RxJava: Tapping Retry

In this talk, I am explaining how RxJava retryWhen operator works, how to use it and a few pitfalls in a real usage.

Besides, I am suggesting screen transitions also can be handled with retryWhen in Android app - say, showing Login screen when receiving 401 Unauthorized and then resume the task after login succeeded.

Links:
- ReactiveX - Retry operator

http://reactivex.io/documentation/operators/retry.html
- ReactiveX - Repeat operator

http://reactivex.io/documentation/operators/repeat.html
- RxJava's repeatWhen and retryWhen, explained

http://blog.danlew.net/2016/01/25/rxjavas-repeatwhen-and-retrywhen-explained/
- Build Your Custom Observable/Operator

https://speakerdeck.com/hkurokawa/operator
- RxNearby
https://github.com/hkurokawa/RxNearby

Transcript

1. RXJAVA TAPPING RETRY RX JA NIGHT #1 2016.02.25 Hiroshi Kurokawa (@hydrakecat)
2. REPEAT AND RETRY ▸ repeat() ▸ resubscribes when it receives onCompleted() ▸ retry() ▸ resubscribes when it receives onError()
3. MORE REQUIREMENTS? ▸ You can specify when it should resubscribe with repeatWhen() or retryWhen() ▸ notificationHandler defines when it should resubscribe
4. MORE REQUIREMENTS? ▸ You can specify when it should resubscribe with repeatWhen() or retryWhen() ▸ notificationHandler defines when it should resubscribe Observable.create(new OnSubscribeThrowError()) .retryWhen(notificationHandler) .subscribe(i -> { // do something }, System.err::println);
5. MORE REQUIREMENTS? ▸ You can specify when it should resubscribe with repeatWhen() or retryWhen() ▸ notificationHandler defines when it should resubscribe Observable.create(new OnSubscribeThrowError()) .retryWhen(notificationHandler) .subscribe(i -> { // do something }, System.err::println); final Func1<? super Observable<? extends Throwable>, ? extends Observable<?>>
6. MORE REQUIREMENTS? ▸ You can specify when it should resubscribe with repeatWhen() or retryWhen() ▸ notificationHandler defines when it should resubscribe Observable.create(new OnSubscribeThrowError()) .retryWhen(notificationHandler) .subscribe(i -> { // do something }, System.err::println); ? extends Observable<?> call(Observable<? extends Throwable> observable)
7. MORE REQUIREMENTS? ▸ You can specify when it should resubscribe with repeatWhen() or retryWhen() ▸ notificationHandler defines when it should resubscribe Observable.create(new OnSubscribeThrowError()) .retryWhen(notificationHandler) .subscribe(i -> { // do something }, System.err::println); Observable<?> call(Observable<Throwable> observable)
8. NOTIFICATION HANDLER ▸ Argument: Observable<Throwable> ▸ Describes what kind of Error happens ▸ Return: Observable<?> ▸ Resubscribes when it emits an event ▸ The event could be anything! Just a tap ▸ If its onError() or onCompleted() is called, just finish Observable<?> call(Observable<Throwable> observable) Note: The returned Observable has to use the argument Observable as its source
9. EXAMPLE Observable.create(new OnSubscribeThrowError()) .retryWhen( ) .subscribe(i -> { }, System.err::println); notificationHandler
10. EXAMPLE Observable.create(new OnSubscribeThrowError()) .retryWhen( ) .subscribe(i -> { }, System.err::println); notificationHandler
11. EXAMPLE Observable.create(new OnSubscribeThrowError()) .retryWhen( ) .subscribe(i -> { }, System.err::println); notificationHandler
12. EXAMPLE Observable.create(new OnSubscribeThrowError()) .retryWhen( ) .subscribe(i -> { }, System.err::println); observable -> observable.flatMap(throwable -> { if (throwable instanceof RecoverableException) return Observable.just(null); return Observable.error(throwable); })
13. EXAMPLE Observable.create(new OnSubscribeThrowError()) .retryWhen( ) .subscribe(i -> { }, System.err::println); observable -> observable.flatMap(throwable -> { if (throwable instanceof RecoverableException) return Observable.just(null); return Observable.error(throwable); })
14. EXAMPLE Observable.create(new OnSubscribeThrowError()) .retryWhen( ) .subscribe(i -> { }, System.err::println); observable -> observable.flatMap(throwable -> { if (throwable instanceof RecoverableException) return Observable.just(null); return Observable.error(throwable); })
15. EXAMPLE Observable.create(new OnSubscribeThrowError()) .retryWhen( ) .subscribe(i -> { }, System.err::println); observable -> observable.flatMap(throwable -> { if (throwable instanceof RecoverableException) return Observable.just(null); return Observable.error(throwable); }) private static class OnSubscribeThrowError implements Observable.OnSubscribe<Integer> { private int count; @Override public void call(Subscriber<? super Integer> subscriber) { System.out.printf("Subscribe count: %d\n", ++count); if (count < 3) subscriber.onError(new RecoverableException()); else subscriber.onError(new UnrecoverableException()); } }
16. EXAMPLE Observable.create(new OnSubscribeThrowError()) .retryWhen( ) .subscribe(i -> { }, System.err::println); observable -> observable.flatMap(throwable -> { if (throwable instanceof RecoverableException) return Observable.just(null); return Observable.error(throwable); }) private static class OnSubscribeThrowError implements Observable.OnSubscribe<Integer> { private int count; @Override public void call(Subscriber<? super Integer> subscriber) { System.out.printf("Subscribe count: %d\n", ++count); if (count < 3) subscriber.onError(new RecoverableException()); else subscriber.onError(new UnrecoverableException()); } } Return an unrecoverable exception only after the first two calls
17. EXAMPLE private static class OnSubscribeThrowError implements Observable.OnSubscribe<Integer> { private int count; @Override public void call(Subscriber<? super Integer> subscriber) { System.out.printf("Subscribe count: %d\n", ++count); if (count < 3) subscriber.onError(new RecoverableException()); else subscriber.onError(new UnrecoverableException()); } } Subscribe count: 1
 Subscribe count: 2
 Subscribe count: 3
 Sample$UnrecoverableException ▸ Output
18. TEXT SOME USES ▸ Delayed Retry retryWhen(observable -> observable.flatMap(throwable -> Observable.timer(5, TimeUnit.SECONDS)))
19. TEXT SOME USES ▸ Delayed Retry ▸ Resubscribe a limited number of times with zip and range retryWhen(observable -> observable.flatMap(throwable -> Observable.timer(5, TimeUnit.SECONDS))) retryWhen(observable -> observable.zipWith(Observable.range(1, 3), (n, i) -> i))
20. TEXT SOME USES ▸ Delayed Retry ▸ Resubscribe a limited number of times with zip and range ▸ Exponential backoff retryWhen(observable -> observable.flatMap(throwable -> Observable.timer(5, TimeUnit.SECONDS))) retryWhen(observable -> observable.zipWith(Observable.range(1, 3), (n, i) -> i)) retryWhen(observable -> observable.zipWith(Observable.range(1, 3), (n, i) -> i) .flatMap(retryCount -> Observable.timer((long) Math.pow(5, retryCount), TimeUnit.SECONDS))
21. TEXT QUIZ 1 ▸ What happens? getAccessToken() .flatMap(Sample::doSomethingWithToken) .retryWhen(observable -> observable.flatMap(throwable -> { if (throwable instanceof TokenExpiredException) { clearCache(); return Observable.just(null); } return Observable.error(throwable); }))
22. TEXT QUIZ 1 ▸ What happens? private static Observable<String> getAccessToken() { String accessToken = getCache(); if (accessToken != null) { return Observable.just(accessToken); } return getNewAccessToken(); } getAccessToken() .flatMap(Sample::doSomethingWithToken) .retryWhen(observable -> observable.flatMap(throwable -> { if (throwable instanceof TokenExpiredException) { clearCache(); return Observable.just(null); } return Observable.error(throwable); }))
23. TEXT QUIZ 1 ▸ What happens? private static Observable<String> getAccessToken() { String accessToken = getCache(); if (accessToken != null) { return Observable.just(accessToken); } return getNewAccessToken(); } getAccessToken() .flatMap(Sample::doSomethingWithToken) .retryWhen(observable -> observable.flatMap(throwable -> { if (throwable instanceof TokenExpiredException) { clearCache(); return Observable.just(null); } return Observable.error(throwable); }))
24. TEXT QUIZ 1 ▸ What happens? private static Observable<String> getAccessToken() { String accessToken = getCache(); if (accessToken != null) { return Observable.just(accessToken); } return getNewAccessToken(); } getAccessToken() .flatMap(Sample::doSomethingWithToken) .retryWhen(observable -> observable.flatMap(throwable -> { if (throwable instanceof TokenExpiredException) { clearCache(); return Observable.just(null); } return Observable.error(throwable); }))
25. TEXT QUIZ 1 ▸ What happens? private static Observable<String> getAccessToken() { String accessToken = getCache(); if (accessToken != null) { return Observable.just(accessToken); } return getNewAccessToken(); } getAccessToken() .flatMap(Sample::doSomethingWithToken) .retryWhen(observable -> observable.flatMap(throwable -> { if (throwable instanceof TokenExpiredException) { clearCache(); return Observable.just(null); } return Observable.error(throwable); }))
26. TEXT QUIZ 1 ▸ What happens? private static Observable<String> getAccessToken() { String accessToken = getCache(); if (accessToken != null) { return Observable.just(accessToken); } return getNewAccessToken(); } getAccessToken() .flatMap(Sample::doSomethingWithToken) .retryWhen(observable -> observable.flatMap(throwable -> { if (throwable instanceof TokenExpiredException) { clearCache(); return Observable.just(null); } return Observable.error(throwable); }))
27. TEXT QUIZ 1 ▸ What happens? private static Observable<String> getAccessToken() { String accessToken = getCache(); if (accessToken != null) { return Observable.just(accessToken); } return getNewAccessToken(); } getAccessToken() .flatMap(Sample::doSomethingWithToken) .retryWhen(observable -> observable.flatMap(throwable -> { if (throwable instanceof TokenExpiredException) { clearCache(); return Observable.just(null); } return Observable.error(throwable); }))
28. TEXT QUIZ 1 ▸ What happens? private static Observable<String> getAccessToken() { String accessToken = getCache(); if (accessToken != null) { return Observable.just(accessToken); } return getNewAccessToken(); } getAccessToken() .flatMap(Sample::doSomethingWithToken) .retryWhen(observable -> observable.flatMap(throwable -> { if (throwable instanceof TokenExpiredException) { clearCache(); return Observable.just(null); } return Observable.error(throwable); }))
29. TEXT QUIZ 1 ANSWER ▸ What happens? ▸ The old token is always returned at the retrial getAccessToken() .flatMap(Sample::doSomethingWithToken) .retryWhen(observable -> observable.flatMap(throwable -> { if (throwable instanceof TokenExpiredException) { clearCache(); return Observable.just(null); } return Observable.error(throwable); }))
30. TEXT QUIZ 1 ANSWER ▸ What happens? ▸ The old token is always returned at the retrial ▸ Infinite loop getAccessToken() .flatMap(Sample::doSomethingWithToken) .retryWhen(observable -> observable.flatMap(throwable -> { if (throwable instanceof TokenExpiredException) { clearCache(); return Observable.just(null); } return Observable.error(throwable); }))
31. TEXT QUIZ 1 ANSWER ▸ What happens? ▸ The old token is always returned at the retrial ▸ Infinite loop ▸ The source Observable should be unique getAccessToken() .flatMap(Sample::doSomethingWithToken) .retryWhen(observable -> observable.flatMap(throwable -> { if (throwable instanceof TokenExpiredException) { clearCache(); return Observable.just(null); } return Observable.error(throwable); }))
32. TEXT QUIZ 2 ▸ In which thread is the MyBackgroundOnSubscribe executed? final PublishSubject<Void> subject = PublishSubject.create(); Observable.create(new MyBackgroundOnSubscribe()) .retryWhen(observable -> observable.flatMap(th -> subject)) .subscribeOn(Schedulers.newThread()) .subscribe(System.out::println, System.err::println); subject.onNext(null);
33. TEXT QUIZ 2 ▸ In which thread is the MyBackgroundOnSubscribe executed? final PublishSubject<Void> subject = PublishSubject.create(); Observable.create(new MyBackgroundOnSubscribe()) .retryWhen(observable -> observable.flatMap(th -> subject)) .subscribeOn(Schedulers.newThread()) .subscribe(System.out::println, System.err::println); subject.onNext(null);
34. TEXT QUIZ 2 ▸ In which thread is the MyBackgroundOnSubscribe executed? final PublishSubject<Void> subject = PublishSubject.create(); Observable.create(new MyBackgroundOnSubscribe()) .retryWhen(observable -> observable.flatMap(th -> subject)) .subscribeOn(Schedulers.newThread()) .subscribe(System.out::println, System.err::println); subject.onNext(null);
35. TEXT QUIZ 2 ▸ In which thread is the MyBackgroundOnSubscribe executed? ▸ At the 1st time? ▸ When resubscribed? final PublishSubject<Void> subject = PublishSubject.create(); Observable.create(new MyBackgroundOnSubscribe()) .retryWhen(observable -> observable.flatMap(th -> subject)) .subscribeOn(Schedulers.newThread()) .subscribe(System.out::println, System.err::println); subject.onNext(null);
36. TEXT QUIZ 2 ▸ In which thread is the MyBackgroundOnSubscribe executed? ▸ At the 1st time? RxNewThreadScheduler-1 ▸ When resubscribed? final PublishSubject<Void> subject = PublishSubject.create(); Observable.create(new MyBackgroundOnSubscribe()) .retryWhen(observable -> observable.flatMap(th -> subject)) .subscribeOn(Schedulers.newThread()) .subscribe(System.out::println, System.err::println); subject.onNext(null);
37. TEXT QUIZ 2 ▸ In which thread is the MyBackgroundOnSubscribe executed? ▸ At the 1st time? RxNewThreadScheduler-1 ▸ When resubscribed? main final PublishSubject<Void> subject = PublishSubject.create(); Observable.create(new MyBackgroundOnSubscribe()) .retryWhen(observable -> observable.flatMap(th -> subject)) .subscribeOn(Schedulers.newThread()) .subscribe(System.out::println, System.err::println); subject.onNext(null);
38. TEXT QUIZ 2 ▸ In which thread is the MyBackgroundOnSubscribe executed? ▸ At the 1st time? RxNewThreadScheduler-1 ▸ When resubscribed? main <= where subject.onNext() is called final PublishSubject<Void> subject = PublishSubject.create(); Observable.create(new MyBackgroundOnSubscribe()) .retryWhen(observable -> observable.flatMap(th -> subject)) .subscribeOn(Schedulers.newThread()) .subscribe(System.out::println, System.err::println); subject.onNext(null);
39. TEXT QUIZ 2 ▸ In which thread is the MyBackgroundOnSubscribe executed? ▸ At the 1st time? RxNewThreadScheduler-1 ▸ When resubscribed? main <= where subject.onNext() is called final PublishSubject<Void> subject = PublishSubject.create(); Observable.create(new MyBackgroundOnSubscribe()) .retryWhen(observable -> observable.flatMap(th -> subject)) .subscribeOn(Schedulers.newThread()) .subscribe(System.out::println, System.err::println); Re-subscription is executed within the retryWhen() operator Thus, the subscribeOn() below retryWhen() does not take effect
40. TEXT QUIZ 2 ▸ In which thread is the MyBackgroundOnSubscribe executed? ▸ At the 1st time? ▸ When resubscribed? final PublishSubject<Void> subject = PublishSubject.create(); Observable.create(new MyBackgroundOnSubscribe()) .retryWhen(observable -> observable.flatMap(th -> subject)) .subscribeOn(Schedulers.newThread()) .subscribe(System.out::println, System.err::println); subject.onNext(null);
41. TEXT QUIZ 2’ ▸ In which thread is the MyBackgroundOnSubscribe executed? ▸ At the 1st time? ▸ When resubscribed? final PublishSubject<Void> subject = PublishSubject.create(); Observable.create(new MyBackgroundOnSubscribe()) .subscribeOn(Schedulers.newThread()) .retryWhen(observable -> observable.flatMap(th -> subject)) .subscribe(System.out::println, System.err::println); subject.onNext(null);
42. TEXT QUIZ 2’ ▸ In which thread is the MyBackgroundOnSubscribe executed? ▸ At the 1st time? RxNewThreadScheduler-1 ▸ When resubscribed? final PublishSubject<Void> subject = PublishSubject.create(); Observable.create(new MyBackgroundOnSubscribe()) .subscribeOn(Schedulers.newThread()) .retryWhen(observable -> observable.flatMap(th -> subject)) .subscribe(System.out::println, System.err::println); subject.onNext(null);
43. TEXT QUIZ 2’ ▸ In which thread is the MyBackgroundOnSubscribe executed? ▸ At the 1st time? RxNewThreadScheduler-1 ▸ When resubscribed? RxNewThreadScheduler-1 final PublishSubject<Void> subject = PublishSubject.create(); Observable.create(new MyBackgroundOnSubscribe()) .subscribeOn(Schedulers.newThread()) .retryWhen(observable -> observable.flatMap(th -> subject)) .subscribe(System.out::println, System.err::println); subject.onNext(null);
44. TEXT QUIZ 2 ANSWER ▸ In which thread is the MyBackgroundOnSubscribe executed? ▸ At the 1st time? ▸ The thread specified with subscribeOn() ▸ When resubscribed? ▸ Depends on the thread where onNext() is called and the position of subscribeOn() ▸ If necessary, use Observable#retryWhen(notificationHandler, scheduler) ▸ Read OnSubscribeRedo.java code!
45. REFERENCE ▸ ReactiveX - Retry operator
 http://reactivex.io/documentation/operators/retry.html ▸ ReactiveX - Repeat operator
 http://reactivex.io/documentation/operators/repeat.html ▸ RxJava's repeatWhen and retryWhen, explained
 http://blog.danlew.net/2016/01/25/rxjavas-repeatwhen-and- retrywhen-explained/ ▸ Build Your Custom Observable/Operator
 https://speakerdeck.com/hkurokawa/operator
46. ON ANDROID (Images are from http://developer.android.com/intl/ja/guide/topics/ui/dialogs.html and http://developer.android.com/intl/ja/design/material/index.html) 401 Unauthorized Application Server clipartist.net SVG by Open Clip Art Library CC
47. ON ANDROID (Images are from http://developer.android.com/intl/ja/guide/topics/ui/dialogs.html and http://developer.android.com/intl/ja/design/material/index.html) Application Server clipartist.net SVG by Open Clip Art Library CC
48. ON ANDROID (Images are from http://developer.android.com/intl/ja/guide/topics/ui/dialogs.html and http://developer.android.com/intl/ja/design/material/index.html) Do Login Request Login Application Server clipartist.net SVG by Open Clip Art Library CC
49. ON ANDROID (Images are from http://developer.android.com/intl/ja/guide/topics/ui/dialogs.html and http://developer.android.com/intl/ja/design/material/index.html) 200 OK Application Server clipartist.net SVG by Open Clip Art Library CC
50. ON ANDROID (Images are from http://developer.android.com/intl/ja/guide/topics/ui/dialogs.html and http://developer.android.com/intl/ja/design/material/index.html) Application Server clipartist.net SVG by Open Clip Art Library CC
51. ON ANDROID (Images are from http://developer.android.com/intl/ja/guide/topics/ui/dialogs.html and http://developer.android.com/intl/ja/design/material/index.html) Resume A Application Server clipartist.net SVG by Open Clip Art Library CC
52. TEXT EXAMPLE private PublishSubject<Void> subject; private void onDoActionA() { apiClient.doActionA() .retryWhen(observable -> observable.flatMap(throwable -> { if (throwable instanceof HttpException && ((HttpException) th).code() == 401) { startLoginActivity(); subject = PublishSubject.create(); return subject; } return Observable.error(throwable); })) .subscribe(data -> { // do something }, System.err::println); }
53. TEXT EXAMPLE private PublishSubject<Void> subject; private void onDoActionA() { apiClient.doActionA() .retryWhen(observable -> observable.flatMap(throwable -> { if (throwable instanceof HttpException && ((HttpException) th).code() == 401) { startLoginActivity(); subject = PublishSubject.create(); return subject; } return Observable.error(throwable); })) .subscribe(data -> { // do something }, System.err::println); }
54. TEXT EXAMPLE @Override public void onActivityResult(int requestCode, int resultCode, Intent data) { switch (requestCode) { case REQ_CODE_LOGIN: if (resultCode == Activity.RESULT_OK) { subject.onNext(user); } else { subject.onError(new LoginCanceledException()); } return; } super.onActivityResult(requestCode, resultCode, data); } if (throwable instanceof HttpException && ((HttpException) th).code() == 401) { startLoginActivity(); subject = PublishSubject.create(); return subject; } return Observable.error(throwable);
55. TEXT EXAMPLE @Override public void onActivityResult(int requestCode, int resultCode, Intent data) { switch (requestCode) { case REQ_CODE_LOGIN: if (resultCode == Activity.RESULT_OK) { subject.onNext(user); } else { subject.onError(new LoginCanceledException()); } return; } super.onActivityResult(requestCode, resultCode, data); } if (throwable instanceof HttpException && ((HttpException) th).code() == 401) { startLoginActivity(); subject = PublishSubject.create(); return subject; } return Observable.error(throwable);
56. REFERENCE ▸ See this repository ▸ It takes advantage of retryWhen() to handle permission request
57. Thank you! Hiroshi Kurokawa (ࠇ઒ ༸) @hydrakecat https://github.com/hkurokawa https://speakerdeck.com/hkurokawa hydrakecat.hatenablog.jp