TJ Dahunsi

Nov 14 2020 · 13 min read

Android Views as a Function of State with ViewBinding Case Study 1: The Live Game Stream

Bugdroid ❤s single line tags

This post is part of a series that shows how Views in android can be represented purely as a function of some state with no side effects helping to build a robust, easy to scale and maintain app with the help of ViewBinding.

This post is also a follow up to Building the right View Abstraction. In that post, I stated it wasn’t XML that was holding the Android View System back, but rather state dichotomy between views and the business layer. I went on further to say that having stateless views would go a long way to improving the View writing process whether programmatically a la Jetpack Compose, or from static declarations via XML.

This post will expand on the latter part; while addressing the sentiment that static view declarations cannot easily represent dynamic content. To do this, I shall be using the basketball game stream feature we just launched at GameChanger as a case study. First let’s go over the high level feature requirements:

1. The basketball game stream should be able to pan between both sides of the court to represent where the action is currently happening.

2. The basketball game stream should be able to animate the basketball according to the play being reflected.

3. In situations like free throws, the backdrop should transition between images of the basketball court and the basketball hoop.

4. After each play, a banner (small or large) should offer a description of the play that just ended.

The basketball game stream experience

The requirements above specify highly dynamic events that are best represented with some sort of state, especially for being able to represent smooth animations for things like panning the court, and animating the basketball. The final state declaration looks like:

1sealed class Target {
2    sealed class Background : Target() {
3        object Hoop : Target.Background()
4        object Court : Target.Background()
5    }
6
7    sealed class Banner : Target() {
8        object Large : Target.Banner()
9        object Small : Target.Banner()
10    }
11
12    sealed class Ball : Target() {
13        object A : Target.Ball()
14        object B : Target.Ball()
15    }
16}
17
18data class State(
19    val dimensions: Action.Dimensions,
20    val bitmapDownSampleFactor: Int,
21    val hoopBitmap: Bitmap,
22    val courtBitmap: Bitmap,
23    val ballBitmapA: Bitmap? = null,
24    val ballBitmapB: Bitmap? = null,
25    val metaData: MetaData = MetaData(),
26    val previousMetaData: MetaData = MetaData(),
27    val courtTranslationX: Float = 0f,
28    val targetAlphaMap: Map<Target, Float> = mapOf(),
29    val targetTranslationYMap: Map<Target, Float> = mapOf(),
30    val ballBitmapCache: LruCache<Int, Bitmap> = object : LruCache<Int, Bitmap>(3) {
31        override fun entryRemoved(evicted: Boolean, key: Int, oldValue: Bitmap, newValue: Bitmap?) =
32            oldValue.recycle()
33    },
34    val banner: Banner? = null
35)

The state machine that drives the production of this state is two way bound as some state parameters are dependent on the dimensions of the phone screen, and the dimension input in turns drives the production of the bitmaps used as the background of the experience.

Next, the static definition of the components that make up this experience:

1<!-- Game Stream Content -->
2
3<?xml version="1.0" encoding="utf-8"?>
4<FrameLayout
5    xmlns:android="http://schemas.android.com/apk/res/android"
6    android:layout_width="match_parent"
7    android:layout_height="match_parent">
8
9    <ImageView
10        android:id="@+id/court_background"
11        android:layout_width="match_parent"
12        android:layout_height="match_parent"
13        android:scaleType="matrix" />
14
15    <ImageView
16        android:id="@+id/hoop_background"
17        android:layout_width="match_parent"
18        android:layout_height="match_parent"
19        android:scaleType="matrix" />
20
21    <ImageView
22        android:id="@+id/ball_image_A"
23        android:layout_width="match_parent"
24        android:layout_height="match_parent"
25        android:scaleType="matrix" />
26
27    <ImageView
28        android:id="@+id/ball_image_B"
29        android:layout_width="match_parent"
30        android:layout_height="match_parent"
31        android:scaleType="matrix" />
32
33    <include
34        android:id="@+id/hoops_large_banner"
35        layout="@layout/viewholder_hoops_large_gamestream_card"
36        android:layout_width="match_parent"
37        android:layout_height="wrap_content"
38        android:layout_gravity="top"
39        android:layout_marginTop="30dp" />
40
41    <include
42        android:id="@+id/hoops_small_banner"
43        layout="@layout/viewholder_hoops_small_gamestream_card"
44        android:layout_width="match_parent"
45        android:layout_height="wrap_content"
46        android:layout_gravity="right|bottom"
47        android:layout_marginBottom="16dp" />
48
49</FrameLayout>
50
51<!-- Large Banner Definition -->
52
53<?xml version="1.0" encoding="utf-8"?>
54<androidx.constraintlayout.widget.ConstraintLayout
55    xmlns:android="http://schemas.android.com/apk/res/android"
56    xmlns:app="http://schemas.android.com/apk/res-auto"
57    xmlns:tools="http://schemas.android.com/tools"
58    android:layout_width="match_parent"
59    android:layout_height="wrap_content"
60    android:layout_gravity="center_horizontal"
61    android:layout_marginTop="24dp"
62    android:alpha="0"
63    android:clipChildren="false"
64    android:clipToPadding="false"
65    android:paddingHorizontal="32dp"
66    tools:alpha="1">
67
68    <com.google.android.material.card.MaterialCardView
69        android:id="@+id/card"
70        android:layout_width="0dp"
71        android:layout_height="100dp"
72        android:layout_marginTop="36dp"
73        app:cardCornerRadius="2dp"
74        app:cardElevation="2dp"
75        app:layout_constraintBottom_toBottomOf="parent"
76        app:layout_constraintLeft_toLeftOf="parent"
77        app:layout_constraintRight_toRightOf="parent"
78        app:layout_constraintTop_toTopOf="parent" />
79
80    <TextView
81        android:id="@+id/primaryText"
82        android:layout_width="0dp"
83        android:layout_height="wrap_content"
84        android:layout_marginTop="8dp"
85        android:elevation="10dp"
86        android:fontFamily="@font/roboto_medium"
87        android:gravity="center_horizontal"
88        android:maxLines="1"
89        android:paddingHorizontal="2dp"
90        android:textColor="@color/off_black"
91        android:textSize="17sp"
92        app:layout_constrainedWidth="true"
93        app:layout_constraintBottom_toTopOf="@+id/secondaryText"
94        app:layout_constraintLeft_toLeftOf="@+id/card"
95        app:layout_constraintRight_toRightOf="@+id/card"
96        app:layout_constraintTop_toBottomOf="@+id/avatar"
97        app:layout_constraintVertical_chainStyle="packed"
98        tools:text="[Event]" />
99
100    <TextView
101        android:id="@+id/secondaryText"
102        android:layout_width="0dp"
103        android:layout_height="wrap_content"
104        android:elevation="10dp"
105        android:fontFamily="@font/roboto_bold"
106        android:gravity="center_horizontal"
107        android:maxLines="1"
108        android:paddingHorizontal="2dp"
109        android:textColor="@color/off_black"
110        android:textSize="17sp"
111        app:layout_constrainedWidth="true"
112        app:layout_constraintBottom_toBottomOf="@+id/card"
113        app:layout_constraintLeft_toLeftOf="@+id/card"
114        app:layout_constraintRight_toRightOf="@+id/card"
115        app:layout_constraintTop_toBottomOf="@id/primaryText"
116        tools:text="by [Player Name]" />
117
118
119    <com.gc.atoms.widgets.AvatarView
120        android:id="@+id/avatar"
121        android:layout_width="72dp"
122        android:layout_height="72dp"
123        android:elevation="10dp"
124        app:layout_constraintEnd_toEndOf="parent"
125        app:layout_constraintStart_toStartOf="parent"
126        app:layout_constraintTop_toTopOf="parent"
127        app:size="large" />
128
129</androidx.constraintlayout.widget.ConstraintLayout>
130
131<!-- Small banner definition is very similar to the large banner def -->

The above by itself is nothing special, it is just a static XML file. The true power comes when it is combined with ViewBinding to create terse, readable two way bindings.

1override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
2        super.onViewCreated(view, savedInstanceState)
3
4        val viewBinding = FragmentHoopsLiveGamestreamBinding.bind(view)
5        val courtImageView = viewBinding.courtBackground
6        val largeBanner = viewBinding.hoopsLargeBanner
7        val smallBanner = viewBinding.hoopsSmallBanner
8
9        viewBinding.root.addOnLayoutChangeListener { _, _, _, _, _, _, _, _, _ ->
10            viewModel.accept(Action.Dimensions(
11                viewHash = System.identityHashCode(view),
12                displaySize = SizeF(
13                    courtImageView.measuredWidth.toFloat(),
14                    courtImageView.measuredHeight.toFloat()
15                ),
16                largeBannerOffscreenTranslationY = -(largeBanner.root.height + largeBanner.root.marginTop).toFloat(),
17                smallBannerOffScreenTranslationY = (smallBanner.root.height + smallBanner.root.marginBottom).toFloat()
18            ))
19        }
20    }

Next up is binding the output of state to the View. Since the intent is to describe the purely as a function of state, the following are pure functional expressions and use method references where possible to communicate the surrounding context of the the class body isn’t leaking into the binding context. The ViewModel exposes state through the state val which is a LiveData<State>, and mapDistinct is syntactic sugar around LiveData Transformations.map() and Transformations.distinctUntilChanged(). Unfortunately there’s some tautology in the syntax of observing difference slices of the state to the methods that consume them, but a DSL can always be written to make it more terse.

1override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
2        ...
3
4        viewModel.state.apply {
5            // Observe the court bitmap first before observing the matrix
6            mapDistinct(State::courtBitmap).observe(viewLifecycleOwner, courtImageView::setImageBitmap)
7            mapDistinct(State::courtAlpha).observe(viewLifecycleOwner, courtImageView::setVisualAlpha)
8            mapDistinct(State::courtMatrix).observe(viewLifecycleOwner, courtImageView::updateMatrix)
9
10            mapDistinct(State::hoopBitmap).observe(viewLifecycleOwner, hoopImageView::setImageBitmap)
11            mapDistinct(State::hoopAlpha).observe(viewLifecycleOwner, hoopImageView::setVisualAlpha)
12            mapDistinct(State::hoopMatrix).observe(viewLifecycleOwner, hoopImageView::updateMatrix)
13
14            mapDistinct(State::ballMirrored).observe(viewLifecycleOwner, ballImageViewA::setScaleX)
15            mapDistinct(State::ballImageAlphaA).observe(viewLifecycleOwner, ballImageViewA::setVisualAlpha)
16            mapDistinct(State::ballBitmapA).observe(viewLifecycleOwner) { it?.let(ballImageViewA::setImageBitmap) }
17            mapDistinct(State::ballMatrixA).observe(viewLifecycleOwner) { it?.let(ballImageViewA::updateMatrix) }
18
19            mapDistinct(State::ballMirrored).observe(viewLifecycleOwner, ballImageViewB::setScaleX)
20            mapDistinct(State::ballImageAlphaB).observe(viewLifecycleOwner, ballImageViewB::setVisualAlpha)
21            mapDistinct(State::ballBitmapB).observe(viewLifecycleOwner) { it?.let(ballImageViewB::setImageBitmap) }
22            mapDistinct(State::ballMatrixB).observe(viewLifecycleOwner) { it?.let(ballImageViewB::updateMatrix) }
23
24            mapDistinct(State::largeBannerAlpha).observe(viewLifecycleOwner, largeBanner.root::setVisualAlpha)
25            mapDistinct(State::smallBannerAlpha).observe(viewLifecycleOwner, smallBanner.root::setVisualAlpha)
26            mapDistinct(State::largeBannerTranslationY).observe(viewLifecycleOwner, largeBanner.root::setTranslationY)
27            mapDistinct(State::smallBannerTranslationY).observe(viewLifecycleOwner, smallBanner.root::setTranslationY)
28            mapDistinct(State::banner).observe(viewLifecycleOwner) { it?.let { if (it.isLarge) largeBanner.bind(it) else smallBanner.bind(it) } }
29        }
30    }

This is practically the entirety of the Fragment that powers the basketball game stream. It is also worth taking a closer look at the implementation of line 28:

1private fun ViewholderHoopsSmallGamestreamCardBinding.bind(banner: Banner) = run {
2    avatar.isVisible = banner.avatar != null
3    secondaryText.isVisible = banner.bottomText != null
4    avatar.config = banner.avatar
5    primaryText.text = banner.topText
6    secondaryText.text = banner.bottomText
7    primaryText.gravity = if (banner.avatar == null) Gravity.CENTER else Gravity.START
8}
9
10private fun ViewholderHoopsLargeGamestreamCardBinding.bind(banner: Banner) = run {
11    avatar.isVisible = banner.avatar != null
12    secondaryText.isVisible = banner.bottomText != null
13    avatar.config = banner.avatar
14    primaryText.text = banner.topText
15    secondaryText.text = banner.bottomText
16}
17
18private fun ImageView.updateMatrix(matrixDef: MatrixDef) {
19    val tuple = matrixTuple
20    // Swap matrix being updated to get around reference checks
21    val matrixToUpdate = if (imageMatrix == tuple.a) tuple.b else tuple.a
22    imageMatrix = matrixToUpdate.apply {
23        reset()
24        setScale(matrixDef.scale, matrixDef.scale)
25        postTranslate(matrixDef.translationX, matrixDef.translationY)
26    }
27}
28
29private val ImageView.matrixTuple get() = tag as? MatrixTuple ?: MatrixTuple().also { tag = it }
30
31/**
32 * ImageViews use reference equality instead of object equality checks when updating their matrix,
33 * so we keep a nice tuple and swap the matrix when we want to update to get around this.
34 *
35 * A [Pair] would have worked as well, but the generic bounds make for ugly casting in use.
36 */
37data class MatrixTuple(
38    val a: Matrix = Matrix(),
39    val b: Matrix = Matrix()
40)

Both the large and small banner are themselves ViewBinding types of ViewholderHoopsLargeGamestreamCardBinding and ViewholderHoopsSmallGamestreamCardBinding respectively. Their state is the Banner data class, which is a slice of the larger State class; just like Jetpack Compose would have you aggregate smaller Composables to “compose” them into larger ones, all with strongly typed, statically defined layouts.

Importantly, the following all hold true:

1. There are no reentrant side effects that attempt to mutate the View after its declaration and binding.

2. The View is declared statically, separating the declaration of the appearance of the View from the logic of driving the View behavior.

3. There is a 1:1 relationship between the ViewBinding and State types, FragmentHoopsLiveGamestreamBinding can be reused anywhere the State is available; subclassing a View to create a custom view for business logic is not necessary.

4. The View is extremely simple; all the complexity is purely in the business layer as the state machine is responsible for driving each animation frame.

The last piece is how the ViewModel is able to drive these animations. I have waxed lyrical and extolled at length the virtues of the Android Dynamic Animation Library, key of which is its ability to be retarget ongoing animations. The above experience is powered by SpringAnimation instances, and specifying the final value the following properties should arrive at:

1. Alpha

2. TranslationX

3. TranslationY

The emissions of the spring can then be piped into a reactive stream, and finally converted to LiveData for consumption by the View.

1/**
2 * Reads existing values from the current [State], configures a [SpringForce] with the specified
3 * params using the [springAnimationOf] extension, and completes when the animation ends.
4 */
5private fun Observable<State>.springObservable(
6    stiffness: Float,
7    finalPosition: (State) -> Float,
8    getter: (State) -> Float,
9    setter: (Float) -> Mutation,
10    shortCircuit: (State) -> Boolean = { false }
11): Observable<Mutation> = take(1).flatMap { state -> // Read the latest state first
12    if (shortCircuit(state)) Observable.just(setter(finalPosition(state)))
13    else Observable.create<Mutation> { emitter ->
14        springAnimationOf(
15            setter = { emitter.onNext(setter(it)) },
16            getter = { getter(state) },
17            finalPosition = 0f
18        ).apply {
19            spring = SpringForce().apply {
20                this.dampingRatio = SpringForce.DAMPING_RATIO_NO_BOUNCY
21                this.stiffness = stiffness
22            }
23            addEndListener { _, _, _, _ -> emitter.onComplete() }
24            animateToFinalPosition(finalPosition(state))
25        }
26    }
27        .subscribeOn(AndroidSchedulers.mainThread())
28        .observeOn(AndroidSchedulers.mainThread())
29}
30
31sealed class Mutation {
32    data class CourtPan(val x: Float) : Mutation()
33    data class SetMetaData(val isFreeThrow: Boolean, val ownTeamInPossession: Boolean?) : Mutation()
34    data class SetBanner(val banner: Banner?) : Mutation()
35    data class SetBallBitmap(val bitmap: Bitmap?, val target: Target.Ball) : Mutation()
36    data class UpdateBitmapCache(val entry: Pair<Int, Bitmap>) : Mutation()
37    data class Alpha(val alpha: Float, val target: Target) : Mutation()
38    data class TranslationY(val translation: Float, val target: Target) : Mutation()
39    data class AnimationComplete(val itemId: String) : Mutation()
40    object ClearBitmaps : Mutation()
41}
42
43fun State.reduce(mutation: Mutation): State = when (mutation) {
44    is Mutation.CourtPan -> copy(courtTranslationX = mutation.x)
45    is Mutation.Alpha -> copy(targetAlphaMap = HashMap(targetAlphaMap).apply {
46        this[mutation.target] = mutation.alpha
47    })
48    is Mutation.TranslationY -> copy(targetTranslationYMap = HashMap(targetTranslationYMap).apply {
49        this[mutation.target] = mutation.translation
50    })
51    is Mutation.SetBanner -> copy(banner = mutation.banner)
52    is Mutation.SetBallBitmap -> when (mutation.target) {
53        Target.Ball.A -> copy(ballBitmapA = mutation.bitmap)
54        Target.Ball.B -> copy(ballBitmapB = mutation.bitmap)
55    }
56    is Mutation.SetMetaData -> copy(
57        metaData = MetaData(
58            isFreeThrow = mutation.isFreeThrow,
59            ownTeamInPossession = mutation.ownTeamInPossession ?: metaData.ownTeamInPossession
60        ),
61        previousMetaData = MetaData(
62            isFreeThrow = metaData.isFreeThrow,
63            ownTeamInPossession = metaData.ownTeamInPossession
64        )
65    )
66    is Mutation.UpdateBitmapCache -> apply {
67        val (key, bitmap) = mutation.entry
68        ballBitmapCache.put(key, bitmap)
69    }
70    is Mutation.ClearBitmaps -> apply {
71        ballBitmapCache.evictAll()
72    }
73    is Mutation.AnimationComplete -> this // No op
74}

With the above each play or action in the game stream creates a Mutation, which in turn modifies a slice of the State. Notice the statelessness of the Mutations; each just overrides a value in the state, and each Mutation creates a brand new SpringAnimation instance. I just need the initial value to animate from, which is read from the state, and specify the final value.

This statelessness also carries on to the View layer, with each UI element is bound once, and only once to the slice of State it cares about. The above illustrates:

1. Immutability / Statelessness is extremely powerful while being incredibly simple.

2. Complex UI absolutely can be represented with static View declarations. A follow up case study will cover the case where nodes in the UI tree need to be removed as part of mutations in state, and how static View declarations are not an obstacle to this.

3. Using ViewBinding as a bridge, it becomes easy to aggregate a collection of Views into typed aggregates, and compose behaviors unto them without having to subclass the View class.

Feel free to try out the game stream yourself! It’s free to create and score a game on your device, you will need another device to see the animated game stream in real time however:

GameChanger Team Manager

The full implementation of the ViewModel that drives the experience and its animation mapping follows below. It’s fairly heavy on rx, but not unfamiliar:

1private typealias StartPayload = Pair<List<ScoringItem>, State>
2
3val StartPayload.seed
4    get() = Action.Cursor(
5        isFirst = true,
6        index = first.lastIndex,
7        itemId = first[first.lastIndex].diffId
8    )
9
10private data class AnimationInput(
11    val cursor: Action.Cursor,
12    val plays: List<ScoringItem>,
13    val state: State
14)
15
16private val AnimationInput.key get() = cursor
17private val AnimationInput.currentItem get() = plays[cursor.index]
18private val AnimationInput.banner get() = currentItem.toBanner
19private val AnimationInput.animatable get() = if (cursor.isFirst) currentItem.initialAnimatable else currentItem.toAnimatable
20
21private typealias AnimationOutput = Triple<Mutation.AnimationComplete, Action.Cursor, List<ScoringItem>>
22
23val AnimationOutput.key get() = first
24val AnimationOutput.cursor get() = second
25val AnimationOutput.plays get() = third
26val AnimationOutput.canStartNext get() = cursor.index < plays.lastIndex
27val AnimationOutput.showLatestState get() = !canStartNext && first.itemId == cursor.itemId
28val AnimationOutput.nextCursor
29    get() = Action.Cursor(
30        isFirst = false,
31        index = cursor.index + 1,
32        itemId = plays[cursor.index + 1].diffId
33    )
34
35class HoopsLiveGameStreamViewModel @Inject constructor(
36    private val controller: HoopsGameStreamController,
37    private val resources: Resources
38) : ViewModel() {
39
40    private val disposable = CompositeDisposable()
41    private val actionRelay: BehaviorRelay<Action> = BehaviorRelay.create()
42    private val mutationRelay: BehaviorRelay<Mutation> = BehaviorRelay.createDefault(Mutation.CourtPan(0f))
43    private val backingState: Observable<State> = actionRelay.filterIsInstance<Action.Dimensions>()
44        // While the UI settles, the dimensions can change frequently, causing us to create and load
45        // multiple background images in memory at once. The operations on the dimensions wait till
46        // the UI is completely settled before committing one way or the other
47        .filter(Action.Dimensions::isValid)
48        .scan(Action.Dimensions::compare)
49        .debounce(200, TimeUnit.MILLISECONDS)
50        .filter(Action.Dimensions::isSameView)
51        .distinct(Action.Dimensions::displaySize)
52        .flatMapSingle(resources::state)
53        .switchMap { state -> mutationRelay.distinctUntilChanged().scan(state, State::reduce) }
54        .replayingShare()
55    private val cursorObservable: Observable<Action.Cursor> = actionRelay.filterIsInstance<Action.Cursor>()
56        .replayingShare()
57    private val playsObservable: Observable<List<ScoringItem>> = controller.latestScorekeepingState
58        .map { state -> state.state.plaByPlay.map { it.convert(state.ownTeam, state.oppTeam, resources, controller.scope) } }
59        .filter(List<ScoringItem>::isNotEmpty)
60        .distinctUntilChanged()
61        .replayingShare()
62
63    val state: LiveData<State> = backingState.toLiveData()
64
65    init {
66        // Run every animation as a sequence one after the other
67        Observables.combineLatest(
68            cursorObservable,
69            playsObservable,
70            backingState,
71            ::AnimationInput
72        )
73            .distinctUntilChanged(AnimationInput::key)
74            .flatMapSingle(this::preAnimate)
75            .map(AnimationInput::animatable)
76            .concatMap(backingState::animate)
77            .subscribe(mutationRelay::accept)
78            .addTo(disposable)
79
80        val completions: Observable<AnimationOutput> = Observables.combineLatest(
81            mutationRelay.filterIsInstance<Mutation.AnimationComplete>(),
82            cursorObservable,
83            playsObservable
84        ).replayingShare()
85
86        // When an animation completes, and there are more items to animate, start the next one
87        completions
88            .filter(AnimationOutput::canStartNext)
89            .distinctUntilChanged(AnimationOutput::key)
90            .map(AnimationOutput::nextCursor)
91            .subscribe(actionRelay::accept)
92            .addTo(disposable)
93
94        // When an animation completes, and there are no more items to animate, show the latest state
95        completions
96            .filter(AnimationOutput::showLatestState)
97            .distinctUntilChanged(AnimationOutput::key)
98            .subscribe { controller.currentEvent = null } // Show latest state
99            .addTo(disposable)
100
101        // Start when plays are available and court has been measured
102        Observables.combineLatest(
103            playsObservable,
104            backingState
105        )
106            .firstOrError()
107            .map(StartPayload::seed)
108            .subscribe(actionRelay::accept)
109            .addTo(disposable)
110
111        actionRelay.filterIsInstance<Action.ViewDestroyed>()
112            .subscribe { mutationRelay.accept(Mutation.ClearBitmaps) }
113            .addTo(disposable)
114    }
115
116    fun accept(input: Action) = actionRelay.accept(input)
117
118    override fun onCleared() = disposable.clear()
119
120    private fun preAnimate(input: AnimationInput): Single<AnimationInput> {
121        if (input.currentItem.event != null) controller.currentEvent = input.currentItem.event
122        mutationRelay.accept(Mutation.SetBanner(input.banner))
123        mutationRelay.accept(Mutation.TranslationY(0f, Target.Ball.A))
124        mutationRelay.accept(Mutation.TranslationY(0f, Target.Ball.B))
125        mutationRelay.accept(Mutation.SetMetaData(
126            isFreeThrow = input.currentItem.isFreeThrow,
127            ownTeamInPossession = input.currentItem.ownTeamInPossession
128        ))
129        return when (val playCode = (input.currentItem as? ScoringItem.PlayByPlayEntry)?.playCode) {
130            null -> Single.just(input)
131            else -> resources.cacheImages(input.state, playCode.ballDrawableResources).andThen(Single.just(input))
132        }
133    }
134
135    private fun Resources.cacheImages(state: State, drawableResources: List<Int>): Completable =
136        Completable.merge(drawableResources.map { id ->
137            if (state.ballBitmapCache[id] != null) Completable.complete()
138            else loadBitMap(id, state.bitmapDownSampleFactor)
139                .doOnSuccess { bitmap ->
140                    mutationRelay.accept(Mutation.UpdateBitmapCache(id to bitmap))
141                }.ignoreElement()
142        })
143}
144
145private fun Resources.state(dimensions: Action.Dimensions) = Single.fromCallable {
146    // Original court image is 5000 * 2238; the display height is larger so we'll use that instead of the width
147    val bitmapDownSampleFactor = max(2, (5000 / dimensions.displaySize.height).toInt())
148    val courtBitmap = downSampleBitmap(this, R.drawable.bg_hoops_court, bitmapDownSampleFactor)
149    val hoopBitmap = downSampleBitmap(this, R.drawable.bg_hoops_ft, bitmapDownSampleFactor)
150    State(
151        dimensions = dimensions,
152        bitmapDownSampleFactor = bitmapDownSampleFactor,
153        courtBitmap = courtBitmap,
154        hoopBitmap = hoopBitmap
155    )
156}
157    .subscribeOn(Schedulers.io())
158    .observeOn(AndroidSchedulers.mainThread())
159    .cache()
160
161private fun Resources.loadBitMap(drawableRes: Int, bitmapDownSampleFactor: Int) = Single.fromCallable {
162    downSampleBitmap(this, drawableRes, bitmapDownSampleFactor)
163}
164
165sealed class Animatable {
166    abstract val isNested: Boolean
167
168    /**
169     * Used to identify the [ScoringItem] being animated
170     */
171    abstract val itemId: String
172
173    /**
174     * Pans the court from one end to the other
175     */
176    data class CourtPan(
177        override val isNested: Boolean = false,
178        override val itemId: String = "",
179        val position: Position
180    ) : Animatable() {
181        sealed class Position {
182            object Left : Position()
183            object Right : Position()
184            object Opposite : Position()
185            object Unchanged : Position()
186        }
187    }
188
189    /**
190     * Fades an item in or out
191     */
192    data class Alpha(
193        override val isNested: Boolean = false,
194        override val itemId: String = "",
195        val springStiffness: Float = SpringForce.STIFFNESS_VERY_LOW,
196        val target: Target,
197        val opacity: Float = 1.0f
198    ) : Animatable()
199
200    /**
201     * Moves an item up or down
202     */
203    data class VerticalTranslation(
204        override val isNested: Boolean = false,
205        override val itemId: String = "",
206        val springStiffness: Float = SpringForce.STIFFNESS_LOW,
207        val target: Target,
208        val atNaturalPosition: Boolean
209    ) : Animatable()
210
211    /**
212     * Sets a ball image into a target
213     */
214    data class BallImage(
215        override val isNested: Boolean = false,
216        override val itemId: String = "",
217        val imageResource: Int?,
218        val target: Target.Ball
219    ) : Animatable()
220
221    /**
222     * Runs the [nested] animations consecutively
223     */
224    data class Series(
225        override val isNested: Boolean = false,
226        override val itemId: String = "",
227        val nested: List<Animatable>
228    ) : Animatable()
229
230    /**
231     * Runs the [nested] animations simultaneously
232     */
233    data class Parallel(
234        override val isNested: Boolean = false,
235        override val itemId: String = "",
236        val nested: List<Animatable>
237    ) : Animatable()
238
239    /**
240     * Delays for [millis] milliseconds
241     */
242    data class Delay(
243        override val isNested: Boolean = false,
244        override val itemId: String = "",
245        val millis: Long
246    ) : Animatable()
247
248    /**
249     * Does not animate at all
250     */
251    data class Drop(
252        override val isNested: Boolean = false,
253        override val itemId: String = ""
254    ) : Animatable()
255}
256
257/**
258 * Creates a [Mutation] stream as the result of running the [animatable] on the [State]
259 * and notifies of the completion.
260 */
261fun Observable<State>.animate(animatable: Animatable): Observable<Mutation> = when (animatable) {
262    is Animatable.CourtPan -> springObservable(
263        stiffness = SpringForce.STIFFNESS_VERY_LOW,
264        finalPosition = { it.courtTranslationX(animatable.position) },
265        getter = State::courtTranslationX,
266        setter = { Mutation.CourtPan(x = it) },
267        // Do not animate if the court it not visible, just snap into place,
268        // used for if there was a free throw background showing prior
269        shortCircuit = { currentState: State -> currentState.previousMetaData.isFreeThrow }
270    )
271    is Animatable.Alpha -> springObservable(
272        stiffness = animatable.springStiffness,
273        finalPosition = { 100f * animatable.opacity },
274        getter = { it.targetAlphaMap.getValue(animatable.target) * 100 },
275        setter = { Mutation.Alpha(alpha = it / 100, target = animatable.target) }
276    )
277    is Animatable.VerticalTranslation -> springObservable(
278        stiffness = animatable.springStiffness,
279        finalPosition = {
280            if (animatable.atNaturalPosition) 0f else when (val target = animatable.target) {
281                is Target.Banner.Large -> it.dimensions.largeBannerOffscreenTranslationY
282                is Target.Banner.Small -> it.dimensions.smallBannerOffScreenTranslationY
283                is Target.Ball.A -> it.ballDropTranslationY(target)
284                is Target.Ball.B -> it.ballDropTranslationY(target)
285                is Target.Background.Court, Target.Background.Hoop -> 0f
286            }
287        },
288        getter = { it.targetTranslationYMap.getValue(animatable.target) },
289        setter = { Mutation.TranslationY(translation = it, target = animatable.target) }
290    )
291    is Animatable.BallImage -> ballImageObservable(animatable.imageResource, animatable.target)
292    is Animatable.Series -> Observable.concat(animatable.nested.map(Animatable::asNested).map(this::animate))
293    is Animatable.Parallel -> Observable.merge(animatable.nested.map(Animatable::asNested).map(this::animate))
294    is Animatable.Delay -> Observable.timer(animatable.millis, TimeUnit.MILLISECONDS, delayScheduler).flatMap { Observable.empty() }
295    is Animatable.Drop -> Observable.empty()
296}.concatWith(if (animatable.isNested) Observable.empty() else Observable.just(Mutation.AnimationComplete(animatable.itemId)))