What is suspendCoroutine?

Question

I am a beginner studying coroutines.

Not exactly, but I have a little understanding of what a coroutine is.

The suspend function is also difficult, but with a little understanding.

I'm studying step by step, but there are parts I don't understand.

That's suspendCoroutine. In the sample code, suspendCoroutine and Continuation used in the block appear, but I have no idea what these two are.

I've looked on other sites, but I can't find anywhere that explains it easily.

Can you explain what suspendCoroutine and Continuation are used for easily and, if possible, with an example?

Answer 1

suspendCoroutine is a builder function that mainly used to convert callbacks into suspend functions. Let's say for example you have some legacy (or not) Api, that uses callbacks. You can easily transform it into a suspend function to call it in a coroutine. For example:

suspend fun getUser(id: String): User  = suspendCoroutine { continuation ->
      Api.getUser(id) { user ->
          continuation.resume(user)
      }
}

Here we have an Api function getUser, which defined in Api class for example like this:

fun getUser(id: String, callback: (User) -> Unit) {...}

suspendCoroutine suspends coroutine in which it executed until we decide to continue by calling appropriate methods - Continuation.resume.... suspendCoroutine mainly used when we have some legacy code with callbacks.

Using suspendCoroutine to convert callbacks into suspend functions makes the code sequential when you work with suspend functions.

For example instead of having a callback hell like this:

Api.getUser(id) { user ->
      Api.getProfile(user) { profile ->
          Api.downloadImage(profile.imageId) { image ->
              // ...
          }
      } 
}

after you apply suspendCoroutine to those callbacks and convert them into suspend functions, the code will look like the following:

val user = getUser(id)
val profile = getProfile(user)
val image = downloadImage(profile.imageId)
//...

Answer 2

suspendCoroutine is Kotlin's name for Scheme's function call-with-current-continuation (abr. call/cc).

It is explained in this video "Fresh Async With Kotlin • Roman Elizarov • GOTO 2018".

The article "The Yin-yang puzzle in Kotlin" explains it based on the Yin-yang puzzle, which is a famous example use case for call/cc. Other use cases are explained in the article "Continuations by example: Exceptions, time-traveling search, generators, threads, and coroutines".