Moshi Json Adapter for Koltinx Immutable List/Collection

DelegatingAdapter.kt

package com.example

import com.squareup.moshi.JsonAdapter
import com.squareup.moshi.JsonAdapter.Factory
import com.squareup.moshi.JsonReader
import com.squareup.moshi.JsonWriter
import com.squareup.moshi.Types
import com.squareup.moshi.rawType
import kotlinx.collections.immutable.ImmutableList

/**
 * A wrapper class that delegates adaption of type [T] (like [ImmutableList]) into [Delegate] (like [List])
 *
 * inspired from https://github.com/square/moshi/issues/449
 * source: https://gist.github.com/beigirad/05802ca153dfa178686063d004a70e0c
 */
class DelegatingAdapter<T, Delegate> private constructor(
    private val delegateAdapter: JsonAdapter<Delegate>,
    private val fromDelegate: (Delegate?) -> T?,
    private val toDelegate: (T?) -> Delegate?,
) : JsonAdapter<T>() {
    override fun fromJson(reader: JsonReader): T? {
        val delegate: Delegate? = delegateAdapter.fromJson(reader)
        return fromDelegate.invoke(delegate)
    }

    override fun toJson(writer: JsonWriter, value: T?) {
        delegateAdapter.toJson(writer, toDelegate.invoke(value))
    }

    companion object {
        /**
         * Creates a factory for [T] using the already-existing factory for [Delegate],
         * and converting between [T] and [Delegate] using the `fromDelegate` and
         * `toDelegate` arguments.
         *
         * todo: this factory is currently only supports parametrized types
         */
        fun <T, Delegate> factory(
            delegatingType: Class<T>,
            delegateType: Class<Delegate>,
            fromDelegate: (Delegate?) -> T?,
            toDelegate: (T?) -> Delegate?,
        ): Factory = Factory { incomingType, annotations, moshi ->
            if (annotations.isNotEmpty()) return@Factory null
            if (incomingType.rawType != delegatingType) return@Factory null

            val delegateRawType = delegateType.rawType
            val elementType = Types.collectionElementType(incomingType, delegateRawType)
            val genericDelegateType = Types.newParameterizedType(delegateRawType, elementType)
            val delegateAdapter = moshi.adapter<Delegate>(genericDelegateType)
            DelegatingAdapter(delegateAdapter, fromDelegate, toDelegate)
        }
    }
}

Setup.kt

Moshi.Builder()
    .add(DelegatingAdapter.factory(ImmutableList::class.java, List::class.java, { it?.toImmutableList() }, { it }))
    .add(DelegatingAdapter.factory(ImmutableSet::class.java, Set::class.java, { it?.toImmutableSet() }, { it }))
    .add(DelegatingAdapter.factory(ImmutableMap::class.java, Map::class.java, { it?.toImmutableMap() }, { it }))
    .build()