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Common RxJava Mistakes
Dan Lew
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Lambda Notation
new Func1() {
@Override public Integer call(String s) {
return s.length();
}
};
s -> s.length();
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Lambda Notation
new Func1() {
@Override public Integer call(String s) {
return s.length();
}
};
s -> {
return s.length();
}
s -> s.length()
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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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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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The Stream
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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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Observable step1 = Observable.just("Hello!");
Observable step2 = step1.map(s -> s.length());
step2.subscribe(i -> System.out.println(i));
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Observable step1 = Observable.just("Hello!");
Observable step2 = step1.map(s -> s.length());
step2.subscribe(i -> System.out.println(i));
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Observable step1 = Observable.just("Hello!");
Observable step2 = step1.map(s -> s.length());
step2.subscribe(i -> System.out.println(i));
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Upstream
Downstream
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Creating Observables 101
• just(object)
• from(iterable)
• Whatever Retrofit is doing
• create()
NO
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Difficulties
• Observable contract
• onNext* -> onCompleted | onError
• Unsubscription
• Backpressure
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class MyClass {
String value;
Observable value() {
return Observable.create(subscriber -> {
subscriber.onNext(value);
subscriber.onCompleted();
});
}
}
???
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class MyClass {
String value;
Observable value() {
return Observable.create(subscriber -> {
subscriber.onNext(value);
subscriber.onCompleted();
});
}
}
BAD
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class MyClass {
String value;
Observable value() {
return Observable.create(subscriber -> {
subscriber.onNext(value);
subscriber.onCompleted();
});
}
}
return Observable.fromCallable(() -> value);
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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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class MyClass {
BehaviorSubject value;
Observable value() {
return value.asObservable();
}
void setValue(String s) {
value.onNext(s);
}
}
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RxRelay
• Safer Subjects
• https://github.com/JakeWharton/RxRelay
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Lessons
• Learn your creation methods!
• Resort to create() when appropriate
• (which is almost never)
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Operator
Misunderstandings
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map vs. flatMap
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Types
• map: T -> R
• flatMap: T -> Observable
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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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flatMap vs. concatMap
• Same, except…
• #ordermatters
• flatMap -> Unordered merge
• concatMap -> Ordered merge
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No content
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toList()
• Item(s) -> List
• “Why does nothing happen?!”
• Missing onCompleted
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Versus
Key!
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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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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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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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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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cache() vs replay()
• Don’t use cache()
• Use replay()
• cache() == replay().autoConnect()
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Custom Operators
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Operators I Have Written
• OperatorNullToDefault
• OperatorSortLists
• OperatorSubscribeUntil
• ContinueWithIfEmptyOperator
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Operators I Should Have
Written
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Why not?
• Each operator —> different flow
• Framework handles common flows
• Resort to custom operators for unique flows
• Operators are hard!
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• OperatorNullToDefault
• OperatorSortLists
• OperatorSubscribeUntil
• ContinueWithIfEmptyOperator
map()
switchIfEmpty()
takeUntil()
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Threading
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Observable.range(0, 5)
.subscribe();
Observable.interval(1, TimeUnit.SECONDS)
.subscribe();
current thread
computation()
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Observable.just("Hello")
.subscribe();
Observable.just("Hello")
.delay(1, TimeUnit.SECONDS)
.subscribe();
current thread
computation()
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Observable.just("Hello")
.subscribeOn(Schedulers.io())
.map()
.subscribeOn(Schedulers.computation())
.subscribe();
io()
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Observable.just("Hello")
.observeOn(Schedulers.io())
.map()
.observeOn(Schedulers.computation())
.subscribe();
io()
computation()
current thread
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No content
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Lessons
• Know your scheduler-based operators
• Use subscribeOn once
• Use observeOn liberally
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Subscriptions
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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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• Observer is an interface
• Subscriber is a class
• Observer is stateless
• Subscriber is stateful
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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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Memory Leaks
• Subscriptions can leak!
• Detect w/ LeakCanary
• Solution: Unsubscribe
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Unsubscribe
Subscription subscription = Observable.just(1).subscribe();
subscription.unsubscribe();
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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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RxLifecycle
• Automatic sequence completion
Observable.interval(1, TimeUnit.SECONDS)
.compose(bindToLifecycle())
.subscribe();
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Error Handling
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onError Uses
• Unexpected exceptions
• Unrecoverable streams
• Flow control
Blame Retrofit 1
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Terminal events suck
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onError ≈ try-catch
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Exception workarounds
• Error state
• onErrorReturn
• onErrorResumeNext
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Debugging onError
Observable.empty()
.first()
.subscribeOn(Schedulers.io())
.subscribe();
Crash!
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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$1.call(OperatorSubscribeOn.java:62)
at rx.internal.schedulers.ScheduledAction.run(ScheduledAction.java:55)
... 7 more
Caused by: java.util.NoSuchElementException: Sequence contains no elements
... 18 more
No references to my code!
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Implement onError
Observable.empty()
.first()
.subscribeOn(Schedulers.io())
.subscribe(o -> System.out.println(o),
err -> {
throw new OnErrorNotImplementedException("Source!", err);
});
Caused by: rx.exceptions.OnErrorNotImplementedException: Source!
at net.danlew.experiments.Tester.lambda$main$1(Tester.java:52)
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Implement onError
Observable.empty()
.first()
.subscribeOn(Schedulers.io())
.subscribe(o -> System.out.println(o),
err -> {
throw new OnErrorNotImplementedException("Source!", err);
});
Caused by: rx.exceptions.OnErrorNotImplementedException: Source!
at net.danlew.experiments.Tester.lambda$main$1(Tester.java:52)
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Implement onError
Observable.empty()
.first()
.subscribeOn(Schedulers.io())
.subscribe(o -> System.out.println(o),
err -> {
throw new OnErrorNotImplementedException("Source!", err);
});
Caused by: rx.exceptions.OnErrorNotImplementedException: Source!
at net.danlew.experiments.Tester.lambda$main$1(Tester.java:52)
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Sharing
• Wanted: Multiple subscribers
retrofitRequest.subscribe(/* do a thing */);
retrofitRequest.subscribe(/* do another thing */);
• Bad: Duplicate expensive operations
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Hot vs. Cold
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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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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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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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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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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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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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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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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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Lessons
• Share cold observables
• Explicitly call connect()
• Use autoConnect()
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it’s over!
@danlew42