Day162 — Convert callback to Coroutine

Here record the notes I took when I am reading “Avoid Callback Hell With Kotlin Coroutines”.

Avoid Callback Hell With Kotlin Coroutines

One-shot callbacks

use suspendCoroutine

suspend fun DatabaseReference.read(): DataSnapshot = suspendCoroutine { continuation ->
    val valueEventListener = object : ValueEventListener {
        override fun onCancelled(error: DatabaseError) {
            continuation.resumeWithException(error.toException())
        }

        override fun onDataChange(snapshot: DataSnapshot) {
            continuation.resume(snapshot)
        }
    }
    addListenerForSingleValueEvent(valueEventListener)
}

suspend fun getGroups(): List<Group> {
    return groupRef.read().mapToGroups()
}

class MyViewModel : ViewModel() {
    fun getGroups() {
        // call it inside a coroutine scope
        viewModelScope.launch {
            val result = getGroups()
            ...
        }
    }
}

wrap callback with suspendCoroutine and make the method, then use resume or resumeWithException at the place where the callback occurs.

Subscription based callbacks

use callbackFlow

suspend fun DatabaseReference.subscribe(): Flow<Player> = callbackFlow {
    val childEventListener = object : ChildEventListener {
        override fun onChildAdded(snapshot: DataSnapshot, previousChildName: String?) {
            trySend(
                Player(
                    snapshot.key
                )
            )
        }
        override fun onChildChanged(snapshot: DataSnapshot, previousChildName: String?) {}
        override fun onChildRemoved(snapshot: DataSnapshot) {}
        override fun onChildMoved(snapshot: DataSnapshot, previousChildName: String?) {}
        override fun onCancelled(error: DatabaseError) {}
    }
    addChildEventListener(childEventListener)
    awaitClose {
        removeEventListener(childEventListener)
    }
}

// subscribe to the data
val flow = DatabaseReference().subscribe()
flow.collect { player ->
    ...
}

wrap callback listener with callbackFlow, use trySend to send the data, and use awaitClose to close the subscription when needed.

Function vs method

Method is a function associated to an object.

Function is a more general term, and all methods are also functions.

class A {
   fun someMethod() {}
}

Kotlin programmer dictionary: Function vs Method vs Procedure