If I’m targeting JVM, Native, and JS, is it at all...

# coroutines

[Adam S]

If I’m targeting JVM, Native, and JS, is it at all possible to write a non-suspending function in commonMain that gets the result from a suspending function?

I want to implement an interface from a 3rd party library with a class in commonMain, and call an expect suspend fun to get the result from each target. But because it’s an external interface I can’t add suspend, and I can’t use runBlocking {} because it’s not available for JS…

MyCommonClass_kt.cpp

[Dmitry Khalanskiy [JB]]

There's always GlobalScope.launch {}, but it does not wait for the code in the block to finish.

[Adam S]

sorry, what do you mean? launch {} returns a Job, but the function needs it to return a String

[Dmitry Khalanskiy [JB]]

Didn't notice the code snippet, sorry, was answering your question directly. No, the way JS works, this is not possible in general.

[Adam S]

ah yes, the first line in my question was ambiguous, I’ve updated it

[Adam S]

is there a YouTrack issue for this? It seems like a pretty big drawback if there’s no way to bridge blocking/non-blocking in JS

[Dmitry Khalanskiy [JB]]

Sure, there are a couple, but the problem is, this is an inherent limitation in JS: https://github.com/Kotlin/kotlinx.coroutines/issues/195

[Adam S]

thanks, I’ll add a comment

[Dmitry Khalanskiy [JB]]

Do you mean, to the issue? I'm pretty sure it won't change anything.

[Adam S]

this is an inherent limitation in JS

I’m not a JS developer, but surely there’s a way that JS code deals with this problem?

I’m not necessarily looking for an equivalent to runBlocking {}, but even if there’s a JS-specific function that can bridge blocking/non-blocking then I could change

expect suspend fun getData(): String

to

expect fun getData(): String

and then use runBlocking {} on JVM/Native, and something else on JS

[Adam S]

Do you mean, to the issue? I’m pretty sure it won’t change anything.

I’d prefer to make a new issue, but if it can’t be done then all I can do is make a comment and stop working on my library

[Dmitry Khalanskiy [JB]]

surely there’s a way that JS code deals with this problem

It does it by working with the equivalent of suspend functions. JS code is single-threaded, and fun getData(): String, since it's not a suspend fun, has to all happen in the same thread. While await() is waiting, the thread can't do anything else. On JS, since this is the only thread, getData has no chance to run.

[Dmitry Khalanskiy [JB]]

On JS, this is done with callbacks, and suspend functions are abstractions over those.

[Dmitry Khalanskiy [JB]]

... well, on the new version of JS, this is done with async functions actually, a direct equivalent of suspend functions, and there, you have the exact same inherent problem. https://stackoverflow.com/a/41976799 EDIT: wrong link

[Adam S]

While await() is waiting, the thread can’t do anything else.

If I could change expect suspend fun getData(): String to expect fun getData(): String, then doesn’t that mean I wouldn’t need an await()?

[Adam S]

also I don’t mind if JS is limited by the single threaded nature - just so long as I can write my code in commonMain, and bridge blocking/non-blocking however is best for each target.

[Dmitry Khalanskiy [JB]]

First, getData is probably a recursive function. You probably want to rename expect suspend fun getData(): String to expect suspend fun getDataImpl(): String or something like that.

[Dmitry Khalanskiy [JB]]

Second, yes, if expect suspend fun getDataImpl(): String becomes expect fun getDataImpl(): String, then you can implement your interface as just override fun getData() = getDataImpl().

[Adam S]

oh yeah, that’s confusing naming on my part :) How about this?

// src/commonMain/kotlin/MyCommonClass.kt

import x.y.z.ThirdPartyInterface

class MyCommonClass : ThirdPartyInterface {
  override fun getData(): String = platformGetData()
}

expect suspend fun platformGetData(): String

[Dmitry Khalanskiy [JB]]

However, if, on JS, you want to call a suspend function from a non-suspend one and obtain its result, it's not possible.

[Dmitry Khalanskiy [JB]]

Kotlin/JS can't do anything that plain JS can not do, it's not magic. And plain JS can't call async functions from non-async ones.

[Adam S]

right, that makes sense, thanks

[Adam S]

the link you shared above helped too https://stackoverflow.com/a/41976799

[Adam S]

The way JavaScript runs on a given thread is that it processes a queue

of jobs:

1. Pick up the next pending job

2. Synchronously execute the code for that job

3. Only when that job completes go back to Step 1 to pick up the next job

(It’s a bit more complicated than that, there are two levels to it, but that’s not relevant to this particular question.)

XHR completions and such are jobs that get scheduled in the queue.

There is no way to pause a job, run another job from the queue, and

then pick up the paused job. `async`/`await` provides dramatically

simpler syntax for handling asynchronous operations, but they don’t

change the nature of the job queue.

could Kotlin implement its own task queue that allows for pausing jobs?

[Dmitry Khalanskiy [JB]]

In theory, it could, but then it wouldn't be able to call JS's async functions/promises, so it would be completely pointless.

[Adam S]

okay, thanks for taking the time to explain