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Why does this code freeze everything and how can I...
# multiplatform
s
Why does this code freeze everything and how can I prevent it? I have this `PhotoSyncService`:
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private val scope = CoroutineScope(Dispatchers.Default)

class PhotoSyncService {

    private var jobMap: MutableMap<PhotoSource, Job> = mutableMapOf()

    fun sync(photoSource: PhotoSource, updateSyncInfo: (SyncInfo) -> Unit) {

        val existingJob = jobMap[photoSource]

        if (existingJob != null && existingJob.isActive)
            return

        val job = scope.launch {

            try {

                updateSyncInfo(
                    SyncInfo(
                        log = "Starting...",
                        status = SyncStatus.SYNCING
                    )
                )

                delay(2500)

                updateSyncInfo(
                    SyncInfo(
                        log = "Sync done.",
                        status = SyncStatus.SYNCED
                    )
                )

            } catch (ex: CancellationException) {

                updateSyncInfo(
                    SyncInfo(
                        log = "Sync canceled.",
                        status = SyncStatus.ERROR
                    )
                )
            }
        }

        jobMap[photoSource] = job
    }

    fun cancelSync(photoSource: PhotoSource) {

        val job = jobMap[photoSource]

        job?.let {

            job.cancel()
            jobMap.remove(photoSource)
        }
    }
}
And I got a 
PhotoStore
class that uses this service like that:
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private fun forceSync(
    oldState: PhotoState,
    photoSource: PhotoSource
): PhotoState {

    photoSyncService.sync(photoSource) { syncInfo ->

        /*
         * This update comes from an background coroutine.
         * So we need to bring it back to the main thread.
         */
        launch(Dispatchers.Main) {
            this@PhotoStore.dispatch(PhotoAction.SyncInfoUpdate(photoSource, syncInfo))
        }
    }

    return oldState
}
The 
forceSync()
is called on the main thread. 
PhotoSyncService
is also initialized on the main thread. As soon as my 
updateSyncInfo
lambda is called in my 
launch
block it freezes the 
PhotoSyncService
& 
PhotoStore
... The line 
jobMap[photoSource] = job
gives me a 
InvalidMutabilityException
. I first thought there were other reasons, but after tracking everything down I know that it must be the lambda call- But why? Because of the call to "this"?
✅ 1
n
In the 
sync
method of 
PhotoSyncService
, try to add 
Dispatchers.Default
when you launch your coroutines :
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val job = scope.launch(Dispatchers.Default) {
    // ...
}
or you can surround your 
delay(2500)
with a 
withContext(Dispatchers.Default)
:
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withContext(Dispatchers.Default) {
    delay(2500)
}
s
Nope, even with 
val job = scope.launch(Dispatchers.Default)
the 
this.dispatch()
seems to capture everything. According to the docs 
launch()
should be in 
Dispatchers.Default
since my scope is there and it will be inherited. Is there a known bug that this does not work?
m
this@PhotoStore
will most likely be freezed from the 
launch {}
s
How can I prevent that?
Is there a way to structure this code that it works with the memory model?
m
Move the 
dispatch
outside the lambda
Something like this:
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val syncInfo = photoSyncService.sync(photoSource)
this@PhotoStore.dispatch(PhotoAction.SyncInfoUpdate(photoSource, syncInfo))
Think of 
SyncInfo
being immutable messages
That are send back to your 
PhotoStore
s
I can't. It's a lambda because during a sync multiple syncInfos are being sent updating a state. SyncInfo itself is an data class.
m
Make 
sync
suspend
s
How does that help preventing Kotlin/Native to freeze my PhotoStore?
m
the 
PhotoStore
will never be referenced from 
PhotoSyncService
PhotoSyncService
does stuff in the background in a 
suspend
function
When it's ready, it resumes in the main thread
You don't have to pass 
PhotoStore
through the 
launch {}
hoops
All the 
PhotoSyncService
sees is a 
Continuation
and it should work
I'm not 100% certain but this is how I'd do it
s
Like this?
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private fun forceSync(
    oldState: PhotoState,
    photoSource: PhotoSource
): PhotoState {

    launch(Dispatchers.Main) {

photoSyncService.sync(photoSource) { syncInfo ->

            launch(Dispatchers.Main) {

                this@PhotoStore.dispatch(PhotoAction.SyncInfoUpdate(photoSource, syncInfo))
            }
        }
    }

    return oldState
}
m
Nope, as long as you have 
this@PhotoStore
called from the lambda, it will be freezed I believe
s
I don't understand how I can update my 
PhotoState
in my 
PhotoStore
without taking the result in my lambda and calling 
dispatch
😕
During a sync the lambda will be called multiple times with a progress and I want to show that in my UI.
m
This is where 
suspend
is useful
suspend
is like a lambda but it will not capture your 
this@PhotoStore
So that ultimately, you'll have something like this:
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val syncInfo = photoSyncService.sync(photoSource)
this@PhotoStore.dispatch(PhotoAction.SyncInfoUpdate(photoSource, syncInfo))
Of course that means 
forceSync
is now suspend too
If you don't want that, you'll have to go through fire & forget 
GlobalScope
stuff
s
But then I only get one SyncInfo as a result from the call to 
sync()
m
Ah sorry, I missed that you needed several
In that case, the primitive to use is 
Flow<PhotoState>
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fun sync(photoSource: PhotoSource): Flow<PhotoState>
s
And the flow won't capture like the lambda?
m
And then:
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photoSyncService.sync(photoSource).collect {
   this@PhotoStore.dispatch(PhotoAction.SyncInfoUpdate(photoSource, it))
}
And the flow won't capture like the lambda?
I hope it won't 😅
But TBH I'm really not sure
If it's all the same dispatcher there's no need to freeze but I don't know how coroutines do that internally
s
Yes, it freezes because 
sync()
switches to 
Default
dispatcher to offload it from 
Main
.
FlowCollector is not thread-safe and concurrent emissions are prohibited.
is the error now. It says I can try to use channelFlow
👀 1
👍 1
Yes, 
channelFlow
actually works. 🎉 I don't understand why, but it prevents the capturing of my PhotoStore. Thank you, @mbonnin 🙏 Currently it's not an ideal solution because the Flow must be captured to go on while a lambda just gives a feedback (like 
onClick()
for a 
Button
) , but maybe I can change other stuff to make it more fitting.
m
Nice 🎉
Yep, this stuff is complex
➕ 1
So depending when you want to ship your product, you might go directly with the new memory model
👍 1
I think it's supposed to go beta in Q1 2022
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