RxJava: A Stream of Joy and Woe

RxJava has become an invaluable tool to many Android developers, aiding in the composition of the several asynchronous systems we must deal with. However, with so much power RxJava can sometimes be the hammer we hold with almost everything looking like a nail. This talk aims to run through anecdotal examples of where RxJava has worked well, and where it maybe wasn’t the best idea.

Transcript

1. RxJava
   A Stream of Joy and Woe
   @chris_h_codes
   

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2. Why have we chased
   the dragon?
   

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3. 
   

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4. 
   

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5. 
   

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6. 
   

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7. 
   

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8. webServices.getSomething()
   .subscribeOn(Schedulers.io())
   .observeOn(AndroidSchedulers.mainThread())
   .subscribe { something ->
   // ...
   }/
   

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9. webServices.getSomething()
   .map { result -> transform(result) }
   .subscribeOn(Schedulers.io())
   .observeOn(AndroidSchedulers.mainThread())
   .subscribe { something ->
   // ...
   }/
   

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10. webServices.getSomething()
    .flatMap { firstResult -> webServices.getSomethingElse(firstResult) }
    .map { result -> transform(result) }
    .subscribeOn(Schedulers.io())
    .observeOn(AndroidSchedulers.mainThread())
    .subscribe { something ->
    // ...
    }/
    

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11. webServices.getSomething()
    .flatMap { firstResult -> webServices.getSomethingElse(firstResult) }
    .map { result -> transform(result) }
    .subscribeOn(Schedulers.io())
    .observeOn(AndroidSchedulers.mainThread())
    .subscribe (
    { something ->
    // ...
    },
    { error ->
    // ...
    }
    )
    

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12. webServices.getSomething()
    .flatMap { firstResult -> webServices.getSomethingElse(firstResult) }
    .map { result -> transform(result) }
    .subscribeOn(Schedulers.io())
    .observeOn(AndroidSchedulers.mainThread())
    .subscribe (
    { something ->
    // ...
    },
    { error ->
    // ...
    }
    )
    OF
    TERMINAL EVENTS
    BEWARE
    

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13. val getSomethingFromWeb = webServices.getSomething()
    .flatMap { firstResult -> webServices.getSomethingElse(firstResult) }
    .map { result -> transform(result) }
    .subscribeOn(Schedulers.io())
    .observeOn(AndroidSchedulers.mainThread())
    

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14. systemEventEmitter()
    .switchMap { getSomethingFromWeb.toObservable() }
    .withLatestFrom(somethingElse) {
    // ...
    }
    

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15. interface WebServices {
    @GET("something")
    fun getSomething(): Single>
    }/
    

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16. interface WebServices {
    @GET("something")
    fun getSomething(): Single>
    }/
    

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17. webServices.getSomething()
    .flatMap { firstResult -> webServices.getSomethingElse(firstResult) }
    .map { result -> transform(result) }
    .subscribeOn(Schedulers.io())
    .observeOn(AndroidSchedulers.mainThread())
    .subscribe { something ->
    // ...
    }/
    

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18. webServices.getSomething()
    .flatMap { firstResult -> webServices.getSomethingElse(firstResult) }
    .map { result -> transform(result) }
    .subscribeOn(Schedulers.io())
    .observeOn(AndroidSchedulers.mainThread())
    .subscribe { something ->
    // ...
    }/
    

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19. webServices.getSomething()
    .flatMap { firstResult ->
    if (!firstResult.isError()) {
    webServices.getSomethingElse(firstResult.response().body())
    } else {
    Single.just(ERROR)
    }/
    }/
    .map { result -> transform(result) }
    .subscribeOn(Schedulers.io())
    .observeOn(AndroidSchedulers.mainThread())
    .subscribe { something ->
    // ...
    }/
    

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20. webServices.getSomething()
    .flatMap { firstResult ->
    if (!firstResult.isError() && firstResult.response().isSuccessful) {
    webServices.getSomethingElse(firstResult.response().body())
    } else {
    Single.just(ERROR)
    }/
    }/
    .map { result -> transform(result) }
    .subscribeOn(Schedulers.io())
    .observeOn(AndroidSchedulers.mainThread())
    .subscribe { something ->
    // ...
    }/
    

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21. webServices.getSomething()
    .flatMap { firstResult ->
    if (!firstResult.isError() && firstResult.response().isSuccessful) {
    webServices.getSomethingElse(firstResult.response().body())
    } else {
    Single.just(ERROR)
    }/
    }/
    .map { result -> transform(result) }
    .subscribeOn(Schedulers.io())
    .observeOn(AndroidSchedulers.mainThread())
    .subscribe { something ->
    // ...
    }/
    

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22. suspend fun doSomeWork() {
    try {
    val firstResponse = webServices.getSomething().await()
    if (firstResponse.isSuccessful) {
    val something = firstResponse.body()
    val secondResponse = webServices.getSomethingElse(something).await()
    // ..
    }
    } catch (e: IOException) {
    // ..
    }
    }
    

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23. 
    

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25. • Interfacing with an engine monitoring system
    • Over bluetooth
    • Which is emulating a serial connection
    • Enabling a (very slow) satellite internet service
    • Via SOAP
    • Sending location and engine data over that
    connection
    • Meanwhile monitoring the accelerometer to check
    for accidents
    

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27. Monitoring for accidents
    

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28. Z
    X
    

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29. Y
    

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30. android.hardware.Sensor.TYPE_GRAVITY
    XYZ XYZ XYZ
    XZ XZ XZ
    XYZ
    Check against some threshold
    !
    Throttle detections
    

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31. sensors.observeSensor(TYPE_GRAVITY)
    .distinctUntilChanged()
    .map { it.sensorEvent.values }
    .filter { it[0] > ROLLOVER_THRESHOLD || it[2] > ROLLOVER_THRESHOLD }
    .map { Emergency.ROLLOVER }
    .throttleFirst(THROTTLE_SECONDS, TimeUnit.SECONDS)
    .observeOn(RealRxSchedulers.mainThread())
    .subscribe { }
    

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32. • Location updates
    • Bluetooth payloads from engine
    • Managing output sockets as network conditions change
    • Shift status timers
    

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33. RxPermissions
    .request(Manifest.permission.CAMERA)
    .subscribe { granted ->
    if (granted) {
    // I can control the camera now.
    } else {
    // Oh no, permission denied.
    }
    }
    

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34. RxActivityResult.on(this)
    .startIntent(takePhoto)
    .subscribe { result, resultCode ->
    if (resultCode == RESULT_OK) {
    result.targetUi().showImage(data)
    } else {
    result.targetUi().printUserCancelled()
    }
    }
    

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35. 
    

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36. 13:37
    chris_h_codes
    

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37. 13:37
    chris_h_codes
    

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38. 13:37
    chris_h_codes
    

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39. fun onViewAttached() {
    val user = userStore.getCurrentValue()
    avatarView.setImage(user.profilePic)
    usernameView.text = user.username
    }/
    

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40. fun onViewAttached() {
    val user = userStore.getCurrentValue()()
    avatarView.setImage(user.profilePic)
    usernameView.text = user.username
    userStore.observe().subscribe { user ->
    avatarView.setImage(user.profilePic)
    usernameView.text = user.usernmame
    }
    }/
    

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41. https:/
    /github.com/Gridstone/RxStore
    

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42. https:/
    /github.com/Gridstone/RxStore
    val userStore = storeProvider.valueStore(file, converter)
    userStore.put(user)
    val observeUser: Observable = userStore.observe()
    val putUser: Single = userStore.observePut(user)
    

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43. webServices.getUser()
    .concatMap { user -> userStore.observePut(user) }
    .subscribeOn(Schedulers.io())
    .observeOn(AndroidSchedulers.mainThread())
    

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44. 
    

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45. Controller
    View
    

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46. Controller
    View
    

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47. State
    Controller
    View
    

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48. Controller
    View
    State
    Event
    

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49. Controller
    View
    Observable
    Observable
    

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50. val events: Observable = Observable.merge(
    loginButton.clicks().map { Event.AttemptLogin(username, password) },
    cancelButton.clicks().map { Event.Cancel }
    )
    https:/
    /github.com/JakeWharton/RxBinding
    

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51. Observables
    .combineLatest(
    usernameView.textChanges().map { it.isNotEmpty() },
    passwordView.textChanges().map { it.isNotEmpty() })
    { hasUsername, hasPassword -> hasUsername && hasPassword }
    .subscribe(loginButton::setEnabled)
    Username
    Password
    Login
    

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52. Controller
    View
    Observable
    Observable
    

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55. View()
    onFinishInflate()
    onRestoreInstanceState()
    draw()
    display(state)
    

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56. stateStream
    .subscribeOn(Schedulers.computation())
    .observeOn(AndroidSchedulers.mainThread())
    .subscribe { state ->
    view.display(state)
    }/
    

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57. stateStream
    .replay(1)
    .autoConnect()
    .subscribeOn(Schedulers.computation())
    .observeOn(AndroidSchedulers.mainThread())
    .subscribe { state ->
    view.display(state)
    }/
    

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58. stateStream
    .subscribeOn(Schedulers.computation())
    .observeOn(AndroidSchedulers.mainThread())
    .replay(1)
    .autoConnect()
    .subscribe { state ->
    view.display(state)
    }/
    

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59. stateStream
    

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60. action
    state
    reduce(action, previousState)
    

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61. Takeaways
    

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62. Takeaways
    • If you only use Rx for callbacks with nice threading, consider coroutines
    • Beware terminal events
    • Be mindful of thread boundaries and state delivery
    • Don’t try to Rx-AllTheThings
    

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64. RxJava
    A Stream of Joy and Woe
    chris_h_codes
    chris-horner
    chrishorner.codes
    

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