SingleSubject.java
/*
* Copyright (c) 2016-present, RxJava Contributors.
*
* 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 io.reactivex.rxjava3.subjects;
import java.util.concurrent.atomic.*;
import io.reactivex.rxjava3.annotations.*;
import io.reactivex.rxjava3.core.*;
import io.reactivex.rxjava3.disposables.Disposable;
import io.reactivex.rxjava3.internal.util.ExceptionHelper;
import io.reactivex.rxjava3.plugins.RxJavaPlugins;
/**
* Represents a hot Single-like source and consumer of events similar to Subjects.
* <p>
* <img width="640" height="236" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/SingleSubject.png" alt="">
* <p>
* This subject does not have a public constructor by design; a new non-terminated instance of this
* {@code SingleSubject} can be created via the {@link #create()} method.
* <p>
* Since the {@code SingleSubject} is conceptionally derived from the {@code Processor} type in the Reactive Streams specification,
* {@code null}s are not allowed (<a href="https://github.com/reactive-streams/reactive-streams-jvm#2.13">Rule 2.13</a>)
* as parameters to {@link #onSuccess(Object)} and {@link #onError(Throwable)}. Such calls will result in a
* {@link NullPointerException} being thrown and the subject's state is not changed.
* <p>
* Since a {@code SingleSubject} is a {@link io.reactivex.rxjava3.core.Single}, calling {@code onSuccess} or {@code onError}
* will move this {@code SingleSubject} into its terminal state atomically.
* <p>
* All methods are thread safe. Calling {@link #onSuccess(Object)} multiple
* times has no effect. Calling {@link #onError(Throwable)} multiple times relays the {@code Throwable} to
* the {@link io.reactivex.rxjava3.plugins.RxJavaPlugins#onError(Throwable)} global error handler.
* <p>
* Even though {@code SingleSubject} implements the {@code SingleObserver} interface, calling
* {@code onSubscribe} is not required (<a href="https://github.com/reactive-streams/reactive-streams-jvm#2.12">Rule 2.12</a>)
* if the subject is used as a standalone source. However, calling {@code onSubscribe}
* after the {@code SingleSubject} reached its terminal state will result in the
* given {@code Disposable} being disposed immediately.
* <p>
* This {@code SingleSubject} supports the standard state-peeking methods {@link #hasThrowable()},
* {@link #getThrowable()} and {@link #hasObservers()} as well as means to read any success item in a non-blocking
* and thread-safe manner via {@link #hasValue()} and {@link #getValue()}.
* <p>
* The {@code SingleSubject} does not support clearing its cached {@code onSuccess} value.
* <dl>
* <dt><b>Scheduler:</b></dt>
* <dd>{@code SingleSubject} does not operate by default on a particular {@link io.reactivex.rxjava3.core.Scheduler} and
* the {@code SingleObserver}s get notified on the thread where the terminating {@code onSuccess} or {@code onError}
* methods were invoked.</dd>
* <dt><b>Error handling:</b></dt>
* <dd>When the {@link #onError(Throwable)} is called, the {@code SingleSubject} enters into a terminal state
* and emits the same {@code Throwable} instance to the last set of {@code SingleObserver}s. During this emission,
* if one or more {@code SingleObserver}s dispose their respective {@code Disposable}s, the
* {@code Throwable} is delivered to the global error handler via
* {@link io.reactivex.rxjava3.plugins.RxJavaPlugins#onError(Throwable)} (multiple times if multiple {@code SingleObserver}s
* cancel at once).
* If there were no {@code SingleObserver}s subscribed to this {@code SingleSubject} when the {@code onError()}
* was called, the global error handler is not invoked.
* </dd>
* </dl>
* <p>
* Example usage:
* <pre><code>
* SingleSubject<Integer> subject1 = SingleSubject.create();
*
* TestObserver<Integer> to1 = subject1.test();
*
* // SingleSubjects are empty by default
* to1.assertEmpty();
*
* subject1.onSuccess(1);
*
* // onSuccess is a terminal event with SingleSubjects
* // TestObserver converts onSuccess into onNext + onComplete
* to1.assertResult(1);
*
* TestObserver<Integer> to2 = subject1.test();
*
* // late Observers receive the terminal signal (onSuccess) too
* to2.assertResult(1);
* </code></pre>
* <p>History: 2.0.5 - experimental
* @param <T> the value type received and emitted
* @since 2.1
*/
public final class SingleSubject<T> extends Single<T> implements SingleObserver<T> {
final AtomicReference<SingleDisposable<T>[]> observers;
@SuppressWarnings("rawtypes")
static final SingleDisposable[] EMPTY = new SingleDisposable[0];
@SuppressWarnings("rawtypes")
static final SingleDisposable[] TERMINATED = new SingleDisposable[0];
final AtomicBoolean once;
T value;
Throwable error;
/**
* Creates a fresh SingleSubject.
* @param <T> the value type received and emitted
* @return the new SingleSubject instance
*/
@CheckReturnValue
@NonNull
public static <T> SingleSubject<T> create() {
return new SingleSubject<>();
}
@SuppressWarnings("unchecked")
SingleSubject() {
once = new AtomicBoolean();
observers = new AtomicReference<>(EMPTY);
}
@Override
public void onSubscribe(@NonNull Disposable d) {
if (observers.get() == TERMINATED) {
d.dispose();
}
}
@SuppressWarnings("unchecked")
@Override
public void onSuccess(@NonNull T value) {
ExceptionHelper.nullCheck(value, "onSuccess called with a null value.");
if (once.compareAndSet(false, true)) {
this.value = value;
for (SingleDisposable<T> md : observers.getAndSet(TERMINATED)) {
md.downstream.onSuccess(value);
}
}
}
@SuppressWarnings("unchecked")
@Override
public void onError(@NonNull Throwable e) {
ExceptionHelper.nullCheck(e, "onError called with a null Throwable.");
if (once.compareAndSet(false, true)) {
this.error = e;
for (SingleDisposable<T> md : observers.getAndSet(TERMINATED)) {
md.downstream.onError(e);
}
} else {
RxJavaPlugins.onError(e);
}
}
@Override
protected void subscribeActual(@NonNull SingleObserver<? super T> observer) {
SingleDisposable<T> md = new SingleDisposable<>(observer, this);
observer.onSubscribe(md);
if (add(md)) {
if (md.isDisposed()) {
remove(md);
}
} else {
Throwable ex = error;
if (ex != null) {
observer.onError(ex);
} else {
observer.onSuccess(value);
}
}
}
boolean add(@NonNull SingleDisposable<T> inner) {
for (;;) {
SingleDisposable<T>[] a = observers.get();
if (a == TERMINATED) {
return false;
}
int n = a.length;
@SuppressWarnings("unchecked")
SingleDisposable<T>[] b = new SingleDisposable[n + 1];
System.arraycopy(a, 0, b, 0, n);
b[n] = inner;
if (observers.compareAndSet(a, b)) {
return true;
}
}
}
@SuppressWarnings("unchecked")
void remove(@NonNull SingleDisposable<T> inner) {
for (;;) {
SingleDisposable<T>[] a = observers.get();
int n = a.length;
if (n == 0) {
return;
}
int j = -1;
for (int i = 0; i < n; i++) {
if (a[i] == inner) {
j = i;
break;
}
}
if (j < 0) {
return;
}
SingleDisposable<T>[] b;
if (n == 1) {
b = EMPTY;
} else {
b = new SingleDisposable[n - 1];
System.arraycopy(a, 0, b, 0, j);
System.arraycopy(a, j + 1, b, j, n - j - 1);
}
if (observers.compareAndSet(a, b)) {
return;
}
}
}
/**
* Returns the success value if this SingleSubject was terminated with a success value.
* @return the success value or null
*/
@Nullable
public T getValue() {
if (observers.get() == TERMINATED) {
return value;
}
return null;
}
/**
* Returns true if this SingleSubject was terminated with a success value.
* @return true if this SingleSubject was terminated with a success value
*/
public boolean hasValue() {
return observers.get() == TERMINATED && value != null;
}
/**
* Returns the terminal error if this SingleSubject has been terminated with an error, null otherwise.
* @return the terminal error or null if not terminated or not with an error
*/
@Nullable
public Throwable getThrowable() {
if (observers.get() == TERMINATED) {
return error;
}
return null;
}
/**
* Returns true if this SingleSubject has been terminated with an error.
* @return true if this SingleSubject has been terminated with an error
*/
public boolean hasThrowable() {
return observers.get() == TERMINATED && error != null;
}
/**
* Returns true if this SingleSubject has observers.
* @return true if this SingleSubject has observers
*/
public boolean hasObservers() {
return observers.get().length != 0;
}
/**
* Returns the number of current observers.
* @return the number of current observers
*/
/* test */ int observerCount() {
return observers.get().length;
}
static final class SingleDisposable<T>
extends AtomicReference<SingleSubject<T>> implements Disposable {
private static final long serialVersionUID = -7650903191002190468L;
final SingleObserver<? super T> downstream;
SingleDisposable(SingleObserver<? super T> actual, SingleSubject<T> parent) {
this.downstream = actual;
lazySet(parent);
}
@Override
public void dispose() {
SingleSubject<T> parent = getAndSet(null);
if (parent != null) {
parent.remove(this);
}
}
@Override
public boolean isDisposed() {
return get() == null;
}
}
}