Why doesn’t `Modifier.testTag("42")` cause a recomposition of the function it’s passed to, but `Modi...

# compose

[Alexander Maryanovsky]

Why doesn’t Modifier.testTag("42") cause a recomposition of the function it’s passed to, but Modifier.testTag(functionThatReturns42()) does?

[Alexander Maryanovsky]

This prints “(Re)composing MyText” twice:

@Composable
private fun MyText(
    modifier: Modifier
) {
    println("(Re)composing MyText")
    Text("Hello", modifier = modifier)
}

private fun makeTestTag(): String {
    return "42"
}


fun main() = singleWindowApplication {
    var number by remember { mutableStateOf(1) }
    Column {
        Text("$number")
        MyText(
            Modifier.testTag(makeTestTag())
        )
    }
    LaunchedEffect(Unit){
        delay(2000)
        number += 1
    }
}

[Alexander Maryanovsky]

But if I replace makeTestTag() with "42" it prints it only once.

[Alexander Maryanovsky]

wow, it prints it twice even if makeTestTag is inline!

[Alexander Maryanovsky]

That has got to be a bug in the compose compiler, no?

[ascii]

Maybe comparison is ===? "42" is constant; function is evaluated every time number changes, thus returns a new string every time.

[ascii]

even if makeTestTag is inline

hmm no idea

[Alexander Maryanovsky]

“42” is an interned constant regardless of where it appears. At least that’s how it is in Java.

[Alexander Maryanovsky]

But regardless, I don’t think it should compare references.

[shikasd]

"42" is inferred to be static (const), and modifier testTag has stable annotation on it, so if expression you pass there is constant, we infer it as never changing
in the second case, modifier is not inferred as static because of the function call, so new instance of modifier causes recomposition.

to fix this, consider hoisting modifier to the scope that does not recompose, or you can technically try applying @stable to the function that returns const value

[Alexander Maryanovsky]

Ah, I forgot functions could also be marked as @Stable. Yeah, that solves it.