Custom implementation of an Android spinner (combo box) with material theme and data binding support by extending AutocompleteTextView

MaterialSpinner.kt

/**
 * Custom implementation of an Android spinner (combo box) with material theme by using AutocompleteTextView.
 * Recommended to use together with [MaterialSpinnerAdapter].
 *
 * Supports listening to and updating the selected position e. g. from a LiveData object bound via
 * the `selectedPosition` two-way binding adapter. The empty text value can be bound via 'emptyText'
 * attr (custom styleable must be declared in attrs.xml).
 *
 * If [MaterialSpinnerAdapter] is used, its [MaterialSpinnerAdapter.selectedItemPosition] is
 * automatically updated when selection changes.
 *
 * Created by Milos Cernilovsky on 12/9/20.
 * @see MaterialSpinnerAdapter
 */
class MaterialSpinner : AppCompatAutoCompleteTextView {
    /**
     * These listeners will receive updates when the selected position changes. Can be used e. g.
     * by adapters to update the selected item's UI.
     */
    private val selectedPositionListeners = mutableSetOf<(Int) -> Unit>()
    private var _selectedPosition: Int = 0
        set(value) {
            field = value
            notifyListenersAboutSelectedPosition()
        }

    private fun notifyListenersAboutSelectedPosition() {
        selectedPositionListeners.forEach {
            it.invoke(_selectedPosition)
        }
    }

    /**
     * Currently selected position in the spinner
     */
    var selectedPosition: Int
        get() {
            return _selectedPosition
        }
        set(value) {
            _selectedPosition = value
            setCurrentPositionItemText()
        }

    /**
     * Empty text will be shown if there are no items in the adapter or if the [selectedPosition]
     * does not exist in the adapter.
     */
    var emptyText: CharSequence? = null
        set(value) {
            field = value
            setCurrentPositionItemText()
        }
    private var wrappedItemClickListener: AdapterView.OnItemClickListener? = null

    init {
        super.setOnItemClickListener { parent, view, position, id ->
            _selectedPosition = position
            wrappedItemClickListener?.onItemClick(parent, view, position, id)
        }
        inputType = InputType.TYPE_NULL
    }

    constructor(context: Context) : this(context, null)
    constructor(context: Context, attrs: AttributeSet?) : this(context, attrs, R.attr.autoCompleteTextViewStyle)
    constructor(context: Context, attrs: AttributeSet?, defStyleAttr: Int) : super(context, attrs, defStyleAttr) {
        val a = context.obtainStyledAttributes(attrs, R.styleable.MaterialSpinner, defStyleAttr, 0)
        emptyText = a.getText(R.styleable.MaterialSpinner_emptyText)
        a.recycle()
    }

    override fun setOnItemClickListener(l: AdapterView.OnItemClickListener?) {
        wrappedItemClickListener = l
    }

    override fun <T> setAdapter(adapter: T) where T : ListAdapter?, T : Filterable? {
        super.setAdapter(adapter)
        setCurrentPositionItemText()
        if (adapter is MaterialSpinnerAdapter<*>) {
            addSelectedPositionListener(adapter::selectedItemPosition::set)
        }
    }

    fun addSelectedPositionListener(listener: (Int) -> Unit) {
        selectedPositionListeners.add(listener)
        listener.invoke(_selectedPosition)
    }

    fun removeSelectedPositionListener(listener: (Int) -> Unit) {
        selectedPositionListeners.remove(listener)
    }

    private fun setCurrentPositionItemText() {
        adapter?.let { adapter ->
            setText(if (adapter.count > selectedPosition) {
                adapter.getItem(selectedPosition)?.toString() ?: emptyText
            } else {
                emptyText
            }, false)
        }
    }
}