Observing the fold

Read the fold state in a view-model with rememberFoldStateFlow, react to posture changes, and animate between postures.

Fold state comes from the window your UI runs in, so it exists only inside a composition. rememberFoldStateFlow() hands it to code that is not composable, such as a view-model.

Pass the state to a view-model

Call it high in the tree and pass the flow down.

import androidx.compose.runtime.Composable
import androidx.compose.runtime.LaunchedEffect
import androidx.compose.runtime.collectAsState
import androidx.compose.runtime.getValue
import com.jantursky.crease.FoldState
import com.jantursky.crease.Posture
import com.jantursky.crease.postures
import com.jantursky.crease.rememberFoldStateFlow
import kotlinx.coroutines.flow.StateFlow

class PlayerModel {
    fun onPosture(posture: Posture) {
        // Pause, resize the camera preview, log an event.
    }
}

@Composable
fun PlayerHost(model: PlayerModel) {
    val foldState: StateFlow<FoldState> = rememberFoldStateFlow()
    LaunchedEffect(foldState) {
        foldState.postures().collect { model.onPosture(it) }
    }
    val current by foldState.collectAsState()
    // current.hinge, current.folds
}

The flow follows rememberFoldState(), so a state set by ProvideFoldState shows up in it too. It updates only while the call is in the composition. When the call leaves, collectors keep the last value.

React to posture changes only

postures() emits the current posture first and then only when it changes. A change of hinge angle inside a posture emits nothing, so a collector that restarts a camera or a player does not fire on every degree.

Animate between postures

rememberPostureTransition() gives the posture before the latest change and the current one.

import androidx.compose.animation.AnimatedContent
import androidx.compose.material3.Text
import androidx.compose.runtime.Composable
import com.jantursky.crease.Posture
import com.jantursky.crease.rememberPostureTransition

@Composable
fun PostureBanner() {
    val transition = rememberPostureTransition()
    AnimatedContent(targetState = transition.current) { posture ->
        Text(
            if (transition.changed && posture == transition.current) {
                "${transition.previous} to $posture"
            } else {
                posture.toString()
            },
        )
    }
}

changed means the posture has changed since the composition began, not that it just changed this frame. It recomposes only when the posture changes.