How to transform an Android Task to a Kotlin Deferred?

Question

Firebase anonymous sign in returns a task (which is basically Google promise implementation):

val task:Task<AuthResult> = FirebaseAuth.getInstance().signInAnonymously()

How it would be possible create a signInAnonymous wrapper where:

• It is a suspend function, waiting for the task completion

  • suspend fun signInAnonymous(): Unit

• It returns a Deferred object, delivering the result asynchronously

  • fun signInAnonymous() : Deferred

Answer 1

The package kotlinx.coroutines.tasks now includes the follwing utility functions:

public suspend fun <T> Task<T>.await(): T { ... }

From the docs:

Awaits for completion of the task without blocking a thread.
This suspending function is cancellable.
If the Job of the current coroutine is cancelled or completed while this suspending function is waiting, this function stops waiting for the completion stage and immediately resumes with CancellationException.

public fun <T> Task<T>.asDeferred(): Deferred<T> { ... }

From the docs:

Converts this task to an instance of Deferred.
If task is cancelled then resulting deferred will be cancelled as well.

---

So you can just do:

suspend fun signInAnonymouslyAwait(): AuthResult {
    return FirebaseAuth.getInstance().signInAnonymously().await()
}

or:

fun signInAnonymouslyDeferred(): Deferred<AuthResult> {
    return FirebaseAuth.getInstance().signInAnonymously().asDeferred()
}

Answer 2

Based on this GitHub library, here's a way to transform a Task into a suspending function in the "usual" way to adapt callback based async calls to coroutines:

suspend fun <T> Task<T>.await(): T = suspendCoroutine { continuation ->
    addOnCompleteListener { task ->
        if (task.isSuccessful) {
            continuation.resume(task.result)
        } else {
            continuation.resumeWithException(task.exception ?: RuntimeException("Unknown task exception"))
        }
    }
}

You can also wrap it in a Deferred of course, CompletableDeferred comes in handy here:

fun <T> Task<T>.asDeferred(): Deferred<T> {
    val deferred = CompletableDeferred<T>()

    deferred.invokeOnCompletion {
        if (deferred.isCancelled) {
            // optional, handle coroutine cancellation however you'd like here
        }
    }

    this.addOnSuccessListener { result -> deferred.complete(result) }
    this.addOnFailureListener { exception -> deferred.completeExceptionally(exception) }

    return deferred
}