This composable (i thought) was supposed to run a ...

# compose-android

[Colton Idle]

This composable (i thought) was supposed to run a block of code onResume of the containing activity. It seems to do that + it will run if I switch tabs, and then back to this one. Am I misunderstanding something (note: I know that new convenience methods for things like this now exist, but im debugging some older code)

@Composable
fun StartOnResume(block: () -> Unit) {
  val lifecycle = LocalLifecycleOwner.current.lifecycle
  DisposableEffect(lifecycle) {
    val observer = LifecycleEventObserver { _, event ->
      if (event == Lifecycle.Event.ON_RESUME) {
        block()
      }
    }
    lifecycle.addObserver(observer)
    onDispose { lifecycle.removeObserver(observer) }
  }
}

[Stylianos Gakis]

If “Switch tabs” you mean navigating from a destination to another one, then that makes total sense yes.
Navigation (at least androidx.navigation) as soon as you start any navigation even turns the local lifecycle of the screen from Resumed to created while the animation is playing, and when it settles the new one that has come on the screen will be turned back into Resumed again.

[Colton Idle]

Interesting. I thought the lifecycle owner here was the activity and so it would only call onresume when the activitiy resumed

[Stylianos Gakis]

Depends where you are. LocalLifecycleOwner if you are inside a androidx.navigation composable is in fact the lifecycle of that destination’s NavBackStackEntry.
See here https://x.com/ianhlake/status/1760066373571797239

[Colton Idle]

interesting....

[Colton Idle]

TIL

[Colton Idle]

so the "screen" composable pretty much gets its own lifecycle

[Colton Idle]

i wonder if thats sorta new, or always worked that way. will investigate

[Stylianos Gakis]

Always has been 🧑‍🚀 🔫 🧑‍🚀

[Colton Idle]

So... I was just thinking about this again. this is kinda like composeable destination are kinda like "fragments" in that they get their own lifecycle. Does that seem right to you?

[Stylianos Gakis]

Yeah that sounds like a reasonable parallel to draw I think.
Look at this test https://cs.android.com/androidx/platform/frameworks/support/+/androidx-main:navigati[…]9?q=NavHostTest&ss=androidx%2Fplatform%2Fframeworks%2Fsupport to see how the backStackEntry?.lifecycle?.currentState is changing as things happen 😄

[Stylianos Gakis]

Basically as a navigation event starts, immediately no screens are in the Resumed state.
The one leaving is going to Created status, the one coming in goes to Started status.
When the animation is over, the new becomes Resumed.
The old one then:
• If it is in the backstack stays as Created
• If it’s a pop, then it just gets Destroyed completely