Add TransitionLink feature

Transitions can now be linked. E.g. when one STL transitions A->B then
the linked STL should automatically play C->D. The configuration can
be passed to SceneTransitionLayoutState via `transitionLinks`.

Test: SceneTransitionLayoutStateTest
Bug: b/320257219
Flag: NONE
Change-Id: I6192ddf489f1d2fd895b20af3b4428aa6e18fa32

4 files changed, 369 insertions, 51 deletions

diff --git a/packages/SystemUI/compose/scene/src/com/android/compose/animation/scene/SceneTransitionLayoutState.kt b/packages/SystemUI/compose/scene/src/com/android/compose/animation/scene/SceneTransitionLayoutState.kt
index 61d9bceef33b..2661301fcb83 100644
--- a/packages/SystemUI/compose/scene/src/com/android/compose/animation/scene/SceneTransitionLayoutState.kt
+++ b/packages/SystemUI/compose/scene/src/com/android/compose/animation/scene/SceneTransitionLayoutState.kt
@@ -24,6 +24,10 @@ import androidx.compose.runtime.getValue
 import androidx.compose.runtime.mutableStateOf
 import androidx.compose.runtime.remember
 import androidx.compose.runtime.setValue
+import androidx.compose.ui.util.fastFilter
+import androidx.compose.ui.util.fastForEach
+import com.android.compose.animation.scene.transition.link.LinkedTransition
+import com.android.compose.animation.scene.transition.link.StateLink
 import kotlin.math.absoluteValue
 import kotlinx.coroutines.CoroutineScope
 import kotlinx.coroutines.channels.Channel
@@ -101,8 +105,9 @@ sealed interface MutableSceneTransitionLayoutState : SceneTransitionLayoutState
 fun MutableSceneTransitionLayoutState(
     initialScene: SceneKey,
     transitions: SceneTransitions = SceneTransitions.Empty,
+    stateLinks: List<StateLink> = emptyList(),
 ): MutableSceneTransitionLayoutState {
-    return MutableSceneTransitionLayoutStateImpl(initialScene, transitions)
+    return MutableSceneTransitionLayoutStateImpl(initialScene, transitions, stateLinks)
 }
 
 /**
@@ -121,9 +126,12 @@ fun updateSceneTransitionLayoutState(
     currentScene: SceneKey,
     onChangeScene: (SceneKey) -> Unit,
     transitions: SceneTransitions = SceneTransitions.Empty,
+    stateLinks: List<StateLink> = emptyList(),
 ): SceneTransitionLayoutState {
-    return remember { HoistedSceneTransitionLayoutScene(currentScene, transitions, onChangeScene) }
-        .apply { update(currentScene, onChangeScene, transitions) }
+    return remember {
+            HoistedSceneTransitionLayoutScene(currentScene, transitions, onChangeScene, stateLinks)
+        }
+        .apply { update(currentScene, onChangeScene, transitions, stateLinks) }
 }
 
 @Stable
@@ -184,8 +192,10 @@ sealed interface TransitionState {
     }
 }
 
-internal abstract class BaseSceneTransitionLayoutState(initialScene: SceneKey) :
-    SceneTransitionLayoutState {
+internal abstract class BaseSceneTransitionLayoutState(
+    initialScene: SceneKey,
+    protected var stateLinks: List<StateLink>,
+) : SceneTransitionLayoutState {
     override var transitionState: TransitionState by
         mutableStateOf(TransitionState.Idle(initialScene))
         protected set
@@ -196,6 +206,8 @@ internal abstract class BaseSceneTransitionLayoutState(initialScene: SceneKey) :
      */
     internal var transformationSpec: TransformationSpecImpl = TransformationSpec.Empty
 
+    private val activeTransitionLinks = mutableMapOf<StateLink, LinkedTransition>()
+
     /**
      * Called when the [current scene][TransitionState.currentScene] should be changed to [scene].
      *
@@ -224,20 +236,68 @@ internal abstract class BaseSceneTransitionLayoutState(initialScene: SceneKey) :
             transitions
                 .transitionSpec(transition.fromScene, transition.toScene, key = transitionKey)
                 .transformationSpec()
-
+        cancelActiveTransitionLinks()
+        setupTransitionLinks(transition)
         transitionState = transition
     }
 
+    private fun cancelActiveTransitionLinks() {
+        for ((link, linkedTransition) in activeTransitionLinks) {
+            link.target.finishTransition(linkedTransition, linkedTransition.currentScene)
+        }
+        activeTransitionLinks.clear()
+    }
+
+    private fun setupTransitionLinks(transitionState: TransitionState) {
+        if (transitionState !is TransitionState.Transition) return
+        stateLinks.fastForEach { stateLink ->
+            val matchingLink =
+                stateLink.transitionLinks.firstOrNull() { it.isMatchingLink(transitionState) } ?: return@fastForEach
+
+            val targetCurrentScene = stateLink.target.transitionState.currentScene
+
+            if (targetCurrentScene != matchingLink.targetFrom) return@fastForEach
+
+            val linkedTransition =
+                LinkedTransition(
+                    originalTransition = transitionState,
+                    fromScene = targetCurrentScene,
+                    toScene = matchingLink.targetTo,
+                )
+
+            stateLink.target.startTransition(linkedTransition, matchingLink.targetTransitionKey)
+            activeTransitionLinks[stateLink] = linkedTransition
+        }
+    }
+
     /**
      * Notify that [transition] was finished and that we should settle to [idleScene]. This will do
      * nothing if [transition] was interrupted since it was started.
      */
     internal fun finishTransition(transition: TransitionState.Transition, idleScene: SceneKey) {
+        resolveActiveTransitionLinks(idleScene)
         if (transitionState == transition) {
             transitionState = TransitionState.Idle(idleScene)
         }
     }
 
+    private fun resolveActiveTransitionLinks(idleScene: SceneKey) {
+        val previousTransition = this.transitionState as? TransitionState.Transition ?: return
+        for ((link, linkedTransition) in activeTransitionLinks) {
+            if (previousTransition.fromScene == idleScene) {
+                // The transition ended by arriving at the fromScene, move link to Idle(fromScene).
+                link.target.finishTransition(linkedTransition, linkedTransition.fromScene)
+            } else if (previousTransition.toScene == idleScene) {
+                // The transition ended by arriving at the toScene, move link to Idle(toScene).
+                link.target.finishTransition(linkedTransition, linkedTransition.toScene)
+            } else {
+                // The transition was interrupted by something else, we reset to initial state.
+                link.target.finishTransition(linkedTransition, linkedTransition.fromScene)
+            }
+        }
+        activeTransitionLinks.clear()
+    }
+
     /**
      * Check if a transition is in progress. If the progress value is near 0 or 1, immediately snap
      * to the closest scene.
@@ -271,7 +331,8 @@ internal class HoistedSceneTransitionLayoutScene(
     initialScene: SceneKey,
     override var transitions: SceneTransitions,
     private var changeScene: (SceneKey) -> Unit,
-) : BaseSceneTransitionLayoutState(initialScene) {
+    stateLinks: List<StateLink> = emptyList(),
+) : BaseSceneTransitionLayoutState(initialScene, stateLinks) {
     private val targetSceneChannel = Channel<SceneKey>(Channel.CONFLATED)
 
     override fun CoroutineScope.onChangeScene(scene: SceneKey) = changeScene(scene)
@@ -281,10 +342,12 @@ internal class HoistedSceneTransitionLayoutScene(
         currentScene: SceneKey,
         onChangeScene: (SceneKey) -> Unit,
         transitions: SceneTransitions,
+        stateLinks: List<StateLink>,
     ) {
         SideEffect {
             this.changeScene = onChangeScene
             this.transitions = transitions
+            this.stateLinks = stateLinks
 
             targetSceneChannel.trySend(currentScene)
         }
@@ -308,7 +371,8 @@ internal class HoistedSceneTransitionLayoutScene(
 internal class MutableSceneTransitionLayoutStateImpl(
     initialScene: SceneKey,
     override var transitions: SceneTransitions,
-) : MutableSceneTransitionLayoutState, BaseSceneTransitionLayoutState(initialScene) {
+    stateLinks: List<StateLink> = emptyList(),
+) : MutableSceneTransitionLayoutState, BaseSceneTransitionLayoutState(initialScene, stateLinks) {
     override fun setTargetScene(
         targetScene: SceneKey,
         coroutineScope: CoroutineScope,
diff --git a/packages/SystemUI/compose/scene/src/com/android/compose/animation/scene/transition/link/LinkedTransition.kt b/packages/SystemUI/compose/scene/src/com/android/compose/animation/scene/transition/link/LinkedTransition.kt
new file mode 100644
index 000000000000..33b57b25fd10
--- /dev/null
+++ b/packages/SystemUI/compose/scene/src/com/android/compose/animation/scene/transition/link/LinkedTransition.kt
@@ -0,0 +1,46 @@
+/*
+ * Copyright (C) 2024 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.android.compose.animation.scene.transition.link
+
+import com.android.compose.animation.scene.SceneKey
+import com.android.compose.animation.scene.TransitionState
+
+/** A linked transition which is driven by a [originalTransition]. */
+internal class LinkedTransition(
+    private val originalTransition: TransitionState.Transition,
+    fromScene: SceneKey,
+    toScene: SceneKey,
+) : TransitionState.Transition(fromScene, toScene) {
+
+    override val currentScene: SceneKey
+        get() {
+            return when (originalTransition.currentScene) {
+                originalTransition.fromScene -> fromScene
+                originalTransition.toScene -> toScene
+                else -> error("Original currentScene is neither FromScene nor ToScene")
+            }
+        }
+
+    override val isInitiatedByUserInput: Boolean
+        get() = originalTransition.isInitiatedByUserInput
+
+    override val isUserInputOngoing: Boolean
+        get() = originalTransition.isUserInputOngoing
+
+    override val progress: Float
+        get() = originalTransition.progress
+}
diff --git a/packages/SystemUI/compose/scene/src/com/android/compose/animation/scene/transition/link/StateLink.kt b/packages/SystemUI/compose/scene/src/com/android/compose/animation/scene/transition/link/StateLink.kt
new file mode 100644
index 000000000000..9b51e447b58b
--- /dev/null
+++ b/packages/SystemUI/compose/scene/src/com/android/compose/animation/scene/transition/link/StateLink.kt
@@ -0,0 +1,62 @@
+/*
+ * Copyright (C) 2024 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.android.compose.animation.scene.transition.link
+
+import com.android.compose.animation.scene.BaseSceneTransitionLayoutState
+import com.android.compose.animation.scene.SceneKey
+import com.android.compose.animation.scene.SceneTransitionLayoutState
+import com.android.compose.animation.scene.TransitionKey
+import com.android.compose.animation.scene.TransitionState
+
+/** A link between a source (implicit) and [target] `SceneTransitionLayoutState`. */
+class StateLink(target: SceneTransitionLayoutState, val transitionLinks: List<TransitionLink>) {
+
+    internal val target = target as BaseSceneTransitionLayoutState
+
+    /**
+     * Links two transitions (source and target) together.
+     *
+     * `null` can be passed to indicate that any SceneKey should match. e.g. passing `null`, `null`,
+     * `null`, `SceneA` means that any transition at the source will trigger a transition in the
+     * target to `SceneA` from any current scene.
+     */
+    class TransitionLink(
+        val sourceFrom: SceneKey,
+        val sourceTo: SceneKey,
+        val targetFrom: SceneKey,
+        val targetTo: SceneKey,
+        val targetTransitionKey: TransitionKey? = null,
+    ) {
+        init {
+            if (
+                (sourceFrom != null && sourceFrom == sourceTo) ||
+                    (targetFrom != null && targetFrom == targetTo)
+            )
+                error("From and To can't be the same")
+        }
+
+        internal fun isMatchingLink(transition: TransitionState.Transition): Boolean {
+            return (sourceFrom == transition.fromScene) &&
+                (sourceTo == transition.toScene)
+        }
+
+        internal fun targetIsInValidState(targetCurrentScene: SceneKey): Boolean {
+            return (targetFrom == targetCurrentScene) &&
+                targetTo != targetCurrentScene
+        }
+    }
+}
diff --git a/packages/SystemUI/compose/scene/tests/src/com/android/compose/animation/scene/SceneTransitionLayoutStateTest.kt b/packages/SystemUI/compose/scene/tests/src/com/android/compose/animation/scene/SceneTransitionLayoutStateTest.kt
index 302fc0b08ab0..2a5a355f67fd 100644
--- a/packages/SystemUI/compose/scene/tests/src/com/android/compose/animation/scene/SceneTransitionLayoutStateTest.kt
+++ b/packages/SystemUI/compose/scene/tests/src/com/android/compose/animation/scene/SceneTransitionLayoutStateTest.kt
@@ -18,10 +18,14 @@ package com.android.compose.animation.scene
 
 import androidx.compose.ui.test.junit4.createComposeRule
 import androidx.test.ext.junit.runners.AndroidJUnit4
+import com.android.compose.animation.scene.TestScenes.SceneA
+import com.android.compose.animation.scene.TestScenes.SceneB
+import com.android.compose.animation.scene.TestScenes.SceneC
+import com.android.compose.animation.scene.TestScenes.SceneD
+import com.android.compose.animation.scene.transition.link.StateLink
 import com.android.compose.test.runMonotonicClockTest
 import com.google.common.truth.Truth.assertThat
 import kotlinx.coroutines.CoroutineStart
-import kotlinx.coroutines.cancel
 import kotlinx.coroutines.launch
 import org.junit.Rule
 import org.junit.Test
@@ -31,93 +35,235 @@ import org.junit.runner.RunWith
 class SceneTransitionLayoutStateTest {
     @get:Rule val rule = createComposeRule()
 
+    class TestableTransition(
+        fromScene: SceneKey,
+        toScene: SceneKey,
+    ) : TransitionState.Transition(fromScene, toScene) {
+        override var currentScene: SceneKey = fromScene
+        override var progress: Float = 0.0f
+        override var isInitiatedByUserInput: Boolean = false
+        override var isUserInputOngoing: Boolean = false
+    }
+
     @Test
     fun isTransitioningTo_idle() {
-        val state = MutableSceneTransitionLayoutStateImpl(TestScenes.SceneA, SceneTransitions.Empty)
+        val state = MutableSceneTransitionLayoutStateImpl(SceneA, SceneTransitions.Empty)
 
         assertThat(state.isTransitioning()).isFalse()
-        assertThat(state.isTransitioning(from = TestScenes.SceneA)).isFalse()
-        assertThat(state.isTransitioning(to = TestScenes.SceneB)).isFalse()
-        assertThat(state.isTransitioning(from = TestScenes.SceneA, to = TestScenes.SceneB))
-            .isFalse()
+        assertThat(state.isTransitioning(from = SceneA)).isFalse()
+        assertThat(state.isTransitioning(to = SceneB)).isFalse()
+        assertThat(state.isTransitioning(from = SceneA, to = SceneB)).isFalse()
     }
 
     @Test
     fun isTransitioningTo_transition() {
-        val state = MutableSceneTransitionLayoutStateImpl(TestScenes.SceneA, SceneTransitions.Empty)
-        state.startTransition(
-            transition(from = TestScenes.SceneA, to = TestScenes.SceneB),
-            transitionKey = null
-        )
+        val state = MutableSceneTransitionLayoutStateImpl(SceneA, SceneTransitions.Empty)
+        state.startTransition(transition(from = SceneA, to = SceneB), transitionKey = null)
 
         assertThat(state.isTransitioning()).isTrue()
-        assertThat(state.isTransitioning(from = TestScenes.SceneA)).isTrue()
-        assertThat(state.isTransitioning(from = TestScenes.SceneB)).isFalse()
-        assertThat(state.isTransitioning(to = TestScenes.SceneB)).isTrue()
-        assertThat(state.isTransitioning(to = TestScenes.SceneA)).isFalse()
-        assertThat(state.isTransitioning(from = TestScenes.SceneA, to = TestScenes.SceneB)).isTrue()
+        assertThat(state.isTransitioning(from = SceneA)).isTrue()
+        assertThat(state.isTransitioning(from = SceneB)).isFalse()
+        assertThat(state.isTransitioning(to = SceneB)).isTrue()
+        assertThat(state.isTransitioning(to = SceneA)).isFalse()
+        assertThat(state.isTransitioning(from = SceneA, to = SceneB)).isTrue()
     }
 
     @Test
     fun setTargetScene_idleToSameScene() = runMonotonicClockTest {
-        val state = MutableSceneTransitionLayoutState(TestScenes.SceneA)
-        assertThat(state.setTargetScene(TestScenes.SceneA, coroutineScope = this)).isNull()
+        val state = MutableSceneTransitionLayoutState(SceneA)
+        assertThat(state.setTargetScene(SceneA, coroutineScope = this)).isNull()
     }
 
     @Test
     fun setTargetScene_idleToDifferentScene() = runMonotonicClockTest {
-        val state = MutableSceneTransitionLayoutState(TestScenes.SceneA)
-        val transition = state.setTargetScene(TestScenes.SceneB, coroutineScope = this)
+        val state = MutableSceneTransitionLayoutState(SceneA)
+        val transition = state.setTargetScene(SceneB, coroutineScope = this)
         assertThat(transition).isNotNull()
         assertThat(state.transitionState).isEqualTo(transition)
 
         testScheduler.advanceUntilIdle()
-        assertThat(state.transitionState).isEqualTo(TransitionState.Idle(TestScenes.SceneB))
+        assertThat(state.transitionState).isEqualTo(TransitionState.Idle(SceneB))
     }
 
     @Test
     fun setTargetScene_transitionToSameScene() = runMonotonicClockTest {
-        val state = MutableSceneTransitionLayoutState(TestScenes.SceneA)
-        assertThat(state.setTargetScene(TestScenes.SceneB, coroutineScope = this)).isNotNull()
-        assertThat(state.setTargetScene(TestScenes.SceneB, coroutineScope = this)).isNull()
+        val state = MutableSceneTransitionLayoutState(SceneA)
+        assertThat(state.setTargetScene(SceneB, coroutineScope = this)).isNotNull()
+        assertThat(state.setTargetScene(SceneB, coroutineScope = this)).isNull()
         testScheduler.advanceUntilIdle()
-        assertThat(state.transitionState).isEqualTo(TransitionState.Idle(TestScenes.SceneB))
+        assertThat(state.transitionState).isEqualTo(TransitionState.Idle(SceneB))
     }
 
     @Test
     fun setTargetScene_transitionToDifferentScene() = runMonotonicClockTest {
-        val state = MutableSceneTransitionLayoutState(TestScenes.SceneA)
-        assertThat(state.setTargetScene(TestScenes.SceneB, coroutineScope = this)).isNotNull()
-        assertThat(state.setTargetScene(TestScenes.SceneC, coroutineScope = this)).isNotNull()
+        val state = MutableSceneTransitionLayoutState(SceneA)
+        assertThat(state.setTargetScene(SceneB, coroutineScope = this)).isNotNull()
+        assertThat(state.setTargetScene(SceneC, coroutineScope = this)).isNotNull()
         testScheduler.advanceUntilIdle()
-        assertThat(state.transitionState).isEqualTo(TransitionState.Idle(TestScenes.SceneC))
+        assertThat(state.transitionState).isEqualTo(TransitionState.Idle(SceneC))
     }
 
     @Test
     fun setTargetScene_transitionToOriginalScene() = runMonotonicClockTest {
-        val state = MutableSceneTransitionLayoutState(TestScenes.SceneA)
-        assertThat(state.setTargetScene(TestScenes.SceneB, coroutineScope = this)).isNotNull()
+        val state = MutableSceneTransitionLayoutState(SceneA)
+        assertThat(state.setTargetScene(SceneB, coroutineScope = this)).isNotNull()
 
         // Progress is 0f, so we don't animate at all and directly snap back to A.
-        assertThat(state.setTargetScene(TestScenes.SceneA, coroutineScope = this)).isNull()
-        assertThat(state.transitionState).isEqualTo(TransitionState.Idle(TestScenes.SceneA))
+        assertThat(state.setTargetScene(SceneA, coroutineScope = this)).isNull()
+        assertThat(state.transitionState).isEqualTo(TransitionState.Idle(SceneA))
     }
 
     @Test
     fun setTargetScene_coroutineScopeCancelled() = runMonotonicClockTest {
-        val state = MutableSceneTransitionLayoutState(TestScenes.SceneA)
+        val state = MutableSceneTransitionLayoutState(SceneA)
 
         lateinit var transition: TransitionState.Transition
         val job =
             launch(start = CoroutineStart.UNDISPATCHED) {
-                transition = state.setTargetScene(TestScenes.SceneB, coroutineScope = this)!!
+                transition = state.setTargetScene(SceneB, coroutineScope = this)!!
             }
         assertThat(state.transitionState).isEqualTo(transition)
 
         // Cancelling the scope/job still sets the state to Idle(targetScene).
         job.cancel()
         testScheduler.advanceUntilIdle()
-        assertThat(state.transitionState).isEqualTo(TransitionState.Idle(TestScenes.SceneB))
+        assertThat(state.transitionState).isEqualTo(TransitionState.Idle(SceneB))
+    }
+
+    private fun setupLinkedStates():
+            Pair<BaseSceneTransitionLayoutState, BaseSceneTransitionLayoutState> {
+        val parentState = MutableSceneTransitionLayoutState(SceneC)
+        val link =
+            listOf(
+                StateLink(
+                    parentState,
+                    listOf(StateLink.TransitionLink(SceneA, SceneB, SceneC, SceneD))
+                )
+            )
+        val childState = MutableSceneTransitionLayoutState(SceneA, stateLinks = link)
+        return Pair(
+            parentState as BaseSceneTransitionLayoutState,
+            childState as BaseSceneTransitionLayoutState
+        )
+    }
+
+    @Test
+    fun linkedTransition_startsLinkAndFinishesLinkInToState() {
+        val (parentState, childState) = setupLinkedStates()
+
+        val childTransition = TestableTransition(SceneA, SceneB)
+
+        childState.startTransition(childTransition, null)
+        assertThat(childState.isTransitioning(SceneA, SceneB)).isTrue()
+        assertThat(parentState.isTransitioning(SceneC, SceneD)).isTrue()
+
+        childState.finishTransition(childTransition, SceneB)
+        assertThat(childState.transitionState).isEqualTo(TransitionState.Idle(SceneB))
+        assertThat(parentState.transitionState).isEqualTo(TransitionState.Idle(SceneD))
+    }
+
+    @Test
+    fun linkedTransition_transitiveLink() {
+        val parentParentState =
+            MutableSceneTransitionLayoutState(SceneB) as BaseSceneTransitionLayoutState
+        val parentLink =
+            listOf(
+                StateLink(
+                    parentParentState,
+                    listOf(StateLink.TransitionLink(SceneC, SceneD, SceneB, SceneC))
+                )
+            )
+        val parentState =
+            MutableSceneTransitionLayoutState(SceneC, stateLinks = parentLink)
+                as BaseSceneTransitionLayoutState
+        val link =
+            listOf(
+                StateLink(
+                    parentState,
+                    listOf(StateLink.TransitionLink(SceneA, SceneB, SceneC, SceneD))
+                )
+            )
+        val childState =
+            MutableSceneTransitionLayoutState(SceneA, stateLinks = link)
+                as BaseSceneTransitionLayoutState
+
+        val childTransition = TestableTransition(SceneA, SceneB)
+
+        childState.startTransition(childTransition, null)
+        assertThat(childState.isTransitioning(SceneA, SceneB)).isTrue()
+        assertThat(parentState.isTransitioning(SceneC, SceneD)).isTrue()
+        assertThat(parentParentState.isTransitioning(SceneB, SceneC)).isTrue()
+
+        childState.finishTransition(childTransition, SceneB)
+        assertThat(childState.transitionState).isEqualTo(TransitionState.Idle(SceneB))
+        assertThat(parentState.transitionState).isEqualTo(TransitionState.Idle(SceneD))
+        assertThat(parentParentState.transitionState).isEqualTo(TransitionState.Idle(SceneC))
+    }
+
+    @Test
+    fun linkedTransition_linkProgressIsEqual() {
+        val (parentState, childState) = setupLinkedStates()
+
+        val childTransition = TestableTransition(SceneA, SceneB)
+
+        childState.startTransition(childTransition, null)
+        assertThat(parentState.currentTransition?.progress).isEqualTo(0f)
+
+        childTransition.progress = .5f
+        assertThat(parentState.currentTransition?.progress).isEqualTo(.5f)
+    }
+
+    @Test
+    fun linkedTransition_reverseTransitionIsNotLinked() {
+        val (parentState, childState) = setupLinkedStates()
+
+        val childTransition = TestableTransition(SceneB, SceneA)
+
+        childState.startTransition(childTransition, null)
+        assertThat(childState.isTransitioning(SceneB, SceneA)).isTrue()
+        assertThat(parentState.transitionState).isEqualTo(TransitionState.Idle(SceneC))
+
+        childState.finishTransition(childTransition, SceneB)
+        assertThat(childState.transitionState).isEqualTo(TransitionState.Idle(SceneB))
+        assertThat(parentState.transitionState).isEqualTo(TransitionState.Idle(SceneC))
+    }
+
+    @Test
+    fun linkedTransition_startsLinkAndFinishesLinkInFromState() {
+        val (parentState, childState) = setupLinkedStates()
+
+        val childTransition = TestableTransition(SceneA, SceneB)
+        childState.startTransition(childTransition, null)
+
+        childState.finishTransition(childTransition, SceneA)
+        assertThat(childState.transitionState).isEqualTo(TransitionState.Idle(SceneA))
+        assertThat(parentState.transitionState).isEqualTo(TransitionState.Idle(SceneC))
+    }
+
+    @Test
+    fun linkedTransition_startsLinkAndFinishesLinkInUnknownState() {
+        val (parentState, childState) = setupLinkedStates()
+
+        val childTransition = TestableTransition(SceneA, SceneB)
+        childState.startTransition(childTransition, null)
+
+        childState.finishTransition(childTransition, SceneD)
+        assertThat(childState.transitionState).isEqualTo(TransitionState.Idle(SceneD))
+        assertThat(parentState.transitionState).isEqualTo(TransitionState.Idle(SceneC))
+    }
+
+    @Test
+    fun linkedTransition_startsLinkButLinkedStateIsTakenOver() {
+        val (parentState, childState) = setupLinkedStates()
+
+        val childTransition = TestableTransition(SceneA, SceneB)
+        val parentTransition = TestableTransition(SceneC, SceneA)
+        childState.startTransition(childTransition, null)
+        parentState.startTransition(parentTransition, null)
+
+        childState.finishTransition(childTransition, SceneB)
+        assertThat(childState.transitionState).isEqualTo(TransitionState.Idle(SceneB))
+        assertThat(parentState.transitionState).isEqualTo(parentTransition)
     }
 
     @Test
@@ -125,11 +271,11 @@ class SceneTransitionLayoutStateTest {
         val transitionkey = TransitionKey(debugName = "foo")
         val state =
             MutableSceneTransitionLayoutState(
-                TestScenes.SceneA,
+                SceneA,
                 transitions =
                     transitions {
-                        from(TestScenes.SceneA, to = TestScenes.SceneB) { fade(TestElements.Foo) }
-                        from(TestScenes.SceneA, to = TestScenes.SceneB, key = transitionkey) {
+                        from(SceneA, to = SceneB) { fade(TestElements.Foo) }
+                        from(SceneA, to = SceneB, key = transitionkey) {
                             fade(TestElements.Foo)
                             fade(TestElements.Bar)
                         }
@@ -138,19 +284,19 @@ class SceneTransitionLayoutStateTest {
                 as MutableSceneTransitionLayoutStateImpl
 
         // Default transition from A to B.
-        assertThat(state.setTargetScene(TestScenes.SceneB, coroutineScope = this)).isNotNull()
+        assertThat(state.setTargetScene(SceneB, coroutineScope = this)).isNotNull()
         assertThat(state.transformationSpec.transformations).hasSize(1)
 
         // Go back to A.
-        state.setTargetScene(TestScenes.SceneA, coroutineScope = this)
+        state.setTargetScene(SceneA, coroutineScope = this)
         testScheduler.advanceUntilIdle()
         assertThat(state.currentTransition).isNull()
-        assertThat(state.transitionState.currentScene).isEqualTo(TestScenes.SceneA)
+        assertThat(state.transitionState.currentScene).isEqualTo(SceneA)
 
         // Specific transition from A to B.
         assertThat(
                 state.setTargetScene(
-                    TestScenes.SceneB,
+                    SceneB,
                     coroutineScope = this,
                     transitionKey = transitionkey,
                 )