Hi guys I m playing with Snapshot System I tried to modify a kotlinlang #compose

Hi guys. I’m playing with Snapshot System. I tried to modify a mutable state in global snapshot, but the apply listener doesn’t get called back. Code in🧵

Here is my code:

val globalWriteObserverHandler = Snapshot.registerGlobalWriteObserver {
  println("on global written")
  Snapshot.sendApplyNotifications()
}
val applyObserverHandler = Snapshot.registerApplyObserver { set, _ ->
  println("on applied - $set")
}
val count = mutableStateOf(0)
count.value = 1
globalWriteObserverHandler.dispose()
applyObserverHandler.dispose()

If I take a snapshot before the modification and dispose the snapshot immediately, then the apply listener can get called back when state is modified:

...
val count = mutableStateOf(0)
// added this line
Snapshot.takeSnapshot().dispose()
count.value = 1
...

Dose anyone know the reason?

I found it’s due to the logic in

fun <T : StateRecord> T.overwritableRecord

internal fun <T : StateRecord> T.overwritableRecord(
    state: StateObject,
    snapshot: Snapshot,
    candidate: T
): T {
    ...

    if (candidate.snapshotId == id) return candidate

    ...

    snapshot.recordModified(state)

    return newData
}

When candidate record is found, it returns, without calling

recordModified

. Is this expected? If yes, what the reason?

Zach Klippenstein (he/him) [MOD]

08/03/2022, 7:08 PM

I’m guessing yea, because the snapshot system is designed only to let observers know if an object has been modified, not how many times it’s been modified, in a given snapshot. If an object is modified many times in a snapshot, there are probably major performance wins to this approach.

fengdai

08/04/2022, 12:59 PM

I see. It makes sense. But I still feel that the behavior is a little bit buggy in my first code snippet. I expect there would be at least one apply notification.

Zach Klippenstein (he/him) [MOD]

08/04/2022, 4:54 PM

Look at the kdoc for initializeObjects and sendApplyNotifications in Snapshot.kt. Changes aren't recorded for brand new state objects until the snapshot is applied or initializeObjects is called.

fengdai

08/05/2022, 12:07 AM

initializeObjects

? I’m guessing you mean

notifyObjectsInitialized

. And it works. Thanks very much!

Zach Klippenstein (he/him) [MOD]

08/05/2022, 6:10 PM

Yep, sorry

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