Rx by example – Volume 3 (the multicast edition)

Given at 360|AnDev

Video: https://academy.realm.io/posts/360-andev-2017-kaushik-gopal-rxjava-by-example-multicasting/

As developer’s understanding of RxJava has matured, they’ve started to peel back the layers and unleash its true power and potential.

One such power is “Multicasting” where you get to share work across your app and reuse a whole bunch of stuff. This is really hard to do in general but Rx makes it really really easy.

Rx itself though has a steep learning curve. But the easiest way to grasp the concepts and features of Rx, is through examples. I’ve given talks in the past explaining Rx usage purely through examples and folks have found this to be the easiest way to warm up to Rx.

Transcript

1. Rx by example

   The Multicast edition
   Vol. 3
   

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2. Multicasting
   What is
   Why
   How
   

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3. Multicasting
   2 ways to do
   ConnectableObservables
   Subjects
   

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4. Multicasting
   with
   ConnectableObservables
   

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5. val source: Observable =
   Observable.interval(1, TimeUnit.SECONDS)
   source.subscribe { Log.v(TAG, "subscriber 1 : received event $it") }
   subscriber 1 : received event 0
   subscriber 2 : received event 0
   subscriber 1 : received event 1
   subscriber 2 : received event 1
   subscriber 1 : received event 2
   subscriber 2 : received event 2
   subscriber 1 : received event 3
   subscriber 2 : received event 3
   subscriber 1 : received event 4
   subscriber 2 : received event 4
   source.subscribe { Log.v(TAG, "subscriber 2 : received event $it") }
   

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6. val source: Observable =
   Observable.interval(1, TimeUnit.SECONDS)
   Thread.sleep(1000)
   source.subscribe { Log.v(TAG, "subscriber 1 : received event $it") }
   source.subscribe { Log.v(TAG, "subscriber 2 : received event $it") }
   subscriber 1 : received event 0
   Skipped 59 frames! app may be doing too much work on its main thread.
   subscriber 1 : received event 1
   subscriber 2 : received event 0
   subscriber 1 : received event 2
   subscriber 2 : received event 1
   subscriber 1 : received event 3
   subscriber 2 : received event 2
   subscriber 1 : received event 4
   subscriber 2 : received event 3
   

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7. val source: Observable =
   Observable.interval(1, TimeUnit.SECONDS)
   Thread.sleep(1000)
   source.subscribe { Log.v(TAG, "subscriber 1 : received event $it") }
   source.subscribe { Log.v(TAG, "subscriber 2 : received event $it") }
   ConnectableObservable =
   .publish()
   source.connect()
   Skipped 59 frames! app may be doing too much work on its main thread.
   subscriber 1 : received event 0
   subscriber 2 : received event 0
   subscriber 1 : received event 1
   subscriber 2 : received event 1
   subscriber 1 : received event 2
   subscriber 2 : received event 2
   subscriber 1 : received event 3
   subscriber 2 : received event 3
   

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8. .publish().refcou
   .replay().autoCo
   .share().cache(
   .replayingShar
   blish().refcoun
   eplay().autoConnect(n
   share().cache()
   .replayingShare()
   h().refcount()
   y().autoConnect(n)
   ).cache()
   are() .publish().refcount
   .replay().autoConn
   re().cache()
   ngShare(
   Multicasting
   operator
   permutations
   

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9. val sourceObservable =
   Observable.interval(1, TimeUnit.SECONDS)

   .doOnSubscribe{ log("observable (subscribed)") }

   .doOnDispose{ log("observable (disposed)") }

   disposable1?.let {

   it.dispose()

   log("subscriber 1 disposed")

   disposable1 = null

   return

   }
   @OnClick(R.id.btn_1)

   fun onBtn1Click() {
   disposable1 =
   sharedObservable.subscribe({ long ->
   _log(“subscriber 1: onNext $long") })
   }
   https://github.com/kaushikgopal/RxJava-Android-Samples/tree/kg/feat/connect_playground
   

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10. val sourceObservable =
    Observable.interval(1, TimeUnit.SECONDS)
    sourceObservable.connect()
    .doOnSubscribe{ log("observable (subscribed)") }

    .doOnDispose{ log("observable (disposed)") }
    .publish()
    https://github.com/kaushikgopal/RxJava-Android-Samples/tree/kg/feat/connect_playground
    

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11. val sourceObservable =
    Observable.interval(1, TimeUnit.SECONDS)
    .doOnSubscribe{ log("observable (subscribed)") }

    .doOnDispose{ log("observable (disposed)") }
    sourceObservable
    .refCount()
    .publish()
    https://github.com/kaushikgopal/RxJava-Android-Samples/tree/kg/feat/connect_playground
    

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12. sourceObservable
    .publish().refCount()
    

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13. sourceObservable
    .publish().refCount()
    subscribe
    1 subscriber
    so connect automatically
    0
    0
    onNext
    onNext
    1
    1
    onNext
    onNext
    subscribe
    2 subscribers
    2
    onNext
    onNext
    2
    onNext
    2
    3
    onNext
    onNext
    3
    onNex
    3
    refcount()
    

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14. ribers
    2
    onNext
    onNext
    2
    onNext
    2
    3
    onNext
    onNext
    3
    onNext
    3
    refcount()
    unsubscribe
    X
    unsubscribe
    1 subscriber left
    4
    4
    onNext
    onNext
    unsubscribe
    X
    X
    0
    subscribers left
    

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15. ext
    refcount()
    unsubscribe
    X
    unsubscribe
    1 subscriber left
    4
    4
    onNext
    onNext
    X
    X
    0
    subscribers left
    subscribe
    0
    0
    onNext
    onNext
    YO LISTEN UP !!
    THIS IS IMPORTANT
    

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16. sourceObservable
    .publish().autoConnect(2)
    

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17. sourceObservable
    .publish().autoConnect(2)
    .autoConnect(2)
    subscribe
    1 subscriber
    (waiting for 2)
    subscribe
    2 subscribers
    0
    onNext
    onNext
    0
    onNext
    0
    1
    onNext
    onNext
    1
    onNext
    1
    unsubscribe
    X
    unsubscribe
    1 subscriber left
    

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18. sourceObservable
    .publish().autoConnect(2)
    ext
    Next
    Next
    1
    onNext
    onNext
    1
    onNext
    1 X
    unsubscribe
    1 subscriber left
    .autoConnect(2)
    2
    2
    onNext
    onNext
    unsubscribe
    X
    0
    subscribers left
    3 4
    YO Me AGAIN !!
    THIS IS IMPORTANT
    

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19. sourceObservable
    .publish().autoConnect(2)
    be
    ber left
    2
    2
    onNext
    onNext
    unsubscribe
    X
    0
    subscribers left
    3 4
    .autoConnect(2)
    subscribe
    5
    5
    onNext
    onNext
    first event seen
    is 5
    

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20. Recap
    .publish()
    .autoConnect(n)
    .refcount()
    Creates ConnectableObservable
    Requires .connect()
    or something else to kickstart
    Source subscription started
    right away
    Source is “restarted” when
    all subscribers leave and 

    new subscribers come in
    Source subscription started

    only after n subscribers start
    Source never stops once
    started (even if all subscribers
    leave)
    

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21. .publish()
    Creates ConnectableObservable
    .replay(n)
    Creates ConnectableObservable
    Another way to create
    ConnectableObservables
    Start with last “n” events
    “n” defaults to all events
    

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22. Pop Quiz !
    .publish().refCount()
    .publish().autoConnect(1)
    Vs
    a.k.a
    .share()
    

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23. Pop Quiz !
    .replay(1).refCount()
    .replay(1).autoConnect(1)
    Vs
    ~
    a.k.a
    .cache()
    *
    

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24. sourceObservable
    .replay(1).refCount()
    .replay(1)
    .refCount()
    subscribe
    1 subscriber
    so connect automatically
    0
    0
    onNext
    onNext
    subscribe
    2 subscribers
    onNext
    0
    unsubscribe
    X
    unsubscribe
    X
    X
    subscribe
    No replay!
    

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25. sourceObservable
    .replay(1).refCount()
    .replay(1)
    .autoConnect()
    subscribe
    1 subscriber
    so connect automatically
    0
    0
    onNext
    onNext
    subscribe
    2 subscribers
    onNext
    0
    unsubscribe
    X
    unsubscribe
    X
    subscribe
    onNext
    0
    

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26. Pop Quiz !
    .replayingShare()
    = .replay(1).refcount()
    But remembers the last item even when restarted
    .replay(1).refCount()
    

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27. Use cases
    Let’s get real
    

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28. Location provider
    Use case 1
    return RxLocation(context)
    .updates(locationRequest)
    // multicast?
    .publish()
    .autoConnect(n)
    .replay(1)
    .refcount()
    .replayingShare()
    .replay(1)
    .refcount()
    

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29. Use case 2
    Persist a network call beyond activity rotation
    ViewModels baby ...
    

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30. Persist a network call beyond activity rotation
    class RotationPersistActivity
    : MyBaseActivity() {
    lateinit var sharedViewModel: SharedViewModel
    override fun onCreate(savedInstanceState: Bundle?) {

    super.onCreate(savedInstanceState)

    sharedViewModel =
    ViewModelProviders.of(activity)
    .get(SharedViewModel::class.java) 

    }

    

    @OnClick(R.id.btn_rotate_persist)

    fun displayEventsOnBtnClick() {

    // …
    compositeDisposables +=
    sharedViewModel

    .networkRequest()

    .subscribe({ l ->

    Use case 2
    

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31. class RotationPersistActivity
    : MyBaseActivity() {
    lateinit var sharedViewModel: SharedViewModel
    override fun onCreate(savedInstanceState: Bundle?) {

    super.onCreate(savedInstanceState)

    sharedViewModel =
    ViewModelProviders.of(activity)
    .get(SharedViewModel::class.java) 

    }

    

    @OnClick(R.id.btn_rotate_persist)

    fun displayEventsOnBtnClick() {

    // …
    compositeDisposables +=
    sharedViewModel

    .networkRequest()

    .subscribe({ l ->

    log("Received element $l")

    })
    }
    }
    Persist a network call beyond activity rotation
    Use case 2
    

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32. Persist a network call beyond activity rotation
    override fun onCleared() {

    super.onCleared()

    // cleanup
    }
    Use case 2
    

    fun networkRequest(): Observable {

    

    // We’ll fill this later
    }
    

    class SharedViewModel : ViewModel() {

    

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33. Use case 2
    Persist a network call beyond activity rotation
    

    fun networkRequest(): Observable {

    
 // We’ll fill this later
    }
    override fun onCleared() {

    super.onCleared()

    // cleanup
    }
    var sharedStream: Observable =
    someApi.makeRequest()
    //.map(…)
    .replay(1)
    .autoConnect()
    

    class SharedViewModel : ViewModel() {

    return sharedStream
    

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34. Persist a network call beyond activity rotation
    Use case 2
    

    fun networkRequest(): Observable {

    

    }
    override fun onCleared() {

    super.onCleared()

    // cleanup
    }
    var sharedStream: Observable =
    someApi.makeRequest()
    //.map(…)
    .replay(1)
    .autoConnect()
    

    class SharedViewModel : ViewModel() {

    return sharedStream
    

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35. Persist a network call beyond activity rotation
    .replay(1)
    .autoConnect(1, disposable -> {
    myDisposable = disposable
    })
    var myDisposable: Disposable? = null
    myDisposable?.dispose
    

    class SharedViewModel : ViewModel() {

    var sharedStream: Observable =
    someApi.makeRequest()
    //.map(…)
    override fun onCleared() {

    super.onCleared()

    // cleanup
    }
    fun networkRequest(): Observable {

    return sharedStream
    }
    Use case 2
    

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36. _paginator
    .map(items ->{ //do something })
    .onBackPressureDrop()
    .doOnNext(

    i -> {

    _progressBar.setVisibility(View.VISIBLE);

    })
    .concatMap(this::itemsFromNetworkCall)
    .observeOn(AndroidSchedulers.mainThread())
    .map(items -> {

    _adapter.addItems(items);

    _adapter.notifyDataSetChanged();

    return items;

    })

    .subscribe(strings -> {

    _progressBar.setVisibility(View.INVISIBLE);

    });

    Use case 3
    Reduce side effects
    

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37. .concatMap(this::itemsFromNetworkCall)
    .observeOn(AndroidSchedulers.mainThread())
    .map(items -> {

    _adapter.addItems(items);

    _adapter.notifyDataSetChanged();

    return items;

    })

    .subscribe(strings -> {

    _progressBar.setVisibility(View.INVISIBLE);

    });

    Use case 3
    Reduce side effects
    .doOnNext(

    i -> {

    _progressBar.setVisibility(View.VISIBLE);

    })
    _paginator
    .map(items ->{ //do something })
    .onBackPressureDrop()
    

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38. .doOnNext(

    i -> {

    _progressBar.setVisibility(View.VISIB
    })
    Use case 3
    Reduce side effects
    _paginator
    .map(items ->{ //do something })
    .onBackPressureDrop()
    .publish()
    ConnectableObservable shared =
    shared
    shared
    .subscribe(

    i -> {

    _progressBar.setVisibility(View.VISIB
    })
    .concatMap(this::itemsFromNetworkCall)
    .observeOn(AndroidSchedulers.mainThread())
    .map(items -> {

    _adapter.addItems(items);

    _adapter.notifyDataSetChanged();

    return items;

    })

    .subscribe(strings -> {

    _progressBar.setVisibility(View.INVISIBLE);

    });

    

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39. .concatMap(this::itemsFromNetworkCall)
    .observeOn(AndroidSchedulers.mainThread())
    .map(items -> {

    _adapter.addItems(items);

    _adapter.notifyDataSetChanged();

    return items;

    })

    .subscribe(strings -> {

    _progressBar.setVisibility(View.INVISIBLE);

    });

    Use case 3
    Reduce side effects
    _paginator
    .map(items ->{ //do something })
    .onBackPressureDrop()
    .publish()
    ConnectableObservable shared =
    shared.connect();
    .subscribe(

    i -> {

    _progressBar.setVisibility(View.VISIBLE);

    })
    shared
    shared
    

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40. ✌
    fragmentedpodcast.com
    @kaushikgopal
    kaush.co
    We're hiring Android devs!
    tech.instacart.com
    

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41. Guided Q&A
    

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42. I'm worried about Multicasting
    leaking things around?
    Should I be?
    If multi-casted stream
    "errors"/"terminates"
    What happens to subscribers?
    How do I dispose of
    ConnectableObs.
    when using autoConnect?
    Where Multicasting begins
    (at which point
    of the Observable) before/after
    publish ?
    When should I use
    ConnectableObsv.
    vs
    Subjects ?
    1
    2
    3
    4
    5
    

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