This document discusses various RxJava concepts and best practices including: 1. RxJava contracts such as not allowing null values in streams, serialized emission of events, and requiring onComplete calls. 2. Threading considerations for operators like flatMap, merge, and zip to ensure proper threading. 3. Retry strategies using RetryWhen to handle errors in different ways like retries or refreshing authentication tokens. 4. Exception handling techniques like wrapping exceptions in other exceptions or returning default values from Single sources. The document provides code examples to illustrate these concepts and compares correct vs incorrect implementations.

1. 다시 보는 RxJava
네이버 노재춘

2. Rx Contract #1-1
no more messages should arrive after an onError or
onComplete message
RxView.clicks(button)
.flatMap(ignored -> getBestSeller()
.subscribeOn(Schedulers.io()))
.observeOn(AndroidSchedulers.mainThread())
.subscribe(bookTitle -> title.setText(bookTitle),
e -> Toast.makeText(this, "문제 발생",
Toast.LENGTH_LONG).show()
);
무한 이벤트에서 문제 발생 가능

3. Rx Contract #1-2
RxView.clicks(button)
.flatMap(ignored -> getBestSeller()
.subscribeOn(Schedulers.io())
.map(title -> new ViewState.Result(title)) // (1)
.cast(ViewState.class) // (2)
.onErrorReturn(e -> new ViewState.Error(e))) // (3)
.observeOn(AndroidSchedulers.mainThread())
.subscribe(viewState -> {
if (viewState instanceof ViewState.Result) { // (4)
title.setText(((ViewState.Result) viewState).title);
} else if (viewState instanceof ViewState.Error) { // (5)
Toast.makeText(this, "문제 발생", Toast.LENGTH_LONG).show();
}
});

4. Rx Contract #1-3
interface ViewState {
class Error implements ViewState {
Throwable e;
Error(Throwable e) {
this.e = e;
}
}
class Result implements ViewState {
String title;
Result(String title) {
this.title = title;
}
}
}
http://hannesdorfmann.com/android/
mosby3-mvi-1
visual bugs such as displaying a loading
indicator (“loading state”) and error indicator
(“error state”) at the same time => exactly
one output

5. Rx Contract #2-1
스트림에 널(null) 허용 안됨
@Test(expected = NullPointerException.class)
public void nullEvent() {
Observable.just("Hello", null, "RxJava")
.subscribe(System.out::println,
System.err::println);
}
Observable 생성
에서 에러 발생

6. Rx Contract #2-2
@Test
public void nullEventOnCreate() {
Observable.create(emitter -> {
emitter.onNext("Hello");
emitter.onNext(null);
emitter.onNext("RxJava");
}).subscribe(System.out::println,
System.err::println);
}
java.lang.NullPointerException:
onNext called with null. Null
values are generally not
allowed in 2.x operators and
sources.

7. Rx Contract #2-3
@Test
public void nullEventPossible() {
Observable.just(1, 2, -1, 1, 2)
.map(dollar -> getCurrentPrice(dollar))
.subscribe(System.out::println,
System.err::println);
}
private String getCurrentPrice(int dollar) {
if (dollar < 0) {
return null;
}
return (dollar * 1000) + " won";
}
1000 won
2000 won
java.lang.NullPointerException:
The mapper function returned
a null value.

8. Rx Contract #2-4
@Test
public void nullEventPossible3() {
Observable.just(1, 2, -1, 1, 2)
.map(dollar -> getCurrentPrice3(dollar))
.onErrorReturnItem("0 won")
.subscribe(System.out::println,
System.err::println,
() -> System.out.println("onComplete"));
}
private String getCurrentPrice3(int dollar) {
if (dollar < 0) {
throw new IllegalArgumentException("dollar should be bigger than 0");
}
return (dollar * 1000) + " won";
} [1000 won, 2000 won, 0 won, onComplete]

9. Rx Contract #2-5
@Test
public void nullEventPossible4() {
Observable.just(1, 2, -1, 1, 2)
.flatMap(dollar -> getCurrentPrice4(dollar)
.onErrorReturnItem("0 won")) // (1)
.subscribe(System.out::println,
System.err::println);
}
private Observable getCurrentPrice4(int dollar) {
if (dollar < 0) {
return Observable.error(
new IllegalArgumentException("dollar should be bigger than 0")); // (2)
}
return Observable.just((dollar * 1000) + " won"); // (3)
}

10. Rx Contract #3-1
Assume observer instances are called in a serialized fashion
Observable obs = Observable.create(emitter -> {
new Thread(() -> {
emitter.onNext(1);
emitter.onNext(3);
. . . .
emitter.onNext(9);
// emitter.onComplete(); // (1)
}).start();
new Thread(() -> {
emitter.onNext(2);
emitter.onNext(4);
. . . .
emitter.onNext(10);
// emitter.onComplete(); // (2)
}).start();
});
onComplete 넣
을 수 없음

11. Rx Contract #3-2
obs.subscribe(value -> System.out.println(
Thread.currentThread().getName() + ":" + value),
System.err::println,
() -> System.out.println("onComplete"));
onComplete 불
리지 않음

12. Rx Contract #3-3
Observable obs1 = Observable.create(emitter -> {
new Thread(() -> {
emitter.onNext(1);
. . . .
emitter.onNext(9);
emitter.onComplete(); // (1)
}).start();
});
Observable obs2 = Observable.create(emitter -> {
new Thread(() -> {
emitter.onNext(2);
. . . .
emitter.onNext(10);
emitter.onComplete(); // (2)
}).start();
});
(1), (2)
모두 필요

13. Rx Contract #3-4
Observable.merge(obs1, obs2).subscribe(value ->
System.out.println(Thread.currentThread().getName() + ":" + value),
System.err::println,
() -> System.out.println("onComplete")
);
onComplete 정
상 호출

14. Rx Contract #3-5
Observable obs1 = Observable.create(emitter -> {
emitter.onNext(1);
emitter.onNext(3);
. . . .
emitter.onNext(9);
emitter.onComplete();
}).subscribeOn(Schedulers.computation());
Observable obs2 = Observable.create(emitter -> {
emitter.onNext(2);
emitter.onNext(4);
. . . .
emitter.onNext(10);
emitter.onComplete();
}).subscribeOn(Schedulers.computation());

15. Rx Contract #3-6
Observable.merge(obs1, obs2).subscribe(value ->
System.out.println(
Thread.currentThread().getName() + ":" + value),
System.err::println,
() -> System.out.println("onComplete")
);

16. flatMap thread #1-1
repository.getBookCategories().toObservable()
.flatMapIterable(categories -> categories)
.flatMapSingle(category -> repository.getCategoryBooks(category.categoryId))
.subscribeOn(Schedulers.io())
.collect(ArrayList::new, ArrayList::addAll)
.observeOn(AndroidSchedulers.mainThread())
.subscribe(books -> title.setText(books.toString()
repository.getBookCategories().toObservable()
.subscribeOn(Schedulers.io())
.flatMapIterable(categories -> categories)
.flatMapSingle(category ->
repository.getCategoryBooks(category.categoryId)
.subscribeOn(Schedulers.io()))
.collect(ArrayList::new, ArrayList::addAll)
.observeOn(AndroidSchedulers.mainThread())
.subscribe(books -> title.setText(books.toString()));

17. flatMap thread #1-2

18. flatMap thread #2 단일 이벤트 연결
repository.getRegion(location)
.map(region -> region.areaCode)
.flatMap(repository::getWeather)
.subscribeOn(Schedulers.io())
.observeOn(AndroidSchedulers.mainThread())
.subscribe(weather -> title.setText(weather.toString()));
repository.getRegion(location)
.subscribeOn(Schedulers.io())
.map(region -> region.areaCode)
.flatMap(areaCode -> repository.getWeather(areaCode)
.subscribeOn(Schedulers.io()))
.observeOn(AndroidSchedulers.mainThread())
.subscribe(weather -> title.setText(weather.toString()));

19. merge thread #1-1
Single.merge(repository.getBestSeller(),
repository.getRecommendBooks(),
repository.getCategoryBooks(7))
.subscribeOn(Schedulers.io())
.collect(ArrayList::new, ArrayList::addAll)
.observeOn(AndroidSchedulers.mainThread())
.subscribe(books -> title.setText(books.toString()));
Single.merge(repository.getBestSeller().subscribeOn(Schedulers.io()),
repository.getRecommendBooks().subscribeOn(Schedulers.io()),
repository.getCategoryBooks(7).subscribeOn(Schedulers.io()))
.collect(ArrayList::new, ArrayList::addAll)
.observeOn(AndroidSchedulers.mainThread())
.subscribe(books -> title.setText(books.toString()));

20. merge thread #1-2

21. zip thread #1-1
Single.zip(repository.getWeather(regionCode),
repository.getWeatherDetail(regionCode),
Pair::new)
.subscribeOn(Schedulers.io())
.observeOn(AndroidSchedulers.mainThread())
.subscribe(pair -> title.setText(pair.first + ", " + pair.second));
Single.zip(repository.getWeather(regionCode).subscribeOn(Schedulers.io()),
repository.getWeatherDetail(regionCode).subscribeOn(Schedulers.io()),
Pair::new)
.observeOn(AndroidSchedulers.mainThread())
.subscribe(pair -> title.setText(pair.first + ", " + pair.second));

22. zip thread #1-2

23. RetryWhen #1
- SocketTimeout에 재시도(3번 재시도)
- 401 status code에는 인증 API 호출

24. RetryWhen #2
public Single<Profile> getProfile() {
return profileRepository.getProfile().retryWhen(errors ->
errors.zipWith(Flowable.range(0, LIMIT),
this::handleRetryAttempt).flatMap(x -> x));
}
private Flowable<Long> handleRetryAttempt(Throwable throwable, int attempt) {
if (throwable instanceof SocketTimeoutException) {
switch (attempt) {
case LIMIT:
return Flowable.error(throwable);
default:
return Flowable.timer(attempt * 3, TimeUnit.SECONDS);
}
} else {else if (throwable instanceof HttpException) {
// 다음 슬라이드
}
OkHttpClient Interceptor도 가능?
원래 흐름으로 돌아올 방법 없음

25. RetryWhen #3
private Flowable<Long> handleRetryAttempt(Throwable throwable, int attempt) {
if (throwable instanceof SocketTimeoutException) {
...
} else if (throwable instanceof HttpException) {
int statusCode = ((HttpException) throwable).code();
if (statusCode == 401) {
return profileRepository.refresh().retry((refreshAttempt, e) ->
refreshAttempt < LIMIT
&& e instanceof SocketTimeoutException)
.map(x -> 1L).toFlowable();
}
}
return Flowable.error(throwable);
}

26. RetryWhen #4
public class RetryTransformer<T> implements SingleTransformer<T, T> {
. . . .
@Override
public SingleSource apply(Single<T> upstream) {
return upstream.retryWhen(errors ->
errors.zipWith(Flowable.range(0, LIMIT),
this::handleRetryAttempt).flatMap(x -> x));
}
}
public Single<Profile> getProfile() {
return profileRepository.getProfile().compose(retryTransformer);
}

27. 예외 처리 #1
public long getTimeInMillis1(String input) throws ParseException {
SimpleDateFormat sdf = new SimpleDateFormat(DATE_FORMAT);
return sdf.parse(input).getTime();
}
public long getTimeInMillis2(String input) {
SimpleDateFormat sdf = new SimpleDateFormat(DATE_FORMAT);
try {
return sdf.parse(input).getTime();
} catch (ParseException e) {
e.printStackTrace();
return -1L; // -1을 상수로 해도 큰 차이가 없음
}
}

28. 예외 처리 #2
public long getTimeInMillis3(String input) {
SimpleDateFormat sdf = new SimpleDateFormat(DATE_FORMAT);
try {
return sdf.parse(input).getTime();
} catch (ParseException e) {
throw new IllegalArgumentException("Illegal input pattern");
}
}

29. 예외 처리 #3
public Single<Long> getTimeInMillis(String input) {
return Single.fromCallable(() -> {
SimpleDateFormat sdf = new SimpleDateFormat(DATE_FORMAT);
return sdf.parse(input).getTime();
});
}
String input = "2017-xx-30 02:20:20";
DateFormatter formatter = new DateFormatter();
long start = formatter.getTimeInMillis(input)
.onErrorReturnItem(0L)
.blockingGet();
long end = formatter.getTimeInMillis(input)
.onErrorReturnItem(new Date().getTime())
.blockingGet();
System.out.println("start=" + start + ", end=" + end);

30. flatMap BiFunction
Observable.range(2, 8)
.flatMap(row -> Observable.range(1, 9)
.map(col -> String.format("%d x %d = %d",
row, col, row * col)))
.subscribe(System.out::println);
Observable.range(2, 8)
.flatMap(row -> Observable.range(1, 9),
(row, col) -> String.format("%d x %d = %d",
row, col, row * col))
.subscribe(System.out::println);

31. collect
Observable<String> obs = Observable.fromIterable(searchOptions)
.filter(SearchOption::isInUse)
.map(SearchOption::getName);
obs.subscribe(name -> {
applySearchOptions.add(name);
}, e -> {
}, () -> {
showData(applySearchOptions);
});
Single<ArrayList<String>> obs = Observable.fromIterable(searchOptions)
.filter(SearchOption::isInUse)
.map(SearchOption::getName)
.collect(ArrayList<String>::new, List::add);
obs.subscribe(this::showData);

32. Q & A