RecyclerView Selection (Long Key)

June 18, 2020
Make Adapter Items Selectable

Dependencies

dependencies {
    implementation 'androidx.recyclerview:recyclerview-selection:1.0.0'
    // implementation 'androidx.recyclerview:recyclerview-selection:1.1.0-rc01'
}

Fragment / Activity

We are using the item position (Long) as Id Key, where everything is selectable.

class TestSelectionFragment : Fragment() {
    private lateinit var adapter: LocalAdapter

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

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

        initList()
    }

    private fun initList() {
        val layoutManager = GridLayoutManager(context, 3)
        list.layoutManager = layoutManager

        if (!::adapter.isInitialized) {
            adapter = LocalAdapter()
            adapter.setHasStableIds(true)
            list.adapter = adapter

            val tracker = SelectionTracker.Builder<Long>(
                "number-selection",
                list,
                StableIdKeyProvider(list),
                LocalItemDetailsLookup(list),
                StorageStrategy.createLongStorage()
            ).withSelectionPredicate(
                SelectionPredicates.createSelectAnything()
            ).build()
            adapter.tracker = tracker

            tracker.addObserver(object : SelectionTracker.SelectionObserver<Long>() {
                override fun onSelectionChanged() {
                    super.onSelectionChanged()
                    Timber.d("selection=${tracker.selection}")
                }
            })
        }

        val items = mutableListOf<ViewItem>()
        for (number in 1..10) {
            items.add(ViewItem.NumberItem(number.toString()))
        }
        adapter.submitList(items)
    }

    private class LocalItemDetailsLookup(private val recyclerView: RecyclerView) :
        ItemDetailsLookup<Long>() {
        override fun getItemDetails(event: MotionEvent): ItemDetails<Long>? {
            val view = recyclerView.findChildViewUnder(event.x, event.y)
            if (view != null) {
                return (recyclerView.getChildViewHolder(view) as LocalAdapter.ViewHolder)
                    .getItemDetails()
            }
            return null
        }
    }

    sealed class ViewItem(open val id: String, val resource: Int) {
        data class NumberItem(override val id: String) : ViewItem(id, R.layout.testselection_item)
    }

    class LocalAdapter() : ListAdapter<ViewItem, LocalAdapter.ViewHolder>(DiffCallback()) {
        lateinit var tracker: SelectionTracker<Long>

        override fun getItemId(position: Int): Long = position.toLong()

        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) {
            Timber.d("position=$position, selected=${tracker.isSelected(position.toLong())}, selection=${tracker.selection}")
            holder.bind(getItem(position), tracker.isSelected(position.toLong()))
        }

        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, isSelected: Boolean) {
                when(item) {
                    is ViewItem.NumberItem -> {
                        numberTextView.text = item.id
                        val res = if (isSelected) R.drawable.ic_baseline_check_circle_24 else R.drawable.ic_baseline_radio_button_unchecked_24
                        selectionImageView.setImageResource(res)
                    }
                }
            }

            fun getItemDetails(): ItemDetailsLookup.ItemDetails<Long> =
                object : ItemDetailsLookup.ItemDetails<Long>() {
                    override fun getPosition(): Int = adapterPosition
                    override fun getSelectionKey(): Long? = itemId
                }
        }

    }
}

Layout

R.layout.testselection

<?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="com.luasoftware.garden.view.TestSelectionFragment">

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

</FrameLayout>

R.layout.testselection_item

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    xmlns:tools="http://schemas.android.com/tools"
    tools:layout_height="500dp">

    <TextView
        android:id="@+id/numberTextView"
        android:layout_width="0dp"
        android:layout_height="0dp"
        android:gravity="center"
        app:layout_constraintDimensionRatio="H,1:1"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        tools:text="Test">

    </TextView>


    <ImageView
        android:id="@+id/selectionImageView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_margin="8dp"
        android:src="@drawable/ic_baseline_radio_button_unchecked_24"
        android:tint="@android:color/black"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintEnd_toEndOf="parent" />
</androidx.constraintlayout.widget.ConstraintLayout>

Note

  • Long press an item to trigger item selection, where the second item can be selected/deselected via click.
  • When no items is selected (after deselect), a long press is required again to trigger item selection
  • SelectionTracker will trigger Adapter.onBindViewHolder for UI update, and you can listen to selection event via SelectionTracker.addObserver.
  • The example above using item postion (Long) as Id Key
  • If you accidentally build SelectionTracker and assigned to Adapter.tracker more than once, there will be strange UI bug where Adapter.onBindViewHolder is not triggered accurately.
  • Refer to RecyclerView Selection With String Key and SelectionPredicate (allow certain items to be selected)

References:

This work is licensed under a
Creative Commons Attribution-NonCommercial 4.0 International License.