What does `resumes` mean for continuations? Where ...

# coroutines

[Loney Chou]

What does resumes mean for continuations? Where and when the code after suspension point run? Like if the continuation runs in an infinite loop, does it mean that resumes will never end?

[efemoney]

A continuation instance is for a particular “suspension”.

If you have:

suspend fun myFun() = suspendCancellableCoroutine { continuation ->
  onWorkComplete { continuation.resume() }
}

// and

1 - suspend fun loop() {
2 -   while(true) {
3 ~     myFun()
4 -   }
5 - }

Execution “suspends” on line 3 when myFun is invoked and the continuation.resume resumes the suspended execution.

So in “reality” you didnt actually “leave” line 3 while myFun was running. Execution is basically
• check condition
• enter while loop
• suspend while loop! (by entering myFun)
• - myFun does its work
• - continuation.resume() after work is done
• resume while loop
• (repeat all over)

[efemoney]

I’m a bit curious if above is what you meant or you mean something like this instead

suspend fun myFun() = suspendCancellableCoroutine { continuation ->
  while(true) { 
    continuation.resume() 
  }
}

🤔 ?

[Nick Allen]

What does resumes mean for continuations?

Continuation is just a callback. resume is how you invoke it.

Where and when the code after suspension point run?

resume passes the callback to a dispatcher, the dispatcher runs the code.

Like if the continuation runs in an infinite loop, does it mean that resumes will never end?

Usually the dispatcher lets resume return right away and invokes the callback later in it's thread pool. If the dispatcher (like Unconfined) runs the callback directly then, resume won't return until the resumed code finishes or suspends and if neither happens then it's blocked forever.

I’m a bit curious if above is what you meant or you mean something like this instead

```suspend fun myFun() = suspendCancellableCoroutine { continuation ->

while(true) {

continuation.resume()

}

}```

No, this doesn't really make any sense. suspendCancellableCoroutine returns with the value you resume with. So calling resume multiple times is like returning multiple times. If you tried, you'd get an exception.

Plenty of articles detail how coroutines work internally which could help. For example: https://kt.academy/article/cc-under-the-hood

[efemoney]

I know it doesnt make sense. I’m just wondering still if thats what OP meant by his question

[Nick Allen]

Oops,I got mixed up, thought that was from the OP 😅

[Loney Chou]

Thank you two for clearing up 😄 Now I know that `resumes`returns on suspension. That's what I don't understand at the first place.