SubscriptionReceiver

internal val customClientKey = ContextPropertyKey<CustomClient>()

val ContextReceiver.customClient: CustomClient?
    get() = get(customClientKey)

var ContextReceiverBuilder.customClient: CustomClient
    get() = error("You cannot retrieve a builder property directly.")
    set(value) = set(customClientKey, value)

inline fun <T> buildCustomSubscriptionKey(
    id: SubscriptionId<T>,
    crossinline subscribe: CustomClient.() -> Flow<T>
): SubscriptionKey<T> = buildSubscriptionKey(
    id = id,
    subscribe = {
        val client = checkNotNull(customClient) { "customClient isn't available. Did you forget to set it up?" }
        with(client) { subscribe() }
    }
)

SubscriptionReceiver {
    customClient = newCustomClient()
}

class ExampleSubscriptionKey(auto: Namespace) : SubscriptionKey<String> by buildCustomSubscriptionKey(
    id = SubscriptionId(auto.value),
    subscribe = { // CustomClient.() -> Flow<String>
        doSomethingFlow()
    }
)