Android Kotlin Coroutines: what is the difference between flow, callbackFlow, channelFlow,… other flow constructors

For callbackFlow:

You cannot use emit() as the simple Flow (because it’s a suspend function) inside a callback. Therefore the callbackFlow offers you a synchronized way to do it with the trySend() option.

Example:

fun observeData() = flow {
 myAwesomeInterface.addListener{ result ->
   emit(result) // NOT ALLOWED
 }
}

So, coroutines offer you the option of callbackFlow:

fun observeData() = callbackFlow {
 myAwesomeInterface.addListener{ result ->
   trySend(result) // ALLOWED
 }
 awaitClose{ myAwesomeInterface.removeListener() }
}

For channelFlow:

The main difference with it and the basic Flow is described in the documentation:

A channel with the default buffer size is used. Use the buffer
operator on the resulting flow to specify a user-defined value and to
control what happens when data is produced faster than consumed, i.e.
to control the back-pressure behavior.

The trySend() still stands for the same thing. It’s just a synchronized way (a non suspending way) for emit() or send()

I suggest you to check Romans Elizarov blog for more detailed information especially this post.

Regarding your code, for callbackFlow you wont’ be needing a coroutine launch:

coroutineScope.launch {
                send(key)
                //trySend(key)
            }

Just use trySend()