Can someone explain why ```LocalConfiguration.current``` line exists in `calculateWindowSizeClass(ac...

# compose

[lilypuchi]

Can someone explain why

LocalConfiguration.current

line exists in calculateWindowSizeClass(activity: Activity) ? Am I wrong in thinking that it does nothing here? 🤔

https://cs.android.com/androidx/platform/frameworks/support/+/androidx-main:compose/[…]ndroidWindowSizeClass.android.kt;l=37?q=calculateWindowSi&sq=

[Filip Wiesner]

I think it's explained in the comment above:

// Observe view configuration changes and recalculate the size class on each change. We can't
// use Activity#onConfigurationChanged as this will sometimes fail to be called on different
// API levels, hence why this function needs to be @Composable so we can observe the
// ComposeView's configuration changes.

[lilypuchi]

Oh so just calling LocalConfiguration.current triggers Recomposition? 🤔

[Filip Wiesner]

I believe so

[Louis Pullen-Freilich [G]]

Yes, reading this will cause this to recompose when LocalConfiguration.current changes. And LocalConfiguration.current changes whenever onConfigurationChanged is called on the ComposeView. So essentially this will recompose everytime onConfigurationChanged is called on the view

[nitrog42]

the same mecanism is used for stringResource() btw !

@Composable
@ReadOnlyComposable
private fun resources(): Resources {
    LocalConfiguration.current
    return LocalContext.current.resources
}

@Composable
@ReadOnlyComposable
fun stringResource(@StringRes id: Int): String {
    val resources = resources()
    return resources.getString(id)
}

[lilypuchi]

Oh you’re right. Well it’s a TIL for me 😃
Thanks all!

[hfhbd]

But why does resources need to call LocalConfiguration.current twice?

[lilypuchi]

Even I misread it the first time. The later one is LocalContext not config blob sweat smile

[hfhbd]

Oh 🤦‍♂️ Thanks 😄

[Landry Norris]

This appears generalizable to any state variable. In one of my projects, I had a really weird edge case where I needed to recalculate something whenever a recomposition occurred, and I needed recomposition to occur when a variable changed, so I just had a line that read the variable with a comment explaining the mechanism and why the variable couldn’t be removed.