iOS-like Predictive Back Animation in Kotlin Multiplatform (Compose)

github-gist-code.kt

import androidx.compose.animation.core.Animatable
import androidx.compose.runtime.getValue
import androidx.compose.runtime.mutableFloatStateOf
import androidx.compose.runtime.setValue
import androidx.compose.ui.Modifier
import androidx.compose.ui.draw.drawWithContent
import androidx.compose.ui.graphics.Color
import androidx.compose.ui.graphics.graphicsLayer
import com.arkivanov.decompose.ExperimentalDecomposeApi
import com.arkivanov.decompose.extensions.compose.stack.animation.predictiveback.PredictiveBackAnimatable
import com.arkivanov.essenty.backhandler.BackEvent

@OptIn(ExperimentalDecomposeApi::class)
internal class IosLikePredictiveBackAnimatableImpl(
    private val scrimColor: Color = Color(0x66000000), // 40% Black overlay
) : PredictiveBackAnimatable {

    // progress: 0f = gesture not started, 1f = gesture fully swiped
    private val progressAnim = Animatable(0f)
    private var rawProgress: Float by mutableFloatStateOf(0f)

    override val exitModifier: Modifier
        get() = Modifier.graphicsLayer {
            // Active screen follows finger: 0% → 100% of its own width
            translationX = rawProgress * size.width
        }

    override val enterModifier: Modifier
        get() = Modifier
            .graphicsLayer {
                // Previous screen parallaxes from -30% at rest toward 0 on complete
                translationX = (1f - rawProgress) * (-0.30f * size.width)
            }
            .drawWithContent {
                drawContent()
                // Scrim fades from 40% at rest to 0 as gesture progresses
                val scrimAlpha = (1f - rawProgress) * scrimColor.alpha
                if (scrimAlpha > 0f) {
                    drawRect(color = scrimColor.copy(alpha = scrimAlpha))
                }
            }

    override suspend fun animate(event: BackEvent) {
        rawProgress = event.progress
    }

    override suspend fun finish() {
        rawProgress = 1f
    }

    override suspend fun cancel() {
        // Snap progressAnim to actual visible position so the spring 
        // starts from where the finger left off.
        progressAnim.snapTo(rawProgress)
        progressAnim.animateTo(targetValue = 0f) {
            rawProgress = value
        }
    }
}