Support animating an overlay out after returning a result

circuit-overlay/src/commonMain/kotlin/com/slack/circuit/overlay/ContentWithOverlays.kt

@@ -2,12 +2,18 @@
 // SPDX-License-Identifier: Apache-2.0
 package com.slack.circuit.overlay
 
+import androidx.compose.animation.AnimatedContent
+import androidx.compose.animation.EnterTransition
+import androidx.compose.animation.ExitTransition
+import androidx.compose.animation.SizeTransform
+import androidx.compose.animation.core.snap
+import androidx.compose.animation.togetherWith
 import androidx.compose.foundation.layout.Box
 import androidx.compose.runtime.Composable
 import androidx.compose.runtime.CompositionLocalProvider
 import androidx.compose.runtime.getValue
-import androidx.compose.runtime.key
 import androidx.compose.runtime.rememberUpdatedState
+import androidx.compose.ui.Alignment
 import androidx.compose.ui.Modifier
 
 /**
@@ -24,11 +30,30 @@ public fun ContentWithOverlays(
   overlayHost: OverlayHost = rememberOverlayHost(),
   content: @Composable () -> Unit
 ) {
+  val overlayHostData by rememberUpdatedState(overlayHost.currentOverlayData)
   CompositionLocalProvider(LocalOverlayHost provides overlayHost) {
-    val overlayHostData by rememberUpdatedState(overlayHost.currentOverlayData)
     Box(modifier) {
       content()
-      key(overlayHostData) { overlayHostData?.let { data -> data.overlay.Content(data::finish) } }
+      AnimatedContent(
+        targetState = overlayHostData,
+        transitionSpec = {
+          val enter =
+            (targetState?.overlay as? AnimatedOverlay)?.enterTransition ?: EnterTransition.None
+          val exit =
+            (initialState?.overlay as? AnimatedOverlay)?.exitTransition ?: ExitTransition.None
+          val sizeTransform = if (targetState != null) SizeTransform { _, _ -> snap(0) } else null
+          (enter togetherWith exit).using(sizeTransform).also {
+            it.targetContentZIndex = targetState?.let { 1f } ?: -1f
+          }
+        },
+        contentAlignment = Alignment.Center
+      ) { data ->
+        when (val overlay = data?.overlay) {
+          null -> Unit
+          is AnimatedOverlay -> with(overlay) { AnimatedContent(data::finish) }
+          else -> overlay.Content(data::finish)
+        }
+      }
     }
   }
 }

circuit-overlay/src/commonMain/kotlin/com/slack/circuit/overlay/Overlay.kt

@@ -2,6 +2,11 @@
 // SPDX-License-Identifier: Apache-2.0
 package com.slack.circuit.overlay
 
+import androidx.compose.animation.AnimatedContent
+import androidx.compose.animation.AnimatedVisibilityScope
+import androidx.compose.animation.EnterTransition
+import androidx.compose.animation.ExitTransition
+import androidx.compose.animation.togetherWith
 import androidx.compose.runtime.Composable
 import androidx.compose.runtime.ProvidableCompositionLocal
 import androidx.compose.runtime.Stable
@@ -166,6 +171,30 @@ public interface Overlay<Result : Any> {
   @Composable public fun Content(navigator: OverlayNavigator<Result>)
 }
 
+/**
+ * An [Overlay] that can be animated in and more interestingly out _after_ it has returned a
+ * [Result]. The [Overlay] is animated in and out by its [enterTransition] and [exitTransition],
+ * [AnimatedContent] is executed with with [AnimatedVisibilityScope] so that child animations can be
+ * coordinated with the overlay's animations.
+ */
+public abstract class AnimatedOverlay<Result : Any>(
+  public val enterTransition: EnterTransition,
+  public val exitTransition: ExitTransition,
+) : Overlay<Result> {
+  @Composable
+  final override fun Content(navigator: OverlayNavigator<Result>) {
+    AnimatedContent(
+      targetState = Unit,
+      transitionSpec = { EnterTransition.None togetherWith ExitTransition.None },
+    ) {
+      AnimatedContent(navigator)
+    }
+  }
+
+  @Composable
+  public abstract fun AnimatedVisibilityScope.AnimatedContent(navigator: OverlayNavigator<Result>)
+}
+
 /** A [ProvidableCompositionLocal] to expose the current [OverlayHost] in the composition tree. */
 public val LocalOverlayHost: ProvidableCompositionLocal<OverlayHost> = compositionLocalOf {
   error("No OverlayHost provided")