Common RxJava Mistakes

Talk given at Droidcon SF 2016.

Recording here: https://www.youtube.com/watch?v=QdmkXL7XikQ

Transcript

1. Common RxJava Mistakes
   Dan Lew
   

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2. Lambda Notation
   new Func1() {

   @Override public Integer call(String s) {

   return s.length();

   }

   };
   s -> s.length();
   

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3. Lambda Notation
   new Func1() {

   @Override public Integer call(String s) {

   return s.length();

   }

   };
   s -> {

   return s.length();

   }
   s -> s.length()
   

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4. Lambda Notation
   new Func1() {

   @Override public Integer call(String s) {

   return s.length();

   }

   };
   (String s) -> {

   return s.length();

   }
   s -> s.length()
   …is the same as…
   

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5. Lambda Notation
   new Func1() {

   @Override public Integer call(String s) {

   return s.length();

   }

   };
   (String s) -> {

   return s.length();

   }
   s -> s.length()
   …is the same as…
   …is the same as…
   

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6. The Stream
   

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7. Observable obs = Observable.just("Hello!");

   obs.map(s -> s.length());

   obs.subscribe(i -> System.out.println(i));
   Observable.just("Hello!")

   .map(s -> s.length())

   .subscribe(i -> System.out.println(i));
   Versus
   DOES NOT WORK
   

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8. Observable step1 = Observable.just("Hello!");
   

   Observable step2 = step1.map(s -> s.length());

   step2.subscribe(i -> System.out.println(i));
   

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9. Observable step1 = Observable.just("Hello!");
   

   Observable step2 = step1.map(s -> s.length());

   step2.subscribe(i -> System.out.println(i));
   

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10. Observable step1 = Observable.just("Hello!");
    

    Observable step2 = step1.map(s -> s.length());

    step2.subscribe(i -> System.out.println(i));
    

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11. Upstream
    Downstream
    

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12. Creating Observables 101
    • just(object)
    • from(iterable)
    • Whatever Retrofit is doing
    • create()
    NO
    

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13. Difficulties
    • Observable contract
    • onNext* -> onCompleted | onError
    • Unsubscription
    • Backpressure
    

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14. class MyClass {

    String value;

    

    Observable value() {

    return Observable.create(subscriber -> {

    subscriber.onNext(value);

    subscriber.onCompleted();

    });

    }

    }
    ???
    

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15. class MyClass {

    String value;

    

    Observable value() {

    return Observable.create(subscriber -> {

    subscriber.onNext(value);

    subscriber.onCompleted();

    });

    }

    }
    BAD
    

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16. class MyClass {

    String value;

    

    Observable value() {

    return Observable.create(subscriber -> {

    subscriber.onNext(value);

    subscriber.onCompleted();

    });

    }

    }
    return Observable.fromCallable(() -> value);
    

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17. class MyClass {

    String value;

    

    Observable value() {

    return Observable.create(subscriber -> {

    subscriber.onNext(value);

    subscriber.onCompleted();

    });

    }

    }
    return Observable.defer(() -> Observable.just(value));
    

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18. class MyClass {

    BehaviorSubject value;

    

    Observable value() {

    return value.asObservable();

    }

    

    void setValue(String s) {

    value.onNext(s);

    }

    }
    

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19. RxRelay
    • Safer Subjects
    • https://github.com/JakeWharton/RxRelay
    

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20. Lessons
    • Learn your creation methods!
    • Resort to create() when appropriate
    • (which is almost never)
    

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21. Operator
    Misunderstandings
    

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22. map vs. flatMap
    

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27. Types
    • map: T -> R
    • flatMap: T -> Observable
    

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28. Exercise
    • Find an API with search
    • Query search results via Observable
    • Request extra data on results via Observable
    • Use only one subscribe()
    

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29. flatMap vs. concatMap
    • Same, except…
    • #ordermatters
    • flatMap -> Unordered merge
    • concatMap -> Ordered merge
    

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31. toList()
    • Item(s) -> List
    • “Why does nothing happen?!”
    • Missing onCompleted
    

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32. Versus
    Key!
    

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33. List of Lists
    Observable> obs = /* ...etc... */;

    

    obs.flatMap(list -> Observable.from(list))

    .map(/* ...something... */)

    .toList();

    

    obs.flatMap(

    list -> Observable.from(list)

    .map(/* ...something... */)

    .toList()

    );
    

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34. List of Lists
    Observable> obs = /* ...etc... */;

    

    obs.flatMap(list -> Observable.from(list))

    .map(/* ...something... */)

    .toList();

    

    obs.flatMap(

    list -> Observable.from(list)

    .map(/* ...something... */)

    .toList()

    );
    result: 1 list
    

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35. List of Lists
    Observable> obs = /* ...etc... */;

    

    obs.flatMap(list -> Observable.from(list))

    .map(/* ...something... */)

    .toList();

    

    obs.flatMap(

    list -> Observable.from(list)

    .map(/* ...something... */)

    .toList()

    );
    result: 1 list
    result: N lists
    

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36. take(1) vs. first()
    • take(1) - emits once, or not at all
    Observable.empty().take(1)
    • first() - emits once, or crashes
    Observable.empty().first()
    crash!
    emits nothing
    

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37. cache() vs replay()
    • Don’t use cache()
    • Use replay()
    • cache() == replay().autoConnect()
    

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38. Custom Operators
    

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39. Operators I Have Written
    • OperatorNullToDefault
    • OperatorSortLists
    • OperatorSubscribeUntil
    • ContinueWithIfEmptyOperator
    

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40. Operators I Should Have
    Written
    

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41. Why not?
    • Each operator —> different flow
    • Framework handles common flows
    • Resort to custom operators for unique flows
    • Operators are hard!
    

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42. • OperatorNullToDefault
    • OperatorSortLists
    • OperatorSubscribeUntil
    • ContinueWithIfEmptyOperator
    map()
    switchIfEmpty()
    takeUntil()
    

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43. Threading
    

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44. Observable.range(0, 5)

    .subscribe();
    Observable.interval(1, TimeUnit.SECONDS)

    .subscribe();
    current thread
    computation()
    

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45. Observable.just("Hello")

    .subscribe();
    Observable.just("Hello")

    .delay(1, TimeUnit.SECONDS)

    .subscribe();
    current thread
    computation()
    

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46. Observable.just("Hello")

    .subscribeOn(Schedulers.io())

    .map()

    .subscribeOn(Schedulers.computation())

    .subscribe();
    io()
    

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47. Observable.just("Hello")

    .observeOn(Schedulers.io())

    .map()

    .observeOn(Schedulers.computation())

    .subscribe();
    io()
    computation()
    current thread
    

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49. Lessons
    • Know your scheduler-based operators
    • Use subscribeOn once
    • Use observeOn liberally
    

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50. Subscriptions
    

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51. Observer observer = /* some Observer */

    o1.subscribe(observer);

    o2.subscribe(observer);
    Subscriber subscriber = /* some Subscriber */
    o1.subscribe(subscriber);

    o2.subscribe(subscriber);
    Versus
    

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52. • Observer is an interface
    • Subscriber is a class
    • Observer is stateless
    • Subscriber is stateful
    

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53. Observer observer = /* some Observer */

    o1.subscribe(observer);

    o2.subscribe(observer);
    Subscriber subscriber = /* some Subscriber */
    o1.subscribe(subscriber);

    o2.subscribe(subscriber);
    Versus
    Can blow up in your face
    

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54. Memory Leaks
    • Subscriptions can leak!
    • Detect w/ LeakCanary
    • Solution: Unsubscribe
    

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55. Unsubscribe
    Subscription subscription = Observable.just(1).subscribe();

    subscription.unsubscribe();
    

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56. CompositeSubscription
    CompositeSubscription cs = new CompositeSubscription();
    

    cs.add(Observable.interval(1, TimeUnit.SECONDS).subscribe());

    cs.add(Observable.never().subscribe());
    

    cs.clear();
    use clear()
    

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57. RxLifecycle
    • Automatic sequence completion
    Observable.interval(1, TimeUnit.SECONDS)

    .compose(bindToLifecycle())

    .subscribe();
    

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58. Error Handling
    

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59. onError Uses
    • Unexpected exceptions
    • Unrecoverable streams
    • Flow control
    Blame Retrofit 1
    

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60. Terminal events suck
    

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61. onError ≈ try-catch
    

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62. Exception workarounds
    • Error state
    • onErrorReturn
    • onErrorResumeNext
    

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63. Debugging onError
    Observable.empty()

    .first()

    .subscribeOn(Schedulers.io())

    .subscribe();
    Crash!
    

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64. Debugging onError
    Exception in thread "RxCachedThreadScheduler-1" java.lang.IllegalStateException: Exception thrown on Scheduler.Worker thread.
    Add `onError` handling.

    at rx.internal.schedulers.ScheduledAction.run(ScheduledAction.java:60)

    at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)

    at java.util.concurrent.FutureTask.run(FutureTask.java:266)

    at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$201(ScheduledThreadPoolExecutor.java:180)

    at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:293)

    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)

    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)

    at java.lang.Thread.run(Thread.java:745)

    Caused by: rx.exceptions.OnErrorNotImplementedException: Sequence contains no elements

    at rx.Observable$26.onError(Observable.java:7881)

    at rx.observers.SafeSubscriber._onError(SafeSubscriber.java:159)

    at rx.observers.SafeSubscriber.onError(SafeSubscriber.java:120)

    at rx.internal.operators.OperatorSubscribeOn$1$1$1.onError(OperatorSubscribeOn.java:71)

    at rx.internal.operators.OperatorSingle$ParentSubscriber.onCompleted(OperatorSingle.java:131)

    at rx.internal.operators.OperatorTake$1.onCompleted(OperatorTake.java:53)

    at rx.Observable$EmptyHolder$1.call(Observable.java:1073)

    at rx.Observable$EmptyHolder$1.call(Observable.java:1070)

    at rx.Observable$2.call(Observable.java:162)

    at rx.Observable$2.call(Observable.java:154)

    at rx.Observable$2.call(Observable.java:162)

    at rx.Observable$2.call(Observable.java:154)

    at rx.Observable.unsafeSubscribe(Observable.java:8098)

    at rx.internal.operators.OperatorSubscribeOn$1$1.call(OperatorSubscribeOn.java:62)

    at rx.internal.schedulers.ScheduledAction.run(ScheduledAction.java:55)

    ... 7 more

    Caused by: java.util.NoSuchElementException: Sequence contains no elements

    ... 18 more
    No references to my code!
    

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65. Implement onError
    Observable.empty()

    .first()

    .subscribeOn(Schedulers.io())

    .subscribe(o -> System.out.println(o),

    err -> {

    throw new OnErrorNotImplementedException("Source!", err);

    });
    Caused by: rx.exceptions.OnErrorNotImplementedException: Source!

    at net.danlew.experiments.Tester.lambda$main$1(Tester.java:52)
    

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66. Implement onError
    Observable.empty()

    .first()

    .subscribeOn(Schedulers.io())

    .subscribe(o -> System.out.println(o),

    err -> {

    throw new OnErrorNotImplementedException("Source!", err);

    });
    Caused by: rx.exceptions.OnErrorNotImplementedException: Source!

    at net.danlew.experiments.Tester.lambda$main$1(Tester.java:52)
    

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67. Implement onError
    Observable.empty()

    .first()

    .subscribeOn(Schedulers.io())

    .subscribe(o -> System.out.println(o),

    err -> {

    throw new OnErrorNotImplementedException("Source!", err);

    });
    Caused by: rx.exceptions.OnErrorNotImplementedException: Source!

    at net.danlew.experiments.Tester.lambda$main$1(Tester.java:52)
    

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68. Sharing
    • Wanted: Multiple subscribers
    retrofitRequest.subscribe(/* do a thing */);

    retrofitRequest.subscribe(/* do another thing */);
    • Bad: Duplicate expensive operations
    

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69. Hot vs. Cold
    

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70. Sharing
    ConnectableObservable obs =

    Observable.fromCallable(() -> System.nanoTime())

    .publish();

    

    obs.subscribe(l -> System.out.println("1: " + l));

    obs.subscribe(l -> System.out.println("2: " + l));

    

    obs.connect();
    

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71. Sharing
    ConnectableObservable obs =

    Observable.fromCallable(() -> System.nanoTime())

    .publish();

    

    obs.subscribe(l -> System.out.println("1: " + l));

    obs.subscribe(l -> System.out.println("2: " + l));

    

    obs.connect();
    

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72. Sharing
    ConnectableObservable obs =

    Observable.fromCallable(() -> System.nanoTime())

    .publish();

    

    obs.subscribe(l -> System.out.println("1: " + l));

    obs.subscribe(l -> System.out.println("2: " + l));

    

    obs.connect();
    

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73. Sharing
    ConnectableObservable obs =

    Observable.fromCallable(() -> System.nanoTime())

    .publish();

    

    obs.subscribe(l -> System.out.println("1: " + l));

    obs.subscribe(l -> System.out.println("2: " + l));

    

    obs.connect();
    

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74. Sharing
    ConnectableObservable obs =

    Observable.fromCallable(() -> System.nanoTime())

    .publish();

    

    obs.subscribe(l -> System.out.println("1: " + l));

    obs.subscribe(l -> System.out.println("2: " + l));

    

    obs.connect();
    

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75. refCount
    Observable obs =

    Observable.fromCallable(() -> System.nanoTime())

    .publish()
    .refCount();

    

    obs.subscribe(l -> System.out.println("1: " + l));

    obs.subscribe(l -> System.out.println("2: " + l));
    // Output
    1: 395093905388878
    2: 395093906717841
    

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76. refCount
    Observable obs =

    Observable.fromCallable(() -> System.nanoTime())

    .publish()
    .refCount();

    

    obs.subscribe(l -> System.out.println("1: " + l));

    obs.subscribe(l -> System.out.println("2: " + l));
    // Output
    1: 395093905388878
    2: 395093906717841
    

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77. refCount
    Observable obs =

    Observable.fromCallable(() -> System.nanoTime())

    .publish()
    .refCount();

    

    obs.subscribe(l -> System.out.println("1: " + l));

    obs.subscribe(l -> System.out.println("2: " + l));
    // Output
    1: 395093905388878
    2: 395093906717841
    huh?
    

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78. Lessons
    • Share cold observables
    • Explicitly call connect()
    • Use autoConnect()
    

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79. it’s over!
    @danlew42
    

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