Is there a coroutine equvalent for `lazy`? I'm not...

# coroutines

[trathschlag]

Is there a coroutine equvalent for lazy? I'm not looking for async. All I want is execution in the current coroutine, if the value was not yet computed.

[marstran]

Why can't you use lazy?

[trathschlag]

Because I want the provider function to be suspendable. Something like:

val userToken = suspendableLazy {
    getUserTokenFromServerWithPotentiallyLongIO()
}

val user = User(userToken.get())
....
val x = getXForUser(userToken.get())

[jimn]

perhaps lateinit is good enough?

[bholota]

Deferred is all you need:

suspend fun currentTimeStamp(): Long  {
    delay(2000L) // simulate background operation
    return System.currentTimeMillis()
}

@Test
fun lazyTest() = runBlocking {
    val time = async {currentTimeStamp() }
    println(time.await())
    delay(2000L)
    println(time.await())
}

second await will print same value

[bholota]

you can put deferred as property in your class if class implements `CoroutineScope`:

class ScopePlayground : CoroutineScope {

    private val job = SupervisorJob()
    override val coroutineContext: CoroutineContext
        get() = job + Dispatchers.Default

    private val time = async { currentTimeStamp() }
// rest of the code

[trathschlag]

Thanks for the reply, but the CoroutineScope is not available during definition time in my case. And I don't want to pass it on every invocation as it is basically unnecessary for what I want to achieve. I went with the following solution for now:

interface SuspendableProvider<T> {
    suspend fun get(): T
}

fun <T : Any> suspendableLazy(provider: suspend () -> T) = object : SuspendableProvider<T> {
    val mutex = Mutex()
    lateinit var computed: T

    override suspend fun get() = mutex.withLock {
        if (!this::computed.isInitialized) {
            computed = provider()
        }

        computed
    }
}

[jimn]

sounds like a lazy contextkey

[trathschlag]

As in coroutineContext[key]? How so?