An experimental implementation of the flatMapLatest operator for StateFlow

FlatMappedStateFlow.kt

/*
 * Copyright (c) 2025 Volodymyr Galandzij
 * 
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 * 
 * http://www.apache.org/licenses/LICENSE-2.0
 * 
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

fun <T, R> StateFlow<T>.flatMapLatest(@BuilderInference transform: (value: T) -> StateFlow<R>): StateFlow<R> =
    FlatMappedStateFlow(this, transform)

class FlatMappedStateFlow<T, R>(
    private val upstream: StateFlow<T>,
    private val mapper: (T) -> StateFlow<R>
) : StateFlow<R> {

    override val replayCache: List<R>
        get() = listOf(value)

    override val value: R
        get() = mapper(upstream.value).value

    override suspend fun collect(collector: FlowCollector<R>): Nothing {
        var previous: Any? = Unset
        upstream
            .map { mapper(it) }
            .distinctUntilChanged()
            .collectLatest { latestFlow ->
                latestFlow.collect { value ->
                    if (value != previous) {
                        previous = value
                        collector.emit(value)
                    }
                }
            }
        error("unreachable code")
    }

    private object Unset
}

Inspired by the MappedStateFlow implementation:
Kotlin/kotlinx.coroutines#2514 (comment)