Why is it that whenever calling a callback passed ...

# compose-android

[dave08]

Why is it that whenever calling a callback passed to a composable inside, say, onClick of a button, I never saw anyone using rememberUpdatedState, whereas when calling it from a LaunchedEffect, it seems to be needed? And when would such a callback be changed? It's usually something like navController.navigate(...) that usually stays the same for that composable...

[vide]

The .clickable modifier updates the saved lambda on every recomposition if it has changed. LaunchedEffect does not restart if its keys do not change.

[vide]

You can check AbstractClickableNode and it's updateCommon method

[dave08]

But why would the lambda ever change?

[dave08]

If in the call site there would be an if to decide what to do based on something outside of it, maybe I'd understand... but if not, then we still need to use rememberUpdatedState?

[vide]

who knows, depends on what you want to implement 😄 it's not restricted to lambdas but any arbitrary values that might change

[yschimke]

Isn't any lambda a really cheap objection allocation on each run through? I forget if it's more efficient with references

[vide]

the point isn't really in allocating the lambda, it's that the LaunchedEffect lambda holds a reference to the old value in its closure even if it changes

[vide]

consider this:

@Composable fun Foo(activate: () -> Unit) {
  LaunchedEffect(Unit) {
    delay(5000)
    activate()
  }
}

vs

@Composable fun Foo(activate: () -> Unit) {
  val updatedActivate = rememberUpdatedState(activate)
  LaunchedEffect(Unit) {
    delay(5000)
    updatedActivate.value()
  }
}

[vide]

this Foo cannot know in advance if it will be ever used by a parent that will recompose it with a different activate before it calls activate()

[dave08]

How could that happen on the one calling Foo?

[dave08]

@Composable fun Bar(nav: NavController) { Foo { nav.navigate("some-destination") } }

would never change... what would be considered that could change?

[dave08]

@Composable fun Bar(nav: NavController, dest: String) { Foo { nav.navigate(dest) } }

?

[dave08]

Then the LaunchedEffect would be cancelled already no?

[vide]

well in that case it wouldn't change unless you create a new navcontroller

[vide]

but consider the case where you want to navigate to a different location based on some other condition

[dave08]

When LaunchedEffect enters the composition it will launch block into the composition's CoroutineContext. The coroutine will be cancelled and re-launched when LaunchedEffect is recomposed with a different key1. The coroutine will be cancelled when the LaunchedEffect leaves the composition.

[dave08]

Unless leaving the composition means that Foo isn't called on recomposition?

[vide]

@Composable fun Qux(isLoggedIn: Boolean) {
    Bar(navController, if (isLoggedIn) "target1" else "target2")
}

[vide]

qux could be called while logged in, but then the user logs out or is logged out by some external event before the timeout completes

[vide]

the first variant of Foo would navigate to target1, while the second variant would navigate to target2

[vide]

you could of course, for example, add the key to the launchedEffect, but then the timeout would also be ran again

[vide]

LaunchedEffect(activate) { ... }

[dave08]

I see... so we're just protecting ourselves from such cases... but when static, it's superfluous.

And so when is it that it "leaves the composition" that the LaunchedEffect gets cancelled?

[vide]

(of course this is just a hypothetical example, but I'm just trying to demonstrate a case where the difference does matter)

[vide]

as a general rule, you would only need rememberUpdatedState with LaunchedEffect when you have a long running operation in LaunchedEffect that you also don't want to restart for some reason

[dave08]

you also don't want to restart for some reason

By using `LaunchedEffect`'s parameter to restart on that lambda change instead? But otherwise, it would only get cancelled when Foo doesn't get called on a future recomposition and the task is still running?

[vide]

yes, exactly

[vide]

if you don't specify any keys, or the key never changes, it will get cancelled only when it's removed from the composition

[dave08]

Wow, thanks for the explanation 😄! It's the kind of thing that's hard to grasp at first...

[vide]

yeah, it takes some time to adjust your mindset to declarative UI

[vide]

glad I could help