I'm using compose material navigation so that I ca...

# compose

[Colton Idle]

I'm using compose material navigation so that I can navigate to a bottom sheet destination. It works great, except on a single composable screen where I have

BackHandler { myLambda() }

Even though the bottom sheet is showing, my backHandler still get triggered. What's the "right" way to prevent this?

[Berkeli Alashov]

did you mean even when not showing? If yes, checkout https://kotlinlang.slack.com/archives/CJLTWPH7S/p1628046266460700 tl;dr: use BackHandler conditionally when sheet is showing

[Colton Idle]

Nope. My issue is that when my bottom sheet is showing, and I hit the back button. The bottom sheet doesn't dismiss, it just calls myLamda()

[Berkeli Alashov]

Don't you change sheet's state in myLambda then?

[Berkeli Alashov]

i.e it won't dismiss automatically, you need to change sheet's state to collapsed in BackHandler#onBack

[Colton Idle]

Nope. compose navigation material with bottom sheet destinations takes care of state changes.

[Berkeli Alashov]

Your BackHandler is probably overriding navigation material's BackHandler, thus only myLambda gets invoked and sheet's state doesn't change

[Berkeli Alashov]

I haven't tried navigation material's bottomSheet yet, so I might be misunderstanding.

[Colton Idle]

Yeah, that's pretty much the conclusion I've come to which is why I came here asking if there's another way I should be approaching this.

[Ian Lake]

I mean, you said you want to handle back, so this seems expected? If you don't want to handle back, why are you using BackHandler at all?

[Colton Idle]

I guess it's not "expected" to me because my screen isn't the one in focus. The bottom sheet is?

[Colton Idle]

So it seems if that's purposeful (that backHandler gets triggered even when the screen isn't top-most) then I guess I need a way to tell when the screen isn't top most and use the enabled arg of BackHandler?

[Ian Lake]

Yep. BackHandler(enabled = navBackStackEntry.lifecycle.currentState == Lifecycle.State.RESUMED)

[Colton Idle]

Ah. Thanks @Ian Lake! Question off of that though. My BackHandler is in my SignInScreen composable which doesn't have access to navBackStackEntry.

Should I be passing that in, or should I be declaring this BackHandler up one level in my NavHost?

[Ian Lake]

LocalLifecycleOwner.current.lifecycle would point to the same object

[Colton Idle]

Oh cool. so this ends up working. Thanks Ian!

BackHandler(
    enabled = LocalLifecycleOwner.current.lifecycle.currentState == Lifecycle.State.RESUMED) {
    myLambda()
}

[Colton Idle]

Okay. so uh. I'm gonna keep playing around with this, but I spoke too soon. The above code doesn't work.

[Colton Idle]

Ian, any idea why the above didn't do what we thought it would do? i.e. stop handling back events when the bottom sheet is opened?

[Ian Lake]

I'm just guessing, but I suspect that you aren't being recomposed when the Lifecycle state actually changes (so what you'd actually want is a DisposableEffect that manages a LifecycleObserver that sets your enabled state object)

[Ian Lake]

I.e., ON_RESUME set enabled to true, ON_PAUSE, back to false (which would be your default)

[Colton Idle]

Oh interesting. i would have that it could recompose based on that code. I'll do a little more reading to see if I could come up with something. thanks for the tip.

[Colton Idle]

For anyone else following along... this is what I came up with.

A bit uglier than I thought it'd be but it works.

var backEnabled by remember { mutableStateOf(false) }
val lifecycle = LocalLifecycleOwner.current.lifecycle
DisposableEffect(LocalLifecycleOwner) {
    backEnabled = lifecycle.currentState == Lifecycle.State.RESUMED
    onDispose {}
}

BackHandler(enabled = backEnabled) { myLabda() }

My main is was that I couldn't figure out how to call LocalLifecycleOwner.current in DisposableEffect because it's a composable itself.

[Ian Lake]

Well, no. That won't recompose when the Lifecycle state changes

[Colton Idle]

Ah snap. You're right. 😅 I was so excited that it didn't call myLamda() when the bottom sheet was up, now to realize that it never calls it.

[Ian Lake]

I meant using a `LifecycleObserver`:

var backEnabled by remember { mutableStateOf(false) }
val lifecycle = LocalLifecycleOwner.current.lifecycle
DisposableEffect(lifecycle) {
    val observer = LifecycleEventObserver { _, event ->
      if (event == Lifecycle.Event.ON_RESUME) {
        backEnabled = true
      else if (event == Lifecycle.Event.ON_PAUSE) {
        backEnabled = false
      }
    }
    lifecycle.addObserver(observer)
    onDispose {
      lifecycle.removeObserver(observer)
    }
}

BackHandler(enabled = backEnabled) { myLabda() }

[Colton Idle]

Thanks. I will play around with this tomorrow. Threw that into my IDE and it can't resolve LifecycleStateObserver so it looks like I'm missing a dependency. Definitely underestimated this.

[Ian Lake]

Ah, that's because it is called LifecycleEventObserver, whoops. Corrected the code snippet