Kotlin create a custom coroutine

Question

I am trying to make a coroutine from a method that I have. to make things simple, let's say I have a class A that I try to connect() and it is connected only after class B that is inside A is connected.

So, I have this code for example, which offcourse doesn't work but it's just to show my use case-

class A {
    fun connect() {
        classB.connect()
        val isConnected = classB.isConnected
    }
}

class B {
    val isConnected: Boolean = false
    fun connect() {
        someObject.connect( SomeListenerInterface {
            override fun onSuccess() {
                isConnected = true
            }
        })
    }
}

I want to make the classB.connect() as a coroutine, and make it suspended, so only when it is done, the line of val isConnected = classB.isConnected would execute and the value would be set properly.

If I would use java and callbacks, I would just pass a callback to the classB.connect() method, and set the class A.isConnected value inside this callback.

is it possible with kotlin coroutines? Thanks

The first thing you need here is convert you callback based `someObject.connect` function into a suspend function. You can do that using `suspendCancellableCoroutine` builder. Now that `someObject.connect` is suspend, you need to mark `B.connect` as `suspend`. The last bit required is a `CoroutineScope` which is a bridge between non-coroutine and coroutine based world. Since you haven't provided the exact code and mentioned what it is doing, it's not possible to suggest the coroutine code but what I wrote earlier is the gist of what you need to do. — Arpit Shukla, Nov 27 '21 at 17:43
@ArpitShukla thanks for the suggestion. I think I will just use something like `val ack = CompletableDeferred()` inside `classB` and I will write `ack.complete(true)` inside `classB.connect()` and in `classA.connect()` I will just do `classB.await()` after the `classB.connect()`. what do you think? — StackerSapper, Nov 28 '21 at 10:57
@StackerSapper this is unnecessary, you should instead follow what Arpit said, which is detailed in Kirill's answer — Joffrey, Dec 06 '21 at 23:32

score 3 · Accepted Answer · answered Dec 06 '21 at 22:37

3

Convert callback to suspend function using suspendCoroutine. The function below returns true when the connection succeeds:

class B {
    suspend fun connect(): Boolean = suspendCoroutine { continuation ->
        someObject.connect(object : SomeListenerInterface {
            override fun onSuccess() {
                continuation.resume(true)
            }
        })
    }
}

connect function in class A should be suspend respectively:

class A {
    suspend fun connect() {
        val isConnected = classB.connect()
    }
}

answered Dec 06 '21 at 22:37

Kirill Kitten

1,759
1
4
5

thank you, though does the solution with `CompletableDeferred()` isn't good also? because I have many callbacks that are relatd to each other – StackerSapper Dec 07 '21 at 11:54
I relied on the [Roman Elizarov](https://stackoverflow.com/users/1051598/roman-elizarov), the Kotlin founder, [opinion](https://stackoverflow.com/questions/48552925/existing-3-function-callback-to-kotlin-coroutines/48562175#48562175). – Kirill Kitten Dec 07 '21 at 13:05

Kotlin create a custom coroutine

1 Answers1