Hey guys, I’m trying to await a result of a callback from the Amazon LWA sdk but it’s a bit advanced...

# coroutines

[Schadenfreude]

Hey guys, I’m trying to await a result of a callback from the Amazon LWA sdk but it’s a bit advanced and confusing for me. This is their method:

AuthorizationManager.getToken(context, SCOPES, tokenListener)

I want to wait for it to get the result in the listener and then continue on with my code but I can’t figure out if it’s at all possible.

[streetsofboston]

suspend fun getAuthManagerToken(context: Context, scopes: Scopes) : Token = suspendCoroutine { cont ->
    AuthorizationManager.getToken(context, scopes) { token ->
        cont.resume(token)
    }
}

[Schadenfreude]

The listener can return either a token or an error, its type is Listener<AuthorizeResult, AuthError>

[streetsofboston]

Is AuthError a Throwable?

[louiscad]

@Schadenfreude Then use resumeWithException when there's an error, and if there's no Throwable, use your own

[Schadenfreude]

@streetsofboston, yes it’s a throwable

[streetsofboston]

suspend fun getAuthManagerToken(context: Context, scopes: Scopes) : Token = suspendCoroutine { cont ->
    AuthorizationManager.getToken(context, scopes) { token, error ->
        if (error != null) resumeWithException(error)
        else cont.resume(token)
    }
}

[streetsofboston]

(I assumed the type(-name)s in my example… I don’t know their exact form)

[Schadenfreude]

So thanks to @streetsofboston I managed to figure it out here’s the final solution:

private suspend fun getAuthManagerToken(context: Context, scopes: Array<Scope>): AuthorizeResult =
        suspendCoroutine { cont ->
            AuthorizationManager.getToken(context, scopes, object : Listener<AuthorizeResult, AuthError> {
                override fun onError(authError: AuthError) {
                    cont.resumeWithException(authError)
                }

                override fun onSuccess(authResult: AuthorizeResult) {
                    cont.resume(authResult)
                }
            })
        }

[Sam]

@Schadenfreude @streetsofboston Is there any reason not to use suspendCancellableCoroutine and cancel getToken() when not needed anymore?

[streetsofboston]

`suspendCancellableCoroutine`: In the code-sample of Daniel, the getAuthManagerToken has no reason to cancel the suspended coroutine itself.
The outer-scope that calls getAuthManagerToken still can cancel the suspended fun.

(suspendCancellableCoroutine will just provide a cont to the implementation of getAuthManagerToken that now has an additional cancel function.)

[Sam]

Okay, if getToken() returns a cancellable call, then it makes sense to use suspendCancellableCoroutine

[Sam]

that way the pending getToken() request and its resources can be dropped early

[streetsofboston]

If a cancelation happens, most of the time the outer-scope that calls the function does the cancelation. The getAuthManager doesn’t need to worry about it in this case.

If the getAuthManagerToken itself needs to cancel it, then it needs to call suspendCancellableCoroutine so that it has a CancelableContinuation (not sure if I have the class-name right) instead of a plain Continuation and now it can call cancel() on it, if necessary, instead of calling resumeWithException(...).

Cancelation is almost(!!) like resumeWithException, because it cancels the suspending call, but it is not ‘fatal’, it has just canceled.

[pakoito]

@louiscad I thought resumeWithExceptions was only for crashing cases, not for control flow

[louiscad]

@pakoito It's to forward `Throwable`s back into the control flow from error/exceptions callbacks