Hello,

I found a bug that cost me one day to understand what triggers it, and I want to thank Adam Powell for helping me a bit to figure out what's going on.

TL;DR

In short, if you call setContentView with the same view hierarchy a second time after the initial render, the composables in the hierarchy will stop recomposing, which breaks/freezes everything in them.

Jetpack Compose release version: 1.1.0-alpha02 (also reproduces with 1.0 and 1.0.1)

Android Studio Build: irrelevant

Kotlin version: irrelevant

Reproducer

I attached a self-contained Kotlin file (MainActivity.kt) that reproduces the issue.

You can paste it in any Compose project over the existing MainActivity and try yourself.

However, I put below reproducing steps anyway.

Steps to Reproduce:

Create a ComposeView that displays a State that changes over time.
Put that instance into a local property.
Call setContentView passing that instance in the onCreate function of the host ComponentActivity.
Launch a coroutine in lifecycleScope
Call awaitFrame(), or delay(1500)
Call setContentView again, with the very same instance of the ComposeView created earlier.

Expected outcome

The composable keeps display the updates of the State instance.

Actual result

Once the setContentView is called the second time, the composable stops displaying the updates of the State instance.

The Slack thread from start to finish

If you want to see what was the process from facing the issue, to finding why it happened, you can take a look at this thread on Kotlin's Slack:

https://kotlinlang.slack.com/archives/CJLTWPH7S/p1629894233127300

What is likely the culprit IMHO

In the Android platform, the setContentView method in Activity is idempotent. Calling it a second time with the same view hierarchy while the Activity is not in the destroyed state will have no effect.

However, that contract seems to have been broken by the ComponentActivity class from AndroidX, as it's also calling a function named initViewTreeOwners().

The name of that function indicates that it's been designed for initialization, something which by definition, is supposed to happen only once. However, a setter doesn't generally have such constraints, at least, not because it's a setter (as the setContentView function is).

I think that this initViewTreeOwners() function being called again is what messes up ComposeView or its underlying mechanisms, putting them in a state they should not be in.

However, the issue might not be in ComponentActivity itself, but in the Compose runtime, or both.

Thanks for reading, and have a great day!

Louis CAD