Observable list

ObservableList.kt

private open class MutableListWrapper<T>(private val source: MutableList<T>) : AbstractMutableList<T>() {
    override val size get() = source.size

    override fun add(index: Int, element: T) = source.add(index, element)

    override fun get(index: Int) = source[index]

    override fun removeAt(index: Int) = source.removeAt(index)

    override fun set(index: Int, element: T) = source.set(index, element)

    override fun toString() = source.toString()
}

fun <T> MutableList<T>.onAdd(onAdded: (element: T) -> Unit): MutableList<T> =
    onAdd { _, element -> onAdded(element) }

fun <T> MutableList<T>.onSet(onSet: (previous: T, new: T) -> Unit): MutableList<T> =
    onSet { _, old, new -> onSet(old, new) }

fun <T> MutableList<T>.onRemove(onRemoved: (element: T) -> Unit): MutableList<T> =
    onRemove { _, element -> onRemoved(element) }

fun <T> MutableList<T>.onAdd(onAdded: (index: Int, element: T) -> Unit): MutableList<T> =
    object : MutableListWrapper<T>(this) {
        override fun add(index: Int, element: T) {
            super.add(index, element)
            onAdded(index, element)
        }
    }

fun <T> MutableList<T>.onSet(onSet: (index: Int, previous: T, new: T) -> Unit): MutableList<T> =
    object : MutableListWrapper<T>(this) {
        override fun set(index: Int, element: T) =
            super.set(index, element)
                .also { onSet(index, it, element) }
    }

fun <T> MutableList<T>.onRemove(onRemoved: (index: Int, element: T) -> Unit): MutableList<T> =
    object : MutableListWrapper<T>(this) {
        override fun removeAt(index: Int) =
            super.removeAt(index)
                .also { onRemoved(index, it) }
    }

Usage.kt

val list = mutableListOf<String>()
    .onAdd { index, element -> println("added [$element] into $index") }
    .onAdd { element -> println("added [$element]") }
    .onSet { index, previuos, new -> println("replace previuos [$previuos] with new [$new] at $index") }
    .onSet { previuos, new -> println("replace previuos [$previuos] with new [$new]") }
    .onRemove { index, element -> println("remove [$element] from $index") }
    .onRemove { element -> println("remove [$element]") }

fun main() = with(list) {
    add("Hello")
    // added [Hello] into 0
    // added [Hello]
    add("world")
    // added [world] into 1
    // added [world]
    set(1, "everybody")
    // replace old [world] with new [everybody] at 1
    // replace old [world] with new [everybody]
    addAll(listOf("who", "is", "there"))
    // added [who] into 2
    // added [who]
    // added [is] into 3
    // added [is]
    // added [there] into 4
    // added [there]
    removeAt(0)
    // remove [Hello] from 0
    // remove [Hello]
    println(this)
    // [everybody, who, is, there]
}