NoInitialStateFlow.kt

NoInitialStateFlow.kt

import kotlinx.coroutines.flow.Flow
import kotlinx.coroutines.flow.MutableStateFlow
import kotlinx.coroutines.flow.StateFlow

/**
 * A variant of [StateFlow] that allows an initial state
 */
interface NoInitialStateFlow<T> : Flow<T> {

    /**
     * Throws an exception if called when this [NoInitialStateFlow] has not been initialized with a state yet.
     * @see [StateFlow.value]
     */
    val value: T

    /**
     * Returns the [value], or `null` if this [NoInitialStateFlow] has not been initialized yet
     */
    val valueOrNull: T?

    /**
     * Returns the [value] immediately if this [NoInitialStateFlow] has been initialized, or suspends until
     * it is initialized
     */
    suspend fun value(): T
}

/**
 * A variant of [MutableStateFlow] that allows an initial state to be set after construction
 */
interface MutableNoInitialStateFlow<T> : NoInitialStateFlow<T> {

    /**
     * See [MutableStateFlow.value]
     */
    override var value: T
}

/**
 * Constructor for [MutableNoInitialStateFlow]
 */
@Suppress("FunctionName")
fun <T> MutableNoInitialStateFlow(): MutableNoInitialStateFlow<T> = NoInitialStateFlowInternalImpl()

NoInitialStateFlowImpl.kt

import kotlinx.coroutines.CompletableDeferred
import kotlinx.coroutines.flow.Flow
import kotlinx.coroutines.flow.MutableStateFlow
import kotlinx.coroutines.flow.conflate
import kotlinx.coroutines.flow.emitAll
import kotlinx.coroutines.flow.flow

@Suppress("FunctionName")
internal fun <T> NoInitialStateFlowInternalImpl(): MutableNoInitialStateFlow<T> {
    val stateFlowDeferred = CompletableDeferred<MutableStateFlow<T>>()

    val flow = flow {
        emitAll(
            stateFlowDeferred.await()
        )
    }
        .conflate()

    return object : MutableNoInitialStateFlow<T>, Flow<T> by flow {
        override var value: T
            get() = if (stateFlowDeferred.isCompleted) {
                stateFlowDeferred.getCompleted().value
            } else {
                error("Called `.value()` before it was set. Maybe try `.valueOrNull()`?")
            }
            set(value) {
                if (stateFlowDeferred.isCompleted) {
                    stateFlowDeferred.getCompleted().value = value
                } else {
                    stateFlowDeferred.complete(MutableStateFlow(value))
                }
            }

        override val valueOrNull: T?
            get() = if (stateFlowDeferred.isCompleted) {
                stateFlowDeferred.getCompleted()
                    .value
            } else {
                null
            }

        override suspend fun value(): T = stateFlowDeferred.await()
            .value
    }
}