Hey guys, I was looking at the `MaterialTheme` obj...

# compose-android

[Masoud Karimi]

Hey guys, I was looking at the MaterialTheme object and also its composable function. Something caught my attention. I don't understand why they are creating a copy of the colorScheme and then call the updateColorSchemeFrom extension function again. I think they are the same. Am I missing something here?

https://cs.android.com/androidx/platform/frameworks/support/+/androidx-main:compose/[…]ialTheme.kt;drc=bcaacc85477f959cac171b60258679b6244f512b;l=69

@Composable
fun MaterialTheme(
    colorScheme: ColorScheme = MaterialTheme.colorScheme,
    shapes: Shapes = MaterialTheme.shapes,
    typography: Typography = MaterialTheme.typography,
    content: @Composable () -> Unit
) {
    val rememberedColorScheme = remember {
        colorScheme.copy()
    }.apply {
        updateColorSchemeFrom(colorScheme) // This function also copy the values from colorScheme just like the copy function called inside the remember block.
    }
    val rippleIndication = rememberRipple()
    val selectionColors = rememberTextSelectionColors(rememberedColorScheme)
    CompositionLocalProvider(
        LocalColorScheme provides rememberedColorScheme,
        LocalIndication provides rippleIndication,
        LocalRippleTheme provides MaterialRippleTheme,
        LocalShapes provides shapes,
        LocalTextSelectionColors provides selectionColors,
        LocalTypography provides typography,
    ) {
        ProvideTextStyle(value = typography.bodyLarge, content = content)
    }
}

[Stylianos Gakis]

They are doing this so that they do not create a new object each time. Instead they create the copy once, and from that point on, on every recomposition if the colorScheme changes, then the resulting colorScheme will have all of its internal values (which are MutableState objects) will be updated accordingly, and since they are state as I said before, all the composable functions that read them will recompose appropriately.

So I think tl;dr of this is that it’s an optimization, not for correctness. That’s my understanding

[Masoud Karimi]

I'm not sure if I understand the point. My question is, why do they make two copy of the colorScheme? First, inside the remember block, and then inside the apply block. 🤔

[Stylianos Gakis]

The apply block will run on each recomposition, the block inside the remember will only run on first composition.
And as far as the first composition goes, yes both will run, but I assume updateColorSchemeFrom when provided with the exact same colors (which it always is the case on first composition) will be a no-op, so it doesn’t matter.

[Stylianos Gakis]

You might notice this pattern too if you look inside rememberUpdatedState
https://cs.android.com/androidx/platform/frameworks/support/+/androidx-main:compose/[…]berUpdatedState&ss=androidx%2Fplatform%2Fframeworks%2Fsupport

[Masoud Karimi]

So if I understand it well, they first make a copy of the ColorScheme in the first composition. Then, on each recomposition, they just update the properties. Am I right?

[Stylianos Gakis]

Yeah, every recomposition, updateColorSchemeFrom is called again, since it’s just part of the composable itself, not inside a lambda or some other side-effect composable. It just happens every single recomposition. It’s more or less the same as if they wrote this

SideEffect {
  rememberedColorScheme.updateColorSchemeFrom(colorScheme)
}

And they must be relying on the fact that updateColorSchemeFrom shouldn’t be expensive if the same colorScheme is passed in there over and over again.

[Stylianos Gakis]

Ah and it does seem like all it does is set some compose state objects https://cs.android.com/androidx/platform/frameworks/support/+/androidx-main:tv/tv-ma[…]ColorSchemeFrom&ss=androidx%2Fplatform%2Fframeworks%2Fsupport
So if the object is equivalent, this simply does nothing, no recompositions happen, it just skips that change.

[Masoud Karimi]

Yes I just saw that. Now I think I know it by heart. I just read about the difference between remember and rememberUpdatedState and I finally got the point. Thank u very much 👍

https://stackoverflow.com/a/70223293
https://proandroiddev.com/jetpack-compose-side-effects-iii-rememberupdatedstate-c8df7b90a01d