Why does it matter if I call a composable from within a `when` in every branch, vs. producing a valu...

# compose

[ursus]

Why does it matter if I call a composable from within a when in every branch, vs. producing a value with the when and calling the composable with the result?

Former seems to lag the scrolling, while latter doesn’t

Code in thread

[ursus]

@Composable
private fun MyContent(
    state: AirPodsBatteryCheckerState,
    scrollState: ScrollState
) {
    when (state) {
        AirPodsBatteryCheckerState.Connecting -> Body(null, scrollState)
        is AirPodsBatteryCheckerState.Connected -> Body(state.airPodsBatteryLevel, scrollState)
        is AirPodsBatteryCheckerState.Error -> Body(AirPodsBatteryLevel(0, 0, 0), scrollState)
    }
}

@Composable
private fun MyContent2(
    state: AirPodsBatteryCheckerState,
    scrollState: ScrollState
) {
    val batteryLevel = when (state) {
        AirPodsBatteryCheckerState.Connecting -> null
        is AirPodsBatteryCheckerState.Connected -> state.airPodsBatteryLevel
        is AirPodsBatteryCheckerState.Error -> AirPodsBatteryLevel(0, 0, 0)
    }
    Body(batteryLevel, scrollState)
}

[Filip Wiesner]

The first variant removes the entire Body UI tree and replaces it with new one. The second variant just updates input params.

[ursus]

I hear what you’re saying but I don’t see it 😄

Isn’t the seconds doing the same? and then it gets diffed internally etc.

[Filip Wiesner]

I don't think this works like this.
If I remember correctly, in MyContent compose compiler inserts replacable group which switches between two different Body nodes.

[ursus]

because of the control flow? if when , something else?

[Filip Wiesner]

Exactly

[ursus]

something else to watch out for?

[Filip Wiesner]

Probably any code with conditional logic where only one output is possible 🤷

[ursus]

Blbe je ze si to vsimnem len tak ze scrollovanie lagne, co moze lagovat aj tak v debug mode

[Filip Wiesner]

Source code KDoc

A replacable group is a group that cannot be moved during execution and can only either inserted, removed, or replaced. For example, the group created by most control flow constructs such as an if statement are replacable groups.

[Filip Wiesner]

Blbe je ze si to vsimnem len tak ze scrollovanie lagne, co moze lagovat aj tak v debug mode

It's not that hard to notice in code. If there is if , when or any other conditional logic, expect the UI to be replaced instead of restarted.

Also please write in english so people can use search and find relevant information 🙏

[Zach Klippenstein (he/him) [MOD]]

Composable identity is always tied to source code location, unless using movableContentOf. Calling the exact same composable with the exact same arguments from two different places in source will result in completely different state being spun up, maintained, and torn down for each.

[Alex Vanyo]

It’s also not just the singular source code location of where a particular composable is being called, it’s the source code locations of the whole call hierarchy.