Make an adapter easily than SimpleExpandableListAdapter

BasicDataExpandableListAdapter.kt

/**
 * SimpleExpandableListAdapterのコンストラクタが受け取る引数が
 * 何を言っているのか全然わからないので、
 * 自然な形のやつ。
 *
 * @param Child CShowによって表示可能な子
 * @param clever リストアップされる親と子の木
 * @param CShow 子を表示する方法
 */
class BasicDataExpandableListAdapter<Child>(
    CShow: Show<Child>,
    context: Context,
    clever: Map<String, List<Child>>,
    groupLayout: Int,
    groupTo: List<Int>,
    childLayout: Int,
    childTo: List<Int>
) : SimpleExpandableListAdapter(
    context,
    takeOnlyGroupName(clever),
    groupLayout,
    arrayOf(uniqueGroupKey),
    groupTo.toIntArray(),
    jsonize(CShow, clever),
    childLayout,
    arrayOf(uniqueChildKey, uniqueGroupKey),
    childTo.toIntArray()
)

private const val uniqueGroupKey = "very-very-uniqueness-group-key"
private const val uniqueChildKey = "very-very-uniqueness-child-key"
const val uniqueInvisibleKey = "very-very-uniqueness-invisible-data-key"

private fun <Child> takeOnlyGroupName(clever: Map<String, List<Child>>): List<Map<String, String>> =
    clever.toList().map { (parent, _) ->
        mapOf(uniqueGroupKey to parent)
    }

/**
 * ```
 * >>> val clever = mapOf(
 *          "x" to listOf("x-1", "x-2"),
 *          "y" to listOf("y-1")
 *     )
 * listOf(
 *      listOf(
 *          mapOf(uniqueGroupKey to "x", uniqueChildKey to "x-1"),
 *          mapOf(uniqueGroupKey to "x", uniqueChildKey to "x-2")
 *      ),
 *      listOf(
 *          mapOf(uniqueGroupKey to "y", uniqueChildKey to "y-1")
 *      )
 * )
 * ```
 */
private fun <Child> jsonize(
    CShow: Show<Child>,
    clever: Map<String, List<Child>>
): List<List<Map<String, Any?>>> {
    val jsonlike: List<List<Pair<String, Child>>> =
        clever.map { (parent, children) ->
            children.map { child ->
                parent to child
            }
        }
    return jsonlike.map { pairs ->
        pairs.map { (parent, child) ->
            mapOf(
                uniqueGroupKey to parent,
                uniqueChildKey to CShow.show(child),
                uniqueInvisibleKey to child
            )
        }
    }
}

BasicExpandableListAdapter.kt

import android.content.Context
import android.widget.SimpleExpandableListAdapter

/**
 * A basic adapter by a basic structure `Map<String, List<String>>`
 */
class BasicExpandableListAdapter(
    context: Context,
    clever: Map<String, List<String>>,
    groupLayout: Int,
    groupTo: List<Int>,
    childLayout: Int,
    childTo: List<Int>
) : SimpleExpandableListAdapter(
    context,
    takeOnlyGroupName(clever),
    groupLayout,
    arrayOf(uniqueGroupKey),
    groupTo.toIntArray(),
    jsonize(clever),
    childLayout,
    arrayOf(uniqueChildKey, uniqueGroupKey),
    childTo.toIntArray()
)

private const val uniqueGroupKey = "very-very-uniqueness-group-key"
private const val uniqueChildKey = "very-very-uniqueness-child-key"

private fun takeOnlyGroupName(clever: Map<String, List<String>>): List<Map<String, String>> =
    clever.toList().map { (parent, _) ->
        mapOf(uniqueGroupKey to parent)
    }

/**
 * ```
 * >>> val clever = mapOf(
 *          "x" to listOf("x-1", "x-2"),
 *          "y" to listOf("y-1")
 *     )
 * listOf(
 *      listOf(
 *          mapOf("very-very-uniqueness-group-key" to "x", "very-very-uniqueness-child-key" to "x-1"),
 *          mapOf("very-very-uniqueness-group-key" to "x", "very-very-uniqueness-child-key" to "x-2")
 *      ),
 *      listOf(
 *          mapOf("very-very-uniqueness-group-key" to "y", "very-very-uniqueness-child-key" to "y-1")
 *      )
 * )
 * ```
 */
private fun jsonize(clever: Map<String, List<String>>): List<List<Map<String, String>>> {
    val jsonlike: List<List<Pair<String, String>>> =
        clever.map { (parent, children) ->
            children.map { child ->
                parent to child
            }
        }
    return jsonlike.map { pairs ->
        pairs.map { (parent, child) ->
            mapOf(uniqueGroupKey to parent, uniqueChildKey to child)
        }
    }
}

Show.kt

/**
 * 表示可能なデータ。
 * toStringを使いたくないときに使う。
 */
interface Show<T> {
    fun show(self: T): String
}

In my opinion, SimpleExpandableListView is a little crazy for a basic usage.

This allows to

val this: Context = aContext
val outline =
    mapOf(
        "x" to listOf("x-1", "x-2"),
        "y" to listOf("y-1")
    )

BasicExpandableListAdapter(
    this,
    outline,
    android.R.layout.simple_expandable_list_item_1,
    listOf(android.R.id.text1),
    android.R.layout.simple_expandable_list_item_2,
    listOf(android.R.id.text1, android.R.id.text2)
)

BasicDataExpandableListAdapter added.
Here is the example 👇

val context: Context = aContext
val view: ExpandableListView = anExpandableListView

data class Foo(val label: String, val value: Int) {
    companion object {
        val show = object : Show<Foo> {
            override fun show(self: Foo) = self.label
        }
    }
}

val outline =
    mapOf(
        "x" to listOf(
            Foo("x-1", 10),
            Foo("x-2", 20),
        ),
        "y" to listOf(
            Foo("y-1", 100)
        )
    )

BasicExpandableListAdapter(
    Foo.show,
    context,
    outline,
    android.R.layout.simple_expandable_list_item_1,
    listOf(android.R.id.text1),
    android.R.layout.simple_expandable_list_item_2,
    listOf(android.R.id.text1, android.R.id.text2)
)

view.setOnChildClickListener { parent, _, groupPos, childPos, _ ->
    println(
        parent.expandableListAdapter.getChild(groupPos, childPos)
    )
    true
}
/* on "x-1" clicked

I/System.out:
 {
    very-very-uniqueness-group-key=x
    very-very-uniqueness-child-key="x-1"
    very-very-uniqueness-invisible-data-key=Foo(label=x-1, value=10
}

*/

or simply, you can use toString instead of Show<T>.