Will `AndroidView` recreate view when `LocalContext.current` changed?

# compose

[fengdai]

Will AndroidView recreate view when LocalContext.current changed?

[fengdai]

Wait… Can LocalContext.current change?

[Filip Wiesner]

LocalContext is StaticCompositionLocal which means that if it changes, it should recompose the whole composition tree.

Unlike compositionLocalOf, reads of a staticCompositionLocalOf are not tracked by the composer and changing the value provided in the CompositionLocalProvider call will cause the entirety of the content to be recomposed instead of just the places where in the composition the local value is used.

[fengdai]

Thanks, Filip. So, the answer is no, since recomposition doesn’t trigger AndroidView to invoke its factory lambda again to recreate view. Am I right?

[Filip Wiesner]

I think that the answer is yes. If I understand it correctly, the whole UI tree including the AndroidView is destroyed and created again.

[fengdai]

Well, the recomposition doesn’t destroy and recreate the whole tree. It just re-execute the root composable function to update the tree.

[fengdai]

The ‘root composable function’ is not accurate. It is the content of the CompositionLocalProvider where the value is provided.

[Filip Wiesner]

I mean yeah but I bet the AndroidView depends on the context so if the context changes (and that is a big if), it should be recreated.
Either way I would think that this is something the creators of Compose and more specifically AndroidView thought of and you should not have to think about things like this.
Or do you have some specific use case that relies on this behavior?

[fengdai]

If it does’t recreate the view, the previous context will be leaked. I believe that the Compose team can guarantee the correctness. I’m just curious about how the recreation is triggered (if it does recreate). But I can’t figure it out with the source code (maybe I missed something).

[tad]

I'm struggling to think of a case where *re*composition can occur with a different Context. If it could, then the View system would be similarly broken.

[tad]

Keep in mind that setContent and similar create a ComposeView.

[fengdai]

I’ve verified. The answer is no. Here is my code:

val localContext = LocalContext.current
var context by remember { mutableStateOf(localContext) }
CompositionLocalProvider(
    LocalContext provides context
) {
    Log.d("AndroidView", "compose: context - ${LocalContext.current}")
    AndroidView(factory = { context ->
        TextView(context).apply {
            Log.d("AndroidView", "create view: ${this.hashCode()}")
        }
    }) {
        Log.d("AndroidView", "update view: ${it.hashCode()}")
    }
}
LaunchedEffect(key1 = Unit) {
    delay(3000)
    context = ContextWrapper(context)
}

And log is:

compose: context - sample.MainActivity@da29b84
create view: 162893151
update view: 162893151
compose: context - android.content.ContextWrapper@d588329

[fengdai]

So, in the real world, LocalContext.current will never be changed?
https://kotlinlang.slack.com/archives/CJLTWPH7S/p1655712069528009?thread_ts=1655710925.339069&cid=CJLTWPH7S

[tad]

In Compose UI, the Context is what hosts ComposeView, which in turn runs the composition. If Context was able to change, then the View class hierarchy would be irreperably broken as it contains references to the containing Context all over the place.

That said, there is no mechanism to enforce this that's exposed in the compose-ui API, so you'll have to either: trust that LocalContext is an immutable property of the composition, or; structure your code to make no assumptions about the runtime value of LocalContext. In practice, both stances are the same, because staticCompositionLocalOf will trigger a recomposition of everything, as it's set within setContent on Android; therefore you can't really avoid being recomposed.

[tad]

In short, it will never change while your Activity is alive, hence all the APIs and years of contention about how to persist UI state on Android.

[fengdai]

Thanks, Tad. Your answer clarifies LocalContext for me.

[fengdai]

For anyone who is interested in my original question:

Will AndroidView recreate view when LocalContext.current changed?

Since:
• LocalContext.current changes triggers recomposition.
• Recomposition doesn’t trigger AndroidView to invoke its factory lambda again.
So, the answer is no.

https://kotlinlang.slack.com/archives/CJLTWPH7S/p1655778343809389?thread_ts=1655710925.339069&cid=CJLTWPH7S