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How to `await` for completion of a suspending func...
# coroutines
a
How to 
await
for completion of a suspending function that wraps a blocking call with 
withContext
b
withContext
itself returns the result, so there's no need to await. If you are wanting to execute the 
withContext
concurrently, use 
async
instead
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val result = withContext(<http://Dispatchers.IO|Dispatchers.IO>) { blockingCall() }
vs
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val resultAsync = async(<http://Dispatchers.IO|Dispatchers.IO>) { blockingCall() }
doSomethingElse()
val result = resultAsync.await()
a
How do I create a suspending function then? Given a blocking call?
g
what do you mean?
see: imagine, you have
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fun myApiBlockingCall(): String {
  Thread.sleep(1000)
  return "Foo"
}
To convert it to non-blocking using withContext:
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suspend fun myApiCall() = withContext(IO) {
   myApiBlockingCall()
}
And that’s all. You can call myApiCall in any suspend block with any dispatcher without blocking
a
@gildor but now how do I await the result?
I can’t do 
myApiCall().await()
right - how do I wait for the result?
g
You don't need await for suspend functions, just call 
myApiCall()
a
How would that work if i m calling it from main? It would just fall through and exit right
l
@asad.awadia 
suspend fun main()
(new in 1.3) can directly call this suspending function
a
Yeah i read
Let me try it
Whaaaaaa
This is some magic right here
Thank you guys
How do i ‘return’ foo from the suspend function
It complains i need another return statement outside the with context
l
The magic you're talking about is similar to 
fun main() = runBlocking(Dispatchers.Default) { … }
fun main()
needs to return 
Unit
, be it 
suspend
or not
a
No no main
How do i assign value of myApiCall to a var
MyApiCall - i can add a return inside the with context - but intellij complains that i need one at the end too
l
Maybe you're looking for 
return@something
?
a
Yeah that’s inside the with context
But apparently the function itself needs a return outside too
l
Yes, you need to return result of 
withContext
call
a
I ll post a code snippet after work to show what I mean
g
withContext is just a function with lambda param. For any real application you probably need not a suspend main, but some coroutine builder, like 
launch
as entry point for your async coroutines code
I would also recommend to use runBlocking instead of suspend main. It may be handy sometimes, but runBlocking also provides CoroutineScope, so you can use launch or async directly and don't worry about running coroutines
a
Would you prefer launch async over suspend functions?
I thought async await is discouraged
g
No, I prefer suspend functions and suspend functions also recommended by official guide
But to use suspend functions you need suspend lambda, so you need to somehow start coroutine, so use launch, async or runBlocking
I'm talking only about entry point usage, that to play with coroutines in 
main
method is more handy to use 
fun main() = runBlocking<Unit>{}
instead of 
suspend fun main() {}
because runBlocking also provides coroutine scope
a
Oh ok ok
I see makes sense
Thank you!
l
Or you can use 
coroutineScope { … }
in 
suspend fun main()
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