rx.observables.AbstractOnSubscribe Maven / Gradle / Ivy
/**
* Copyright 2014 Netflix, Inc.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package rx.observables;
import java.util.Arrays;
import java.util.concurrent.atomic.*;
import rx.*;
import rx.Observable.OnSubscribe;
import rx.annotations.Experimental;
import rx.exceptions.CompositeException;
import rx.functions.*;
import rx.internal.operators.BackpressureUtils;
/**
* Abstract base class for the {@link OnSubscribe} interface that helps you build Observable sources one
* {@code onNext} at a time, and automatically supports unsubscription and backpressure.
*
*
Usage rules
* When you implement the {@code next()} method, you
*
* - should either
*
* - create the next value and signal it via {@link SubscriptionState#onNext state.onNext()},
* - signal a terminal condition via {@link SubscriptionState#onError state.onError()}, or
* {@link SubscriptionState#onCompleted state.onCompleted()}, or
* - signal a stop condition via {@link SubscriptionState#stop state.stop()} indicating no further values
* will be sent.
*
*
* - may
*
* - call {@link SubscriptionState#onNext state.onNext()} and either
* {@link SubscriptionState#onError state.onError()} or
* {@link SubscriptionState#onCompleted state.onCompleted()} together, and
*
- block or sleep.
*
*
* - should not
*
* - do nothing or do async work and not produce any event or request stopping. If neither of
* the methods are called, an {@link IllegalStateException} is forwarded to the {@code Subscriber} and
* the Observable is terminated;
* - call the {@code state.on}foo() methods more than once (yields
* {@link IllegalStateException}).
*
*
*
*
* The {@link SubscriptionState} object features counters that may help implement a state machine:
*
* - A call counter, accessible via {@link SubscriptionState#calls state.calls()} tells how many times the
* {@code next()} was run (zero based).
* - You can use a phase counter, accessible via {@link SubscriptionState#phase state.phase}, that helps track
* the current emission phase, in a {@code switch()} statement to implement the state machine. (It is named
* {@code phase} to avoid confusion with the per-subscriber state.)
* - You can arbitrarily change the current phase with
* {@link SubscriptionState#advancePhase state.advancePhase()},
* {@link SubscriptionState#advancePhaseBy(int) state.advancedPhaseBy(int)} and
* {@link SubscriptionState#phase(int) state.phase(int)}.
*
*
* When you implement {@code AbstractOnSubscribe}, you may override {@link AbstractOnSubscribe#onSubscribe} to
* perform special actions (such as registering {@code Subscription}s with {@code Subscriber.add()}) and return
* additional state for each subscriber subscribing. You can access this custom state with the
* {@link SubscriptionState#state state.state()} method. If you need to do some cleanup, you can override the
* {@link #onTerminated} method.
*
* For convenience, a lambda-accepting static factory method, {@link #create}, is available.
* Another convenience is {@link #toObservable} which turns an {@code AbstractOnSubscribe}
* instance into an {@code Observable} fluently.
*
*
Examples
* Note: these examples use the lambda-helper factories to avoid boilerplate.
*
* Implement: just
*
* AbstractOnSubscribe.create(s -> {
* s.onNext(1);
* s.onCompleted();
* }).toObservable().subscribe(System.out::println);
*
* Implement: from Iterable
*
* Iterable iterable = ...;
* AbstractOnSubscribe.create(s -> {
* Iterator it = s.state();
* if (it.hasNext()) {
* s.onNext(it.next());
* }
* if (!it.hasNext()) {
* s.onCompleted();
* }
* }, u -> iterable.iterator()).subscribe(System.out::println);
*
*
* Implement source that fails a number of times before succeeding
*
* AtomicInteger fails = new AtomicInteger();
* int numFails = 50;
* AbstractOnSubscribe.create(s -> {
* long c = s.calls();
* switch (s.phase()) {
* case 0:
* s.onNext("Beginning");
* s.onError(new RuntimeException("Oh, failure.");
* if (c == numFails.getAndIncrement()) {
* s.advancePhase();
* }
* break;
* case 1:
* s.onNext("Beginning");
* s.advancePhase();
* case 2:
* s.onNext("Finally working");
* s.onCompleted();
* s.advancePhase();
* default:
* throw new IllegalStateException("How did we get here?");
* }
* }).subscribe(System.out::println);
*
* Implement: never
*
* AbstractOnSubscribe.create(s -> {
* s.stop();
* }).toObservable()
* .timeout(1, TimeUnit.SECONDS)
* .subscribe(System.out::println, Throwable::printStacktrace, () -> System.out.println("Done"));
*
*
* @param the value type
* @param the per-subscriber user-defined state type
* @since (if this graduates from Experimental/Beta to supported, replace this parenthetical with the release number)
* @Experimental
*/
@Experimental
public abstract class AbstractOnSubscribe implements OnSubscribe {
/**
* Called when a Subscriber subscribes and lets the implementor create a per-subscriber custom state.
*
* Override this method to have custom state per-subscriber. The default implementation returns
* {@code null}.
*
* @param subscriber the subscriber who is subscribing
* @return the custom state
*/
protected S onSubscribe(Subscriber subscriber) {
return null;
}
/**
* Called after the terminal emission or when the downstream unsubscribes.
*
* This is called only once and no {@code onNext} call will run concurrently with it. The default
* implementation does nothing.
*
* @param state the user-provided state
*/
protected void onTerminated(S state) {
}
/**
* Override this method to create an emission state-machine.
*
* @param state the per-subscriber subscription state
*/
protected abstract void next(SubscriptionState state);
@Override
public final void call(final Subscriber subscriber) {
final S custom = onSubscribe(subscriber);
final SubscriptionState state = new SubscriptionState(this, subscriber, custom);
subscriber.add(new SubscriptionCompleter(state));
subscriber.setProducer(new SubscriptionProducer(state));
}
/**
* Convenience method to create an Observable from this implemented instance.
*
* @return the created observable
*/
public final Observable toObservable() {
return Observable.create(this);
}
/** Function that returns null. */
private static final Func1 NULL_FUNC1 = new Func1() {
@Override
public Object call(Object t1) {
return null;
}
};
/**
* Creates an {@code AbstractOnSubscribe} instance which calls the provided {@code next} action.
*
* This is a convenience method to help create {@code AbstractOnSubscribe} instances with the help of
* lambdas.
*
* @param the value type
* @param the per-subscriber user-defined state type
* @param next the next action to call
* @return an {@code AbstractOnSubscribe} instance
*/
public static AbstractOnSubscribe create(Action1> next) {
@SuppressWarnings("unchecked")
Func1, ? extends S> nullFunc =
(Func1, ? extends S>)NULL_FUNC1;
return create(next, nullFunc, Actions.empty());
}
/**
* Creates an {@code AbstractOnSubscribe} instance which creates a custom state with the {@code onSubscribe}
* function and calls the provided {@code next} action.
*
* This is a convenience method to help create {@code AbstractOnSubscribe} instances with the help of
* lambdas.
*
* @param the value type
* @param the per-subscriber user-defined state type
* @param next the next action to call
* @param onSubscribe the function that returns a per-subscriber state to be used by {@code next}
* @return an {@code AbstractOnSubscribe} instance
*/
public static AbstractOnSubscribe create(Action1> next,
Func1, ? extends S> onSubscribe) {
return create(next, onSubscribe, Actions.empty());
}
/**
* Creates an {@code AbstractOnSubscribe} instance which creates a custom state with the {@code onSubscribe}
* function, calls the provided {@code next} action and calls the {@code onTerminated} action to release the
* state when its no longer needed.
*
* This is a convenience method to help create {@code AbstractOnSubscribe} instances with the help of
* lambdas.
*
* @param the value type
* @param the per-subscriber user-defined state type
* @param next the next action to call
* @param onSubscribe the function that returns a per-subscriber state to be used by {@code next}
* @param onTerminated the action to call to release the state created by the {@code onSubscribe} function
* @return an {@code AbstractOnSubscribe} instance
*/
public static AbstractOnSubscribe create(Action1> next,
Func1, ? extends S> onSubscribe, Action1 onTerminated) {
return new LambdaOnSubscribe(next, onSubscribe, onTerminated);
}
/**
* An implementation that forwards the three main methods ({@code next}, {@code onSubscribe}, and
* {@code onTermianted}) to functional callbacks.
*
* @param the value type
* @param the per-subscriber user-defined state type
*/
private static final class LambdaOnSubscribe extends AbstractOnSubscribe {
final Action1> next;
final Func1, ? extends S> onSubscribe;
final Action1 onTerminated;
private LambdaOnSubscribe(Action1> next,
Func1, ? extends S> onSubscribe, Action1 onTerminated) {
this.next = next;
this.onSubscribe = onSubscribe;
this.onTerminated = onTerminated;
}
@Override
protected S onSubscribe(Subscriber subscriber) {
return onSubscribe.call(subscriber);
}
@Override
protected void onTerminated(S state) {
onTerminated.call(state);
}
@Override
protected void next(SubscriptionState state) {
next.call(state);
}
}
/**
* Manages unsubscription of the state.
*
* @param the value type
* @param the per-subscriber user-defined state type
*/
private static final class SubscriptionCompleter extends AtomicBoolean implements Subscription {
private static final long serialVersionUID = 7993888274897325004L;
private final SubscriptionState state;
private SubscriptionCompleter(SubscriptionState state) {
this.state = state;
}
@Override
public boolean isUnsubscribed() {
return get();
}
@Override
public void unsubscribe() {
if (compareAndSet(false, true)) {
state.free();
}
}
}
/**
* Contains the producer loop that reacts to downstream requests of work.
*
* @param the value type
* @param the per-subscriber user-defined state type
*/
private static final class SubscriptionProducer implements Producer {
final SubscriptionState state;
private SubscriptionProducer(SubscriptionState state) {
this.state = state;
}
@Override
public void request(long n) {
if (n > 0 && BackpressureUtils.getAndAddRequest(state.requestCount, n) == 0) {
if (n == Long.MAX_VALUE) {
// fast-path
for (; !state.subscriber.isUnsubscribed(); ) {
if (!doNext()) {
break;
}
}
} else
if (!state.subscriber.isUnsubscribed()) {
do {
if (!doNext()) {
break;
}
} while (state.requestCount.decrementAndGet() > 0 && !state.subscriber.isUnsubscribed());
}
}
}
/**
* Executes the user-overridden next() method and performs state bookkeeping and
* verification.
*
* @return true if the outer loop may continue
*/
protected boolean doNext() {
if (state.use()) {
try {
int p = state.phase();
state.parent.next(state);
if (!state.verify()) {
throw new IllegalStateException("No event produced or stop called @ Phase: " + p + " -> " + state.phase() + ", Calls: " + state.calls());
}
if (state.accept() || state.stopRequested()) {
state.terminate();
return false;
}
state.calls++;
} catch (Throwable t) {
state.terminate();
state.subscriber.onError(t);
return false;
} finally {
state.free();
}
return true;
}
return false;
}
}
/**
* Represents a per-subscription state for the {@code AbstractOnSubscribe} operation. It supports phasing
* and counts the number of times a value was requested by the downstream.
*
* @param the value type
* @param the per-subscriber user-defined state type
* @since (if this graduates from Experimental/Beta to supported, replace this parenthetical with the release number)
* @Experimental
*/
public static final class SubscriptionState {
private final AbstractOnSubscribe parent;
private final Subscriber subscriber;
private final S state;
private final AtomicLong requestCount;
private final AtomicInteger inUse;
private int phase;
private long calls;
private T theValue;
private boolean hasOnNext;
private boolean hasCompleted;
private boolean stopRequested;
private Throwable theException;
private SubscriptionState(AbstractOnSubscribe parent, Subscriber subscriber, S state) {
this.parent = parent;
this.subscriber = subscriber;
this.state = state;
this.requestCount = new AtomicLong();
this.inUse = new AtomicInteger(1);
}
/**
* @return the per-subscriber specific user-defined state created via
* {@link AbstractOnSubscribe#onSubscribe}
*/
public S state() {
return state;
}
/**
* @return the current phase value
*/
public int phase() {
return phase;
}
/**
* Sets a new phase value.
*
* @param newPhase
*/
public void phase(int newPhase) {
phase = newPhase;
}
/**
* Advance the current phase by 1.
*/
public void advancePhase() {
advancePhaseBy(1);
}
/**
* Advance the current phase by the given amount (can be negative).
*
* @param amount the amount to advance the phase
*/
public void advancePhaseBy(int amount) {
phase += amount;
}
/**
* @return the number of times {@link AbstractOnSubscribe#next} was called so far, starting at 0 for the
* very first call
*/
public long calls() {
return calls;
}
/**
* Call this method to offer the next {@code onNext} value for the subscriber.
*
* @param value the value to {@code onNext}
* @throws IllegalStateException if there is a value already offered but not taken or a terminal state
* is reached
*/
public void onNext(T value) {
if (hasOnNext) {
throw new IllegalStateException("onNext not consumed yet!");
} else
if (hasCompleted) {
throw new IllegalStateException("Already terminated", theException);
}
theValue = value;
hasOnNext = true;
}
/**
* Call this method to send an {@code onError} to the subscriber and terminate all further activities.
* If there is a pending {@code onNext}, that value is emitted to the subscriber followed by this
* exception.
*
* @param e the exception to deliver to the client
* @throws IllegalStateException if the terminal state has been reached already
*/
public void onError(Throwable e) {
if (e == null) {
throw new NullPointerException("e != null required");
}
if (hasCompleted) {
throw new IllegalStateException("Already terminated", theException);
}
theException = e;
hasCompleted = true;
}
/**
* Call this method to send an {@code onCompleted} to the subscriber and terminate all further
* activities. If there is a pending {@code onNext}, that value is emitted to the subscriber followed by
* this exception.
*
* @throws IllegalStateException if the terminal state has been reached already
*/
public void onCompleted() {
if (hasCompleted) {
throw new IllegalStateException("Already terminated", theException);
}
hasCompleted = true;
}
/**
* Signals that there won't be any further events.
*/
public void stop() {
stopRequested = true;
}
/**
* Emits the {@code onNext} and/or the terminal value to the actual subscriber.
*
* @return {@code true} if the event was a terminal event
*/
protected boolean accept() {
if (hasOnNext) {
T value = theValue;
theValue = null;
hasOnNext = false;
try {
subscriber.onNext(value);
} catch (Throwable t) {
hasCompleted = true;
Throwable e = theException;
theException = null;
if (e == null) {
subscriber.onError(t);
} else {
subscriber.onError(new CompositeException(Arrays.asList(t, e)));
}
return true;
}
}
if (hasCompleted) {
Throwable e = theException;
theException = null;
if (e != null) {
subscriber.onError(e);
} else {
subscriber.onCompleted();
}
return true;
}
return false;
}
/**
* Verify if the {@code next()} generated an event or requested a stop.
*
* @return true if either event was generated or stop was requested
*/
protected boolean verify() {
return hasOnNext || hasCompleted || stopRequested;
}
/** @return true if the {@code next()} requested a stop */
protected boolean stopRequested() {
return stopRequested;
}
/**
* Request the state to be used by {@code onNext} or returns {@code false} if the downstream has
* unsubscribed.
*
* @return {@code true} if the state can be used exclusively
* @throws IllegalStateEception
* @warn "throws" section incomplete
*/
protected boolean use() {
int i = inUse.get();
if (i == 0) {
return false;
} else
if (i == 1 && inUse.compareAndSet(1, 2)) {
return true;
}
throw new IllegalStateException("This is not reentrant nor threadsafe!");
}
/**
* Release the state if there are no more interest in it and it is not in use.
*/
protected void free() {
int i = inUse.get();
if (i <= 0) {
return;
} else
if (inUse.decrementAndGet() == 0) {
parent.onTerminated(state);
}
}
/**
* Terminates the state immediately and calls {@link AbstractOnSubscribe#onTerminated} with the custom
* state.
*/
protected void terminate() {
for (;;) {
int i = inUse.get();
if (i <= 0) {
return;
}
if (inUse.compareAndSet(i, 0)) {
parent.onTerminated(state);
break;
}
}
}
}
}