Kotlin Flow - custom result retry

RetryRequest.MD

Kotlin Flow - custom result retry

Maybe I will write article about this

API:

internal fun <T, R> Flow<T>.retryMap(
    map: suspend (T) -> R,
    predicate: suspend FlowCollector<R>.(value: T, result: R, attempt: Int) -> Boolean
): Flow<R> {
    var retryCount = 1

    suspend fun FlowCollector<R>.collector(
        value: T,
        result: R
    ) {
        if (predicate(value, result, retryCount)) {
            retryCount++
            collector(value, map(value))
        } else {
            emit(result)
        }
    }

    return flow {
        collect { request ->
            collector(request, map(request))
        }
    }
}

internal fun <T, R> Flow<T>.retryMapFlow(
    map: Flow<T>.() -> Flow<R>,
    predicate: suspend FlowCollector<R>.(value: T, result: R, attempt: Int) -> Boolean
): Flow<R> {
    return retryMap(
        map = { request -> flowOf(request).map().first() },
        predicate = predicate
    )
}

Usage:

data class Request(val id: String)

sealed class Result<out T> {
    data class Success<T>(val data: T) : Result<T>()
    data class Error(val cause: Throwable) : Result<Nothing>()
    data class Loading(val retryCount: Int = 0) : Result<Nothing>()
}

suspend fun getResult(request : Request): Result<Unit> {
    delay(500)
    return Result.Error(Exception("Error while getting result for [${request.id}]"))
}

fun main() = runBlocking {
    flowOf(Request("id1"))
        .retryRequest(
            getResult = ::getResult,
            predicate = { _, result, attempt ->
                val shouldRetry = result is Result.Error && count < 3
                if (shouldRetry) {
                    delay(1000)
                    emit(Result.Loading(retryCount = attempt))
                }
                shouldRetry
            }
        )
        .stateIn(this, started = SharingStarted.Eagerly, initialValue = Result.Loading())
        .collect { println(it) }
}

Prints:

Loading(retryCount=0)
Loading(retryCount=1)
Loading(retryCount=2)
Loading(retryCount=3)
Error(cause=java.lang.Exception: Error while getting result for [id1])