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Molecule: Using Compose for presentation logic

Chris Horner
September 27, 2022

Molecule: Using Compose for presentation logic

Jetpack Compose can be used for more than just emitting a user interface. Molecule is a library that allows `Flow` or `StateFlow` streams to be built using Compose.

This talk covers:
- Some historical approaches to presentation logic on Android
- Comparing Compose to Rx and Flow APIs
- The benefits Compose can have on readability
- How Molecule helps decouple Compose from Compose UI
- How to build a StateFlow using Compose
- Testing strategies and gotchas

Chris Horner

September 27, 2022
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Transcript

  1. @chris_h_codes
    Molecule
    Using Compose for presentation logic
    Chris Horner

    View Slide

  2. Compose. Not Compose UI.

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  3. Presentation logic?
    MV Whatever
    Model
    Event

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  4. What does Molecule enable?
    @Composable


    fun models(events: Flow): Model {


    //
    Calculate Model here.


    }
    Why is this interesting?

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  5. 2017

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  6. 2017
    RxJava

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  7. class MyActivity : Activity() {


    override fun onCreate(savedInstanceState: Bundle?) {


    button.setOnClickListener {




    }


    }


    }
    2010

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  8. class MyActivity : Activity() {


    override fun onCreate(savedInstanceState: Bundle?) {


    button.setOnClickListener {


    object : AsyncTask() {


    override fun doInBackground(vararg arg: Void?) {


    //
    Update "state" somehow?


    }


    }


    }


    }


    }
    2010

    View Slide

  9. <>
    Observable
    Observable
    Observable
    Observable
    The State of Managing State with RxJava
    https://youtu.be/0IKHxjkgop4
    Jake Wharton
    2017

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  10. 2017
    Single.just(input)


    .flatMap { longRunningOp(it) }


    .map { it.asSomethingElse() }


    .filter { someCheck(it) }


    .subscribe { … }

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  11. 2017
    Single.just(input)


    .flatMap { longRunningOp(it) }


    .map { it.asSomethingElse() }


    .filter { someCheck(it) }


    .subscribe { … }
    val result = longRunningOp(input)


    val output = result.asSomethingElse()


    return if someCheck(output) output else null

    View Slide

  12. 2017
    Single.just(input)


    .flatMap { longRunningOp(it) }


    .map { it.asSomethingElse() }


    .filter { someCheck(it) }


    .subscribeOn(Schedulers.Io)


    .observeOn(Schedulers.Main)


    .subscribe { … }
    val result = longRunningOp(input)


    val output = result.asSomethingElse()


    return if someCheck(output) output else null

    View Slide

  13. Single Maybe Completable
    suspend
    2017

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  14. 2019
    fun map(mapper: (T)
    ->
    R): Flowable
    fun flatMapSingle(mapper: (T)
    ->
    SingleSource): Flowable
    Mapper
    Asynchronous Data Streams with Kotlin Flow
    https://youtu.be/tYcqn48SMT8
    Roman Elizarov

    View Slide

  15. 2019
    Sync
    Async
    fun map(mapper: (T)
    ->
    R): Flowable
    fun flatMapSingle(mapper: (T)
    ->
    SingleSource): Flowable
    Mapper
    Asynchronous Data Streams with Kotlin Flow
    https://youtu.be/tYcqn48SMT8
    Roman Elizarov

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  16. fun map(mapper: (T)
    ->
    R): Flowable
    fun flatMapSingle(mapper: (T)
    ->
    SingleSource): Flowable
    Mapper
    2019
    fun filter(predicate: (T)
    ->
    Boolean): Flowable
    fun …
    🤯
    Predicate
    Sync
    Async
    Sync
    Async
    Asynchronous Data Streams with Kotlin Flow
    https://youtu.be/tYcqn48SMT8
    Roman Elizarov
    Asynchronous Data Streams with Kotlin Flow
    https://youtu.be/tYcqn48SMT8
    Roman Elizarov

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  17. flow.map {


    }

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  18. Operator Avoidance
    startWith(observable)
    startWith(value)
    delaySubscription(time)
    onStart { emitAll(flow) }
    onStart { emit(value) }
    onStart { delay(time) }

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  19. Operator Avoidance
    RxJava API surface
    Flow API surface

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  20. Operator Avoidance
    RxJava API surface
    Flow API surface

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  21. Operator Power
    val queries: Flowable
    sealed interface Model {


    object Loading : Model


    data class Loaded(


    val results: List


    ): Model


    }

    View Slide

  22. Operator Power
    Mel
    val queries: Flowable
    sealed interface Model {


    object Loading : Model


    data class Loaded(


    val results: List


    ): Model


    }

    View Slide

  23. Operator Power
    Mel
    val queries: Flowable
    sealed interface Model {


    object Loading : Model


    data class Loaded(


    val results: List


    ): Model


    }

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  24. Operator Power
    queries.switchMap { query
    -
    >

    search(query)


    .delaySubscription(300, TimeUnit.MILLISECONDS)


    .toObservable()


    .startWith { Model.Loading }


    }

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  25. Operator Power
    queries.switchMap { query
    -
    >

    search(query)


    .delaySubscription(300, TimeUnit.MILLISECONDS)


    .toObservable()


    .startWith { Model.Loading }


    }
    queries.transformLatest { query
    ->


    emit(Model.Loading)


    delay(300)


    emit(search(query))


    }

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  26. Operator Power
    queries.switchMap { query
    -
    >

    search(query)


    .delaySubscription(300, TimeUnit.MILLISECONDS)


    .toObservable()


    .startWith { Model.Loading }


    }
    queries.transformLatest { query
    ->


    emit(Model.Loading)


    delay(300)


    emit(search(query))


    }
    1

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  27. Operator Power
    queries.switchMap { query
    -
    >

    search(query)


    .delaySubscription(300, TimeUnit.MILLISECONDS)


    .toObservable()


    .startWith { Model.Loading }


    }
    queries.transformLatest { query
    ->


    emit(Model.Loading)


    delay(300)


    emit(search(query))


    }
    2

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  28. Operator Power
    queries.switchMap { query
    -
    >

    search(query)


    .delaySubscription(300, TimeUnit.MILLISECONDS)


    .toObservable()


    .startWith { Model.Loading }


    }
    queries.transformLatest { query
    ->


    emit(Model.Loading)


    delay(300)


    emit(search(query))


    }
    3

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  29. Operator Power
    queries.switchMap { query
    -
    >

    search(query)


    .delaySubscription(300, TimeUnit.MILLISECONDS)


    .toObservable()


    .startWith { Model.Loading }


    }
    queries.transformLatest { query
    ->


    emit(Model.Loading)


    delay(300)


    emit(search(query))


    }
    1

    View Slide

  30. Operator Power
    queries.switchMap { query
    -
    >

    search(query)


    .delaySubscription(300, TimeUnit.MILLISECONDS)


    .toObservable()


    .startWith { Model.Loading }


    }
    queries.transformLatest { query
    ->


    emit(Model.Loading)


    delay(300)


    emit(search(query))


    }
    2

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  31. Operator Power
    queries.switchMap { query
    -
    >

    search(query)


    .delaySubscription(300, TimeUnit.MILLISECONDS)


    .toObservable()


    .startWith { Model.Loading }


    }
    queries.transformLatest { query
    ->


    emit(Model.Loading)


    delay(300)


    emit(search(query))


    }
    3

    View Slide

  32. So suspend + Flow wins. End of story?
    • suspend is great because we can write imperative code


    • Flow has advantages over Rx because it’s powered by coroutines


    • It still has many operators to learn


    • It’s still a slightly different way of writing code

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  33. View Slide

  34. queries.transformLatest { query
    ->


    emit(State.Loading)


    delay(300)


    emit(search(query))


    }

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  35. queries


    .map {
    ...
    }


    .transformLatest { query
    ->


    emit(State.Loading)


    delay(300)


    emit(search(query))


    }


    .flowOn(
    .. .
    )


    .onEmpty {
    ...
    }


    .catch {
    .. .
    }


    .scan(emptyList()) { list, items
    ->


    list + items


    .map {
    ...
    }


    .zip()


    }


    .flatMapMerge(concurrency = 4) {
    ..
    .
    }


    .distinctUntilChanged()

    View Slide

  36. What are we doing?
    We’re composing a model in response to
    events and values changing over time.

    View Slide

  37. What if we actually used Compose to
    build that model?
    Molecule asks the question:
    What are we doing?
    We’re composing a model in response to
    events and values changing over time.

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  38. Column
    Text Row
    Column Image
    Text Text

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  39. Column
    Text Row
    Column Image
    Text Text
    @Composable


    fun UserInterface(model: Model) {




    }

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  40. Column
    Text Row
    Box Image
    Image Icon
    @Composable


    fun UserInterface(model: Model) {




    }

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  41. Column
    Text Row
    Image
    @Composable


    fun UserInterface(model: Model) {


    var someState: Int by remember {


    mutableStateOf(1)


    }


    }
    Box
    Image Icon

    View Slide

  42. @Composable


    fun UserInterface(model: Model) {


    val someState: MutableState = remember {


    mutableStateOf(1)


    }


    }

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  43. @Composable


    fun UserInterface(model: Model) {


    val state: State


    }
    Reactive!

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  44. @Composable


    fun UserInterface(model: Model) {


    var someState: Int by remember {


    mutableStateOf(1)


    }


    }

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  45. @Composable


    fun UserInterface() {


    var someState: Int by remember {


    mutableStateOf(1)


    }


    LaunchedEffect(Unit) {


    while (true) {


    delay(1_000)


    someState
    ++

    }


    }


    }

    View Slide

  46. @Composable


    fun UserInterface() {


    var someState: Int by remember {


    mutableStateOf(1)


    }


    LaunchedEffect(Unit) {


    while (true) {


    delay(1_000)


    someState
    ++

    }


    }


    Text(someState.toString())


    }

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  47. @Composable


    fun UserInterface() {
    var someState: Int by remember {


    mutableStateOf(1)


    }


    LaunchedEffect(Unit) {


    while (true) {


    delay(1_000)


    someState
    ++

    }


    }


    Text(someState.toString())


    }
    Presentation logic

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  48. @Composable


    fun UserInterface() {
    }
    Flow Example

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  49. @Composable


    fun UserInterface() {
    }
    sealed interface Model {


    object Loading : Model


    data class Loaded(


    val results: List


    ) : Model


    }

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  50. @Composable


    fun UserInterface(queries: Flow) {
    }

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  51. @Composable


    fun UserInterface(queries: Flow) {
    val models = queries


    .onStart { emit("") }


    }

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  52. @Composable


    fun UserInterface(queries: Flow) {
    val models = queries


    .onStart { emit("") }


    .transformLatest { query
    ->

    emit(Model.Loading)


    }


    }

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  53. @Composable


    fun UserInterface(queries: Flow) {
    val models = queries


    .onStart { emit("") }


    .transformLatest { query
    ->

    emit(Model.Loading)


    delay(300)


    val results = search(query)


    emit(Model.Loaded(results))


    }


    }

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  54. e


    terface(queries: Flow) {
    ls = queries


    rt { emit("") }


    formLatest { query
    ->

    (Model.Loading)


    y(300)


    results = search(query)


    (Model.Loaded(results))




    interface Database {


    fun observeDataset(): Flow


    }

    View Slide

  55. e


    terface(queries: Flow) {
    ls = queries


    rt { emit("") }


    formLatest { query
    ->

    (Model.Loading)


    y(300)


    results = search(query, dataset)


    (Model.Loaded(results))




    interface Database {


    fun observeDataset(): Flow


    }

    View Slide

  56. @Composable


    fun UserInterface(queries: Flow) {
    val models = queries


    .onStart { emit("") }


    .transformLatest { query
    ->

    emit(Model.Loading)


    delay(300)


    val results = search(query, dataset)


    emit(Model.Loaded(results))


    }


    }

    View Slide

  57. @Composable


    fun UserInterface(queries: Flow, datasets: Flow) {
    val models = queries


    .onStart { emit("") }


    .transformLatest { query
    ->

    emit(Model.Loading)


    delay(300)


    val results = search(query, dataset)


    emit(Model.Loaded(results))


    }


    }

    View Slide

  58. @Composable


    fun UserInterface(queries: Flow, datasets: Flow) {
    combineTransform(queries, datasets)
    val models = queries


    .onStart { emit("") }


    .transformLatest { query
    ->

    emit(Model.Loading)


    delay(300)


    val results = search(query, dataset)


    emit(Model.Loaded(results))


    }


    }
    ?

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  59. @Composable


    fun UserInterface(queries: Flow, datasets: Flow) {
    combineTransformLatest(queries, dataset)
    val models = queries


    .onStart { emit("") }


    .transformLatest { query
    ->

    emit(Model.Loading)


    delay(300)


    val results = search(query, dataset)


    emit(Model.Loaded(results))


    }


    }
    ?

    View Slide

  60. @Composable


    fun UserInterface(queries: Flow, datasets: Flow) {


    val models = datasets.flatMapLatest { dataset
    ->

    queries


    .onStart { emit("") }


    .transformLatest { query
    -
    >

    emit(Model.Loading)


    delay(300)


    val results = search(query, dataset)


    emit(Model.Loaded(results))


    }


    }


    }

    View Slide

  61. What if we used Compose?

    View Slide

  62. @Composable


    fun UserInterface(queries: Flow, datasets: Flow) {


    val models = datasets.flatMapLatest { dataset
    ->

    queries


    .onStart { emit("") }


    .transformLatest { query
    -
    >

    emit(Model.Loading)


    delay(300)


    val results = search(query, dataset)


    emit(Model.Loaded(results))


    }


    }


    }
    What if we used Compose?

    View Slide

  63. @Composable


    fun UserInterface(queries: Flow, datasets: Flow) {
    val query by queries.collectAsState("")


    val models = dataset
    s​
    .flatMapLatest { dataset
    ->

    queries


    .onStart { emit("") }


    .transformLatest { query
    -
    >

    emit(Model.Loading)


    delay(300)


    val results = search(query, dataset)


    emit(Model.Loaded(results))


    }


    }


    }

    View Slide

  64. @Composable


    fun UserInterface(queries: Flow, datasets: Flow) {
    val query by queries.collectAsState("")


    val dataset by datasets.collectAsState(InitialSet)


    val models = dataset
    s​
    .flatMapLatest { dataset
    ->

    queries


    .onStart { emit("") }


    .transformLatest { query
    -
    >

    emit(Model.Loading)


    delay(300)


    val results = search(query, dataset)


    emit(Model.Loaded(results))


    }


    }


    }

    View Slide

  65. @Composable


    fun UserInterface(queries: Flow, datasets: Flow) {
    val query by queries.collectAsState("")


    val dataset by datasets.collectAsState(InitialSet)


    var model by remember { mutableStateOf(Model.Loaded(emptyList())) }


    val models = dataset
    s​
    .flatMapLatest { dataset
    ->

    queries


    .onStart { emit("") }


    .transformLatest { query
    -
    >

    emit(Model.Loading)


    delay(300)


    val results = search(query, dataset)


    emit(Model.Loaded(results))


    }


    }


    }

    View Slide

  66. @Composable


    fun UserInterface(queries: Flow, datasets: Flow) {
    val query by queries.collectAsState("")


    val dataset by datasets.collectAsState(InitialSet)


    var model by remember { mutableStateOf(Model.Loaded(emptyList())) }


    val models = dataset
    s​
    .flatMapLatest { dataset
    ->

    queries


    .onStart { emit("") }


    .transformLatest { query
    -
    >

    emit(Model.Loading)


    delay(300)


    val results = search(query, dataset)


    emit(Model.Loaded(results
    )​
    )


    }


    }


    }

    View Slide

  67. @Composable


    fun UserInterface(queries: Flow, datasets: Flow) {
    val query by queries.collectAsState("")


    val dataset by datasets.collectAsState(InitialSet)


    var model by remember { mutableStateOf(Model.Loaded(emptyList())) }


    LaunchedEffect(query, dataset) {


    emit(Model.Loading)


    delay(300)


    val results = search(query, dataset)


    emit(Model.Loaded(results
    )​
    )


    }


    }

    View Slide

  68. @Composable


    fun UserInterface(queries: Flow, datasets: Flow) {
    val query by queries.collectAsState("")


    val dataset by datasets.collectAsState(InitialSet)


    var model by remember { mutableStateOf(Model.Loaded(emptyList())) }


    LaunchedEffect(query, dataset) {


    model = Model.Loading


    delay(300)


    val results = search(query, dataset)


    model = Model.Loaded(results)


    }


    }
    Still reactive!

    View Slide

  69. @Composable


    fun UserInterface(queries: Flow, datasets: Flow) {
    }
    • flatMap


    • scan


    • debounce


    • zip

    View Slide

  70. @Composable


    fun UserInterface(queries: Flow, datasets: Flow) {
    }
    • remember


    • LaunchedEffect


    • collectAsState


    • mutableStateOf

    View Slide

  71. @Composable


    fun UserInterface(queries: Flow, datasets: Flow) {
    }
    • remember


    • LaunchedEffect


    • collectAsState


    • mutableStateOf


    • if, else, when, for, while

    View Slide

  72. @Composable


    fun UserInterface(queries: Flow, datasets: Flow) {
    val query by queries.collectAsState("")


    val dataset by datasets.collectAsState(InitialSet)


    var model by remember { mutableStateOf(Model.Loaded(emptyList())) }


    LaunchedEffect(query, dataset) {


    model = Model.Loading


    delay(300)


    val results = search(query, dataset)


    model = Model.Loaded(results)


    }


    }

    View Slide

  73. @Composable


    fun UserInterface(queries: Flow, datasets: Flow) {
    val query by queries.collectAsState("")


    val dataset by datasets.collectAsState(InitialSet)


    var model by remember { mutableStateOf(Model.Loaded(emptyList())) }


    LaunchedEffect(query, dataset) {


    model =

    Model.Loadin
    g​

    delay(300)


    val results = search(query, dataset)


    model =

    Model.Loaded(results)


    }


    Column {


    // .. .

    }


    }

    View Slide

  74. @Composable


    fun UserInterface(queries: Flow, datasets: Flow) {
    val query by queries.collectAsState("")


    val dataset by datasets.collectAsState(InitialSet)


    var model by remember { mutableStateOf(Model.Loaded(emptyList())) }


    LaunchedEffect(query, dataset) {


    model = Model.Loading


    delay(300)


    val results = search(query, dataset)


    model = Model.Loaded(results)


    }


    Column {


    // .. .

    }


    }
    Presentation logic

    View Slide

  75. Why do we need Molecule?

    View Slide

  76. Why do we need Molecule?
    presentation-logic
    ui
    @Composable
    StateFlow
    junit

    View Slide

  77. Why do we need Molecule?
    presentation-logic
    ui
    @Composable
    StateFlow
    junit

    View Slide

  78. How do we use it?

    View Slide

  79. How do we use it?
    apply plugin: 'app.cash.molecule'

    View Slide

  80. How do we use it?
    Flow StateFlow

    View Slide

  81. How do we use it?
    Flow StateFlow

    View Slide

  82. How do we use it?
    val scope = CoroutineScope(AndroidUiDispatcher.Main)

    View Slide

  83. How do we use it?
    val scope = CoroutineScope(AndroidUiDispatcher.Main)


    val models: StateFlow = scope.launchMolecule() {




    }

    View Slide

  84. How do we use it?
    val scope = CoroutineScope(AndroidUiDispatcher.Main)


    val models: StateFlow = scope.launchMolecule() {


    var model by remember { mutableStateOf(Model(A)) }


    }

    View Slide

  85. val scope = CoroutineScope(AndroidUiDispatcher.Main)


    val models: StateFlow = scope.launchMolecule() {


    var model by remember { mutableStateOf(Model(A)) }




    LaunchedEffect(Unit) {


    model = loadModelB()


    }


    }

    View Slide

  86. val scope = CoroutineScope(AndroidUiDispatcher.Main)


    val models: StateFlow = scope.launchMolecule() {


    var model by remember { mutableStateOf(Model(A)) }




    LaunchedEffect(Unit) {


    model = loadModelB()


    }


    model


    }

    View Slide

  87. val scope = CoroutineScope(AndroidUiDispatcher.Main)


    val models: StateFlow = scope.launchMolecule() {


    var model by remember { mutableStateOf(Model(A)) }




    LaunchedEffect(Unit) {


    model = loadModelB()


    }


    model


    }
    1

    View Slide

  88. val scope = CoroutineScope(AndroidUiDispatcher.Main)


    val models: StateFlow = scope.launchMolecule() {


    var model by remember { mutableStateOf(Model(A)) }




    LaunchedEffect(Unit) {


    model = loadModelB()


    }


    model


    }
    2

    View Slide

  89. val scope = CoroutineScope(AndroidUiDispatcher.Main)


    val models: StateFlow = scope.launchMolecule() {


    var model by remember { mutableStateOf(Model(A)) }




    LaunchedEffect(Unit) {


    model = loadModelB()


    }


    model


    }
    3
    A
    Emissions

    View Slide

  90. val scope = CoroutineScope(AndroidUiDispatcher.Main)


    val models: StateFlow = scope.launchMolecule() {


    var model by remember { mutableStateOf(Model(A)) }




    LaunchedEffect(Unit) {


    model = loadModelB()


    }


    model


    }
    4
    A
    Emissions

    View Slide

  91. val scope = CoroutineScope(AndroidUiDispatcher.Main)


    val models: StateFlow = scope.launchMolecule() {


    var model by remember { mutableStateOf(Model(A)) }




    LaunchedEffect(Unit) {


    model = loadModelB()


    }


    model


    }
    5
    A
    Emissions

    View Slide

  92. 6
    A B
    val scope = CoroutineScope(AndroidUiDispatcher.Main)


    val models: StateFlow = scope.launchMolecule() {


    var model by remember { mutableStateOf(Model(A)) }




    LaunchedEffect(Unit) {


    model = loadModelB()


    }


    model


    }
    Emissions

    View Slide

  93. val scope = CoroutineScope(AndroidUiDispatcher.Main)


    val models: StateFlow = scope.launchMolecule() {


    var model by remember { mutableStateOf(Model(A)) }




    LaunchedEffect(Unit) {


    model = loadModelB()


    model = Model(C)


    }


    model


    }
    A B
    Emissions
    ?

    View Slide

  94. val scope = CoroutineScope(AndroidUiDispatcher.Main)


    val models: StateFlow = scope.launchMolecule() {


    var model by remember { mutableStateOf(Model(A)) }




    LaunchedEffect(Unit) {


    model = loadModelB()


    model = Model(C)


    }


    model


    }
    A B C
    Emissions

    View Slide

  95. val scope = CoroutineScope(AndroidUiDispatcher.Main)


    val models: StateFlow = scope.launchMolecule() {


    model


    }
    • Some State is invalidated


    • A MonotonicFrameClock ticks
    For a new emission, two things must happen
    Emissions

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  96. val scope = CoroutineScope(AndroidUiDispatcher.Main)


    val models: StateFlow = scope.launchMolecule() {


    var model by remember { mutableStateOf(Model(A)) }




    LaunchedEffect(Unit) {


    model = loadModelB()


    model = Model(C)


    }


    model


    }
    A B C
    Emissions

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  97. A B C
    Emissions
    val scope = CoroutineScope(AndroidUiDispatcher.Main)


    val models: StateFlow = scope.launchMolecule() {


    var model by remember { mutableStateOf(Model(A)) }




    LaunchedEffect(Unit) {


    model = loadModelB()


    model = Model(C)


    }


    model


    }

    View Slide

  98. A B C
    Frame tick
    Emissions
    val scope = CoroutineScope(AndroidUiDispatcher.Main)


    val models: StateFlow = scope.launchMolecule() {


    var model by remember { mutableStateOf(Model(A)) }




    LaunchedEffect(Unit) {


    model = loadModelB()


    model = Model(C)


    }


    model


    }

    View Slide

  99. A B C
    Emissions
    val scope = CoroutineScope(AndroidUiDispatcher.Main)


    val models: StateFlow = scope.launchMolecule() {


    var model by remember { mutableStateOf(Model(A)) }




    LaunchedEffect(Unit) {


    model = loadModelB()


    model = Model(C)


    }


    model


    }

    View Slide

  100. A B C
    Emissions
    C
    val scope = CoroutineScope(AndroidUiDispatcher.Main)


    val models: StateFlow = scope.launchMolecule() {


    var model by remember { mutableStateOf(Model(A)) }




    LaunchedEffect(Unit) {


    model = loadModelB()


    model = Model(C)


    }


    model


    }

    View Slide

  101. A B C
    Emissions
    val scope = CoroutineScope(AndroidUiDispatcher.Main)


    val models: StateFlow = scope.launchMolecule() {


    var model by remember { mutableStateOf(Model(A)) }




    LaunchedEffect(Unit) {


    model = loadModelB()


    model = Model(C)


    }


    model


    }

    View Slide

  102. val scope = CoroutineScope(AndroidUiDispatcher.Main)


    val models: StateFlow = scope.launchMolecule() {


    var model by remember { mutableStateOf(Model(A)) }




    LaunchedEffect(Unit) {


    model = loadModelB()


    model = Model(C)


    }


    model


    }

    View Slide

  103. val scope = CoroutineScope(AndroidUiDispatcher.Main)


    val models: StateFlow = scope.launchMolecule(


    clock = RecompositionClock.ContextClock


    ) {


    var model by remember { mutableStateOf(Model(A)) }




    LaunchedEffect(Unit) {


    model = loadModelB()


    model = Model(C)


    }


    model


    }

    View Slide

  104. val scope = CoroutineScope(AndroidUiDispatcher.Main)


    val models: StateFlow = scope.launchMolecule(


    clock = RecompositionClock.ContextClock


    ) {


    var model by remember { mutableStateOf(Model(A)) }




    LaunchedEffect(Unit) {


    model = loadModelB()


    model = Model(C)


    }


    model


    }

    View Slide

  105. val scope = CoroutineScope
    (​
    )


    val models: StateFlow = scope.launchMolecule(


    clock = RecompositionClock.ContextClock


    ) {


    var model by remember { mutableStateOf(Model(A)) }




    LaunchedEffect(Unit) {


    model = loadModelB()


    model = Model(C)


    }


    model


    }

    View Slide

  106. fun unitTest() = runBlocking {


    val scope = this


    val models: StateFlow = scope.launchMolecule(


    clock = RecompositionClock.ContextClock


    ) {


    var model by remember { mutableStateOf(Model(A)) }




    LaunchedEffect(Unit) {


    model = loadModelB()


    model = Model(C)


    }


    model


    }

    View Slide

  107. fun unitTest() = runBlocking {


    val scope = this


    val models: StateFlow = scope.launchMolecule(


    clock = RecompositionClock.Immediate


    ) {


    var model by remember { mutableStateOf(Model(A)) }




    LaunchedEffect(Unit) {


    model = loadModelB()


    model = Model(C)


    }


    model


    }

    View Slide

  108. fun unitTest() = runBlocking {


    val scope = this


    val models: StateFlow = scope.launchMolecule(


    clock = RecompositionClock.Immediate


    ) {


    var model by remember { mutableStateOf(Model(A)) }




    LaunchedEffect(Unit) {


    model = loadModelB()


    model = Model(C)


    }


    model


    }

    View Slide

  109. ContextClock
    Immediate
    • Emissions match Android’s built in frame clock


    • Need to control emissions using BroadcastFrameClock


    • Unit tests requiring time manipulation
    Choosing a RecompositionClock
    • Frames tick automatically when snapshot state changes


    • Unit tests that don’t require time manipulation

    View Slide

  110. You only get one emission per
    frame

    View Slide

  111. Realistic example

    View Slide

  112. Realistic example
    class Presenter {


    @Composable


    fun models(events: Flow): Model {


    //
    Calculate Model here.


    }


    }

    View Slide

  113. Realistic example
    class Presenter {


    @Composable


    fun models(events: Flow): Model {


    //
    Calculate Model here.


    }


    }

    View Slide

  114. class SearchPresenter : Presenter {


    @Composable


    fun models(events: Flow): Model {


    //
    Calculate Model here.


    }


    }
    Realistic example

    View Slide

  115. sealed interface Event {


    data class EnterText(val text: String) : Event


    }


    sealed interface Model {


    object Loading : Model


    data class Loaded(


    val results: List = emptyList()


    ) : Model


    }

    View Slide

  116. class SearchPresenter : Presenter {


    @Composable


    fun models(events: Flow): Model {


    //
    Calculate Model here.


    }


    }
    Realistic example

    View Slide

  117. class SearchPresenter(


    val service: Service


    ) : Presenter {


    @Composable


    fun models(events: Flow): Model {


    //
    Calculate Model here.


    }


    }
    Realistic example

    View Slide

  118. class SearchPresenter(


    val service: Service


    ) : Presenter {


    @Composable


    fun models(events: Flow): Model {


    val modelState = remember { mutableStateOf(Model.Loaded()) }


    }


    }
    Realistic example

    View Slide

  119. class SearchPresenter(


    val service: Service


    ) : Presenter {


    @Composable


    fun models(events: Flow): Model {


    val modelState: MutableState = remember { mutableStateOf(Mod
    }


    }
    Realistic example

    View Slide

  120. class SearchPresenter(


    val service: Service


    ) : Presenter {


    @Composable


    fun models(events: Flow): Model {


    val modelState = remember { mutableStateOf(Model.Loaded()) }


    }


    }
    Realistic example

    View Slide

  121. class SearchPresenter(


    val service: Service


    ) : Presenter {


    @Composable


    fun models(events: Flow): Model {


    val modelState = remember { mutableStateOf(Model.Loaded()) }


    return modelState.value


    }


    }
    Realistic example

    View Slide

  122. class SearchPresenter(


    val service: Service


    ) : Presenter {


    @Composable


    fun models(events: Flow): Model {


    val modelState = remember { mutableStateOf(Model.Loaded()) }


    LaunchedEffect(Unit) {


    events.collect { event
    ->

    when (event) {


    ...

    }


    }


    }


    return modelState.value


    }


    }

    View Slide

  123. class SearchPresenter(


    val service: Service


    ) : Presenter {


    @Composable


    fun models(events: Flow): Model {


    val modelState = remember { mutableStateOf(Model.Loaded()) }


    var query by remember { mutableStateOf("") }


    LaunchedEffect(Unit) {


    events.collect { event
    ->

    when (event) {


    EnterText
    ->
    query = event.text


    }


    }


    }


    return modelState.value


    }


    View Slide

  124. @Composable


    fun models(events: Flow): Model {


    val modelState = remember { mutableStateOf(Model.Loaded()) }


    var query by remember { mutableStateOf("") }


    LaunchedEffect(Unit) {


    events.collect { event
    ->

    when (event) {


    EnterText
    ->
    query = event.text


    }


    }


    }


    LaunchedEffect(query) {


    runSearch(query, modelState)


    }


    return modelState.value


    }


    View Slide

  125. private suspend fun runSearch(


    query: String,


    state: MutableState,


    ) {


    state.value = Model.Loading


    val results = service.search(query)


    state.value = Model.Loaded(results)


    }
    LaunchedEffect(query) {


    runSearch(query, modelState)


    }


    View Slide

  126. private suspend fun runSearch(


    query: String,


    state: MutableState,


    ) {


    state.value = Model.Loading


    val results = service.search(query)


    state.value = Model.Loaded(results)


    }
    LaunchedEffect(query) {


    runSearch(query, modelState)


    }


    View Slide

  127. private suspend fun runSearch(


    query: String,


    state: MutableState,


    ) {


    state.value = Model.Loading


    val results = service.search(query)


    state.value = Model.Loaded(results)


    }
    LaunchedEffect(query) {


    runSearch(query, modelState)


    }


    View Slide

  128. Writing a test
    cashapp/turbine

    View Slide

  129. cashapp/turbine
    Writing a test
    flowOf("one", "two").test {


    assertEquals("one", awaitItem())


    assertEquals("two", awaitItem())


    awaitComplete()


    }

    View Slide

  130. Writing a test
    @Test


    fun `entering text runs search`() = runBlocking {


    }

    View Slide

  131. Writing a test
    @Test


    fun `entering text runs search`() = runBlocking {


    val events = MutableSharedFlow(replay = 1)


    }

    View Slide

  132. Writing a test
    @Test


    fun `entering text runs search`() = runBlocking {


    val events = MutableSharedFlow(replay = 1)


    launchMolecule(RecompositionClock.Immediate) {


    presenter.models(events)


    }.test {


    }


    }

    View Slide

  133. Writing a test
    @Test


    fun `entering text runs search`() = runBlocking {


    val events = MutableSharedFlow(replay = 1)


    launchMolecule(RecompositionClock.Immediate) {


    presenter.models(events)


    }.test {


    assertThat(awaitItem()).isEqualTo(Model.Loading)


    }


    }

    View Slide

  134. Writing a test
    @Test


    fun `entering text runs search`() = runBlocking {


    val events = MutableSharedFlow(replay = 1)


    launchMolecule(RecompositionClock.Immediate) {


    presenter.models(events)


    }.test {


    assertThat(awaitItem()).isEqualTo(Model.Loading)


    events.emit(Event.EnterText("query"))


    fakeService.setResults(
    ..
    .
    )


    }


    }

    View Slide

  135. Writing a test
    @Test


    fun `entering text runs search`() = runBlocking {


    val events = MutableSharedFlow(replay = 1)


    launchMolecule(RecompositionClock.Immediate) {


    presenter.models(events)


    }.test {


    assertThat(awaitItem()).isEqualTo(Model.Loading)


    events.emit(Event.EnterText("query"))


    fakeService.setResults(
    ..
    .
    )


    assertThat(awaitItem()).isEqualTo(Model.Loaded(
    ..
    .
    ))


    }


    }

    View Slide

  136. Writing a test
    @Test


    fun `entering text runs search`() = runBlocking {


    presenter.test {


    assertThat(awaitItem()).isEqualTo(Model.Loading)


    sendEvent(Event.EnterText("query"))


    fakeService.setResults(
    ..
    .
    )


    assertThat(awaitItem()).isEqualTo(Model.Loaded(
    ..
    .
    ))


    }


    }

    View Slide

  137. Takeaways

    View Slide

  138. Takeaways
    • Compose manages a tree of nodes - it doesn’t have to be UI


    • Managing state involves tying together streams


    • Compose can do to streams what suspend did to Single


    • State is reactive. Think of it like a stream


    • There’s still a learning curve, but it’s less steep compared to Rx/Flow


    • Tricks you’ve learnt in Compose UI work in Molecule too

    View Slide

  139. Interested in more?
    Building StateFlows with Jetpack Compose
    droidcon.com/2022/09/29/building-stateflows-in-android-with-jetpack-compose
    Mohit Sarveiya
    Demystifying Molecule
    droidcon.com/2022/09/29/demystifying-molecule-running-your-own-compositions-for-fun-and-profit
    Bill Phillips & Ash Davies
    Opening the Shutter on Snapshots
    droidcon.com/2022/09/29/opening-the-shutter-on-snapshots
    Zach Klippenstein

    View Slide

  140. chris_h_codes
    github.com/cashapp/molecule
    Molecule
    Using Compose for presentation logic

    View Slide