Hi, can anybody help me understand how does compos...

# compose

[Davide Bertola]

Hi, can anybody help me understand how does compose subscribe to state changes for knowing when to recompose ?

[Davide Bertola]

for example liveObject.observeAsState() returns a State<T> which is not something that can be subscribed to

[Brandon Trautmann]

Have you read this? https://developer.android.com/jetpack/compose/state

[Brandon Trautmann]

Specifically:

Note: observeAsState observes a LiveData<T> and returns a State<T> object that is updated whenever the LiveData changes. State<T> is an observable type that Jetpack Compose can use directly.

observeAsState will observe the LiveData only while it is in the Composition.

[Davide Bertola]

yes i have read all of that and this is where i do not see anybody actually subscribing/observing

[Davide Bertola]

observeAsState internally subscribes, but it just returns a State<T>

[Davide Bertola]

so I am guessing there could be some compiler magic here ?

[Davide Bertola]

@Composable
fun <R, T : R> LiveData<T>.observeAsState(initial: R): State<R> {
    val lifecycleOwner = LifecycleOwnerAmbient.current
    val observer = remember { DisposableObserver<R, T>(initial, lifecycleOwner) }
    observer.source = this
    return observer.state
}

[Halil Ozercan]

I'd suggest this talk by LeLand. It helped me understand the basics behind compose-runtime and "compiler magic" that you referred. I'm not sure if there is any other up-to-date talk with the latest changes but I'd assume the basics stayed the same.

https://www.youtube.com/watch?v=Q9MtlmmN4Q0▾

[Davide Bertola]

maybe it injiects into setValue(), somehow

[Adam Powell]

This conversation from a little while ago talks about how it works https://kotlinlang.slack.com/archives/CJLTWPH7S/p1604328608205800?thread_ts=1604327643.203200&amp;cid=CJLTWPH7S

[Davide Bertola]

i'll try the video first as the conversation seems to start from some concepts I do not have in my mind

[Davide Bertola]

ok so the video does not really explain who and when triggers the recomposition

[Davide Bertola]

liveData.observe(this) { recompose() }

[Davide Bertola]

I would expect this somewhere if there was no compiler magic 🙂

[Adam Powell]

there is, it just happens on a different level 🙂 check snapshotFlow {} here: https://cs.android.com/androidx/platform/frameworks/support/+/androidx-master-dev:compose/runtime/runtime/src/commonMain/kotlin/androidx/compose/runtime/snapshots/SnapshotFlow.kt

[Adam Powell]

snapshot observers observe all snapshot-backed objects in the process at once

[Adam Powell]

it happens here:

val unregisterApplyObserver = Snapshot.registerApplyObserver { changed, _ ->
    appliedChanges.offer(changed)
}

[Adam Powell]

changed is the set of snapshot-backed objects (e.g. mutableStateOf) that changed in a single transaction

[Davide Bertola]

so when i do val foo: String by l.oserveAsState("")

[Davide Bertola]

is foo a snapshot-backed-object ?

[Adam Powell]

yes

[Adam Powell]

you can see the LiveData observe call here: https://cs.android.com/androidx/platform/frameworks/support/+/androidx-master-dev:compose/runtime/runtime-livedata/src/main/java/androidx/compose/runtime/livedata/LiveDataAdapter.kt;l=55

[Davide Bertola]

that's what i have read in the first place

[Davide Bertola]

state.value = it

[Davide Bertola]

how do you know state has changed

[Davide Bertola]

(i am looking for the source of mutableStateOf() ) too see if the magic is there

[Adam Powell]

https://cs.android.com/androidx/platform/frameworks/support/+/androidx-master-dev:compose/runtime/runtime/src/commonMain/kotlin/androidx/compose/runtime/MutableState.kt;l=124

[Davide Bertola]

ooohh

[Davide Bertola]

ok

[Davide Bertola]

thank you