IssueTracker

The reason this is happening is that,

1. BoxWithConstraints creates a separate composition that is registered with the Recomposer
2. When composition starts the Recomposer collects all invalidated compistions (in thie case, all composition that read value) and schedules them to recompose.
3. It then recomposes all the recompositions with no regard to any dependency order (it is in order of registration which is loosly dependency order but not guaranteed to be).
4. The primary composition detects that BoxWithConstraints should be removed and schedules it and the sub-composition to be disposed but the disposal doesn't occur until applyChanges()
5. The sub-composition then recomposes with the value which violates the implied assumption.

We do not currently guarantee that the code invoked in a sub-composition of content that is removed from the parent composition prevents the contained sub-compositions from executing.

To fix this would require,

1. Ensuring all composition is performed in strict dependency order (parent first, sub-compositions after parents).
2. Communicate to the sub-compsoitions when they are to be disposed by the parent and ignore compositions in the round they are to be disposed.

As we didn't intended this constraint to hold I am changing this is not technically a bug. As the constraint seems reasonable and ensuring it holds does not seem too expensive nor would break currently working code, we should enforce this constraint and I am changing this to be a feature request and keeping it open.

This is a critical bug please fix as soon as possible! I get a lot of crashes because of this! This is not a Feature Request!

If crashes appear in your own code you can mitigate the issue by moving all crash-causing state reads outside of SubcomposeLayout, e.g. my sample from the issue description would may look like this:

Column {
    var value by remember { mutableStateOf(true) }
    Button(
        onClick = {
            value = false
        }
    ) {
        Text("Change state")
    }

    if (value) {
        val actualValue = value
        BoxWithConstraints() {
            assert(actualValue)
        }
    }
}

In this case the issue doesn't occur, because the state is read in the parent composition. Alternatively, you can extract the SubcomposeLayout into a separate function and pass values as parameters. It does the same trick:

Column {
    var value by remember { mutableStateOf(true) }
    Button(
        onClick = {
            value = false
        }
    ) {
        Text("Change state")
    }

    if (value) {
        SomeLayout(value)
    }
}

@Composable
private fun SomeLayout(value: Boolean) {
    BoxWithConstraints() {
        assert(value)
    }
}

I have nested NavHost and this is what happens in the inner one.

But does NavHost/AnimatedNavHost even use SubcomposeLayout? Are you sure it is related to this issue?

Compose does not ensure repeatable read consistency across a sub-composition boundary (that is we do not guarantee that a mutable value read by the parent composition will be the same value read by the sub-composition).

However, as I noted above, we will be changing the runtime to avoid recomposing sub-compositions that have been detected in the parent composition as being removed, point (2) in comment #3 above. This change should land this week. This changes requires an API change to how composition interacts with the recomposer so it is not a candidate for cherry-picks to earlier versions and, as it is a behavior change, requires going though the our normal release cycle to ensure it doesn't regress behavior or performance. We will also be landing, as a separate change, a solution to (1) above, which will also require API changes. (1) is riskier and we are doing it as a separate change as (2) is strictly an improvement over current behavior (much less likely to cause a regression) than (1) and can land independently of (1).

These two changes will change the rules so that the assumption how data dependencies are handled across sub-composition boundaries, as is implied by the assert above in comment #1, will be honored for composition scheduled by the recomposer. However, as composition can occur at anytime for a sub-composition, there are cases where the assert above will throw and assuming it will always being true should not be relied on. These changes make it so the recomposer, at least, will not introduce violations to this assumption, and will require explicit calls to setContent of a Composition to be violated. When and why setContent is called is in the hands to the code that created the composition, not the recomposer, or the rest of the runtime, which is why we cannot guarantee the assert assumption will always hold.

Theses changes will not ensure repeatable read consistency across a sub-composition boundary, but they will make it much easier for code to avoid requiring it by avoiding recomposing code that is being removed. Because repeatable read consistency is not guaranteed it is recommended that code not rely on it and avoid it by avoiding repeating reads as shown in the examples in comment #5.

I am sorry if my calling this a feature request above implied a lack of priority. We are treating this change as a high priority change for the next feature release. As it requires an API change and a change to the rules of how composition works I think of it as a feature request but will leave it marked as a bug type to avoid confusion.

As Ben and I found a relatively fast and safe way to fix this that didn't involve an API changes, we have started the process to cherry-pick this to earlier releases.

Which version is this in / going to be in?

This issue will be updated when it lands in a stable release.

I could not reproduce the crash above with 1.6.0-alpha01

The steps I took where:

1. Created a new android project using Android Studio Giraffe | 2022.3.1 RC 1
2. Replaced Greeting with the code in #1 (fixing the imports)
3. Ran the program
4. Pressed the button (crash reproduced)
5. Added the line implementation("androidx.compose.runtime:runtime:1.6.0-alpha01") to the dependencies
6. Update the compile sdk to 34.
7. Sync'ed gradle
8. Ran the program
9. Pressed the button (no crash)

Note the above fix is only a partial fix. There are cases that will continue to violate the implied constraint until the second part of the fix lands (explained above).

Also note that this doesn't guarantee the constraint implied by the assert will always hold, only that the runtime will try to avoid recomposing sub-compositions that might be exposed to the constraint. Any use of sub-composition can violate this constraint as the value might change between the time the main composition composes and the sub-composition composes and before the main composition can be recomposed to remove the sub-composition.

If you require consistency between a composition and its subcomposition you need to capture the state as #5, above. This change just avoids a common case would violate this constraint.