_parent"
        android:layout_height="match_parent" />

</FrameLayout>

R.layout.albumlist_item

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:fresco="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:orientation="vertical"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:layout_margin="@dimen/margin16"
    >

    <TextView
        android:id="@+id/titleTextView"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginTop="@dimen/margin8"
        android:layout_marginBottom="@dimen/margin8"
        tools:text="Title" />

    <TextView
        android:id="@+id/detailTextView"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:textAppearance="@style/TextAppearance.AppCompat.Small"
        tools:text="10 items" />
</LinearLayout>
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