RxJava Subscription 自动取消订阅

在RxJava Observer与Subscriber的关系 一文中，我们提到：

subscribe(mObserver)和subscribe(mSubscriber)执行结果就会有区别：

  • subscribe(mSubscriber)这种订阅方式在第二次请求数据时就不会执行了，原因就是第一次onNext后自动取消了订阅；
  • subscribe(mObserver)则不出现此问题。

今天我们分析下原因。

先来看看源码。

Subscription
RxJava中有个叫做Subscription的接口,可以用来取消订阅.

public interface Subscription {
    /**
     * Stops the receipt of notifications on the {@link Subscriber} that was registered when this Subscription
     * was received.
     * <p>
     * This allows unregistering an {@link Subscriber} before it has finished receiving all events (i.e. before
     * onCompleted is called).
     */
    void unsubscribe();

    /**
     * Indicates whether this {@code Subscription} is currently unsubscribed.
     *
     * @return {@code true} if this {@code Subscription} is currently unsubscribed, {@code false} otherwise
     */
    boolean isUnsubscribed();
}
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从上面可以看到,我们只需要调用unsubscribe就可以取消订阅,那么如何得到一个Subscription对象呢?

其实很简单:
Observable.subscribe()方法可以返回一个Subscription的对象,即我们每次订阅都会返回.
感觉Subscription就像一个订单,你下单了就会生成一个订单,而你也可以用这个订单取消订单.

实战

我先写了以下代码来测试:

Subscription subscription = Observable.just("Hello subscription")
        .subscribe(new Action1<String>() {
            @Override
            public void call(String s) {
                System.out.println(s);
            }
        });
System.out.println(subscription.isUnsubscribed());
subscription.unsubscribe();
System.out.println(subscription.isUnsubscribed());
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在我想来输出的日志应该是这样的:

Hello subscription
false
true
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但是,结果出乎我的意料,我运行之后是这样的:

Hello subscription
true
true
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明明我没有取消订阅啊，怎么就true了呢?

接下去我进源码探索了一下发现:
在Observable.subscribe()源码：

public final Subscription subscribe(Subscriber<? super T> subscriber) {
        return Observable.subscribe(subscriber, this);
    }

    static <T> Subscription subscribe(Subscriber<? super T> subscriber, Observable<T> observable) {
     // validate and proceed
        if (subscriber == null) {
            throw new IllegalArgumentException("observer can not be null");
        }
        if (observable.onSubscribe == null) {
            throw new IllegalStateException("onSubscribe function can not be null.");
            /*
             * the subscribe function can also be overridden but generally that‘s not the appropriate approach
             * so I won‘t mention that in the exception
             */
        }

        // new Subscriber so onStart it
        subscriber.onStart();

        /*
         * See https://github.com/ReactiveX/RxJava/issues/216 for discussion on "Guideline 6.4: Protect calls
         * to user code from within an Observer"
         */
        // if not already wrapped
        if (!(subscriber instanceof SafeSubscriber)) {
            // assign to `observer` so we return the protected version
            subscriber = new SafeSubscriber<T>(subscriber);
        }

        // The code below is exactly the same an unsafeSubscribe but not used because it would
        // add a significant depth to already huge call stacks.
        try {
            // allow the hook to intercept and/or decorate
            hook.onSubscribeStart(observable, observable.onSubscribe).call(subscriber);
            return hook.onSubscribeReturn(subscriber);
        } catch (Throwable e) {
            // special handling for certain Throwable/Error/Exception types
            Exceptions.throwIfFatal(e);
            // in case the subscriber can‘t listen to exceptions anymore
            if (subscriber.isUnsubscribed()) {
                RxJavaPluginUtils.handleException(hook.onSubscribeError(e));
            } else {
                // if an unhandled error occurs executing the onSubscribe we will propagate it
                try {
                    subscriber.onError(hook.onSubscribeError(e));
                } catch (Throwable e2) {
                    Exceptions.throwIfFatal(e2);
                    // if this happens it means the onError itself failed (perhaps an invalid function implementation)
                    // so we are unable to propagate the error correctly and will just throw
                    RuntimeException r = new OnErrorFailedException("Error occurred attempting to subscribe [" + e.getMessage() + "] and then again while trying to pass to onError.", e2);
                    // TODO could the hook be the cause of the error in the on error handling.
                    hook.onSubscribeError(r);
                    // TODO why aren‘t we throwing the hook‘s return value.
                    throw r;
                }
            }
            return Subscriptions.unsubscribed();
        }
    }
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关键代码：

if (!(subscriber instanceof SafeSubscriber)) {
    // assign to `observer` so we return the protected version
    subscriber = new SafeSubscriber<T>(subscriber);
}
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它会把我们传递的subscriber转成SafeSubscriber，SafeSubcriber源码如下：

public class SafeSubscriber<T> extends Subscriber<T> {

    private final Subscriber<? super T> actual;

    boolean done = false;

    public SafeSubscriber(Subscriber<? super T> actual) {
        super(actual);
        this.actual = actual;
    }

    /**
     * Notifies the Subscriber that the {@code Observable} has finished sending push-based notifications.
     * <p>
     * The {@code Observable} will not call this method if it calls {@link #onError}.
     */
    @Override
    public void onCompleted() {
        if (!done) {
            done = true;
            try {
                actual.onCompleted();
            } catch (Throwable e) {
                // we handle here instead of another method so we don‘t add stacks to the frame
                // which can prevent it from being able to handle StackOverflow
                Exceptions.throwIfFatal(e);
                RxJavaPluginUtils.handleException(e);
                throw new OnCompletedFailedException(e.getMessage(), e);
            } finally {
                try {
                    // Similarly to onError if failure occurs in unsubscribe then Rx contract is broken
                    // and we throw an UnsubscribeFailureException.
                    unsubscribe();
                } catch (Throwable e) {
                    RxJavaPluginUtils.handleException(e);
                    throw new UnsubscribeFailedException(e.getMessage(), e);
                }
            }
        }
    }

    /**
     * Notifies the Subscriber that the {@code Observable} has experienced an error condition.
     * <p>
     * If the {@code Observable} calls this method, it will not thereafter call {@link #onNext} or
     * {@link #onCompleted}.
     *
     * @param e
     *          the exception encountered by the Observable
     */
    @Override
    public void onError(Throwable e) {
        // we handle here instead of another method so we don‘t add stacks to the frame
        // which can prevent it from being able to handle StackOverflow
        Exceptions.throwIfFatal(e);
        if (!done) {
            done = true;
            _onError(e);
        }
    }

    /**
     * Provides the Subscriber with a new item to observe.
     * <p>
     * The {@code Observable} may call this method 0 or more times.
     * <p>
     * The {@code Observable} will not call this method again after it calls either {@link #onCompleted} or
     * {@link #onError}.
     *
     * @param args
     *          the item emitted by the Observable
     */
    @Override
    public void onNext(T args) {
        try {
            if (!done) {
                actual.onNext(args);
            }
        } catch (Throwable e) {
            // we handle here instead of another method so we don‘t add stacks to the frame
            // which can prevent it from being able to handle StackOverflow
            Exceptions.throwOrReport(e, this);
        }
    }

    /**
     * The logic for {@code onError} without the {@code isFinished} check so it can be called from within
     * {@code onCompleted}.
     *
     * @see <a href="https://github.com/ReactiveX/RxJava/issues/630">the report of this bug</a>
     */
    protected void _onError(Throwable e) {
        RxJavaPluginUtils.handleException(e);
        try {
            actual.onError(e);
        } catch (Throwable e2) {
            if (e2 instanceof OnErrorNotImplementedException) {
                /*
                 * onError isn‘t implemented so throw
                 *
                 * https://github.com/ReactiveX/RxJava/issues/198
                 *
                 * Rx Design Guidelines 5.2
                 *
                 * "when calling the Subscribe method that only has an onNext argument, the OnError behavior
                 * will be to rethrow the exception on the thread that the message comes out from the observable
                 * sequence. The OnCompleted behavior in this case is to do nothing."
                 */
                try {
                    unsubscribe();
                } catch (Throwable unsubscribeException) {
                    RxJavaPluginUtils.handleException(unsubscribeException);
                    throw new RuntimeException("Observer.onError not implemented and error while unsubscribing.", new CompositeException(Arrays.asList(e, unsubscribeException)));
                }
                throw (OnErrorNotImplementedException) e2;
            } else {
                /*
                 * throw since the Rx contract is broken if onError failed
                 *
                 * https://github.com/ReactiveX/RxJava/issues/198
                 */
                RxJavaPluginUtils.handleException(e2);
                try {
                    unsubscribe();
                } catch (Throwable unsubscribeException) {
                    RxJavaPluginUtils.handleException(unsubscribeException);
                    throw new OnErrorFailedException("Error occurred when trying to propagate error to Observer.onError and during unsubscription.", new CompositeException(Arrays.asList(e, e2, unsubscribeException)));
                }

                throw new OnErrorFailedException("Error occurred when trying to propagate error to Observer.onError", new CompositeException(Arrays.asList(e, e2)));
            }
        }
        // if we did not throw above we will unsubscribe here, if onError failed then unsubscribe happens in the catch
        try {
            unsubscribe();
        } catch (RuntimeException unsubscribeException) {
            RxJavaPluginUtils.handleException(unsubscribeException);
            throw new OnErrorFailedException(unsubscribeException);
        }
    }

    /**
     * Returns the {@link Subscriber} underlying this {@code SafeSubscriber}.
     *
     * @return the {@link Subscriber} that was used to create this {@code SafeSubscriber}
     */
    public Subscriber<? super T> getActual() {
        return actual;
    }
}
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从源码中不能发现：

public void onCompleted() {
    if (!done) {
        done = true;
        try {
            actual.onCompleted();
        } catch (Throwable e) {
            // we handle here instead of another method so we don‘t add stacks to the frame
            // which can prevent it from being able to handle StackOverflow
            Exceptions.throwIfFatal(e);
            // handle errors if the onCompleted implementation fails, not just if the Observable fails
            _onError(e);
        } finally {
            // auto-unsubscribe
            unsubscribe();
        }
    }
}
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原来它在finally里自动取消了订阅!!

同样，在出现error时，也会自动取消订阅。

protected void _onError(Throwable e) {
        RxJavaPluginUtils.handleException(e);
        try {
            actual.onError(e);
        } catch (Throwable e2) {
            if (e2 instanceof OnErrorNotImplementedException) {
                /*
                 * onError isn‘t implemented so throw
                 *
                 * https://github.com/ReactiveX/RxJava/issues/198
                 *
                 * Rx Design Guidelines 5.2
                 *
                 * "when calling the Subscribe method that only has an onNext argument, the OnError behavior
                 * will be to rethrow the exception on the thread that the message comes out from the observable
                 * sequence. The OnCompleted behavior in this case is to do nothing."
                 */
                try {
                    unsubscribe();
                } catch (Throwable unsubscribeException) {
                    RxJavaPluginUtils.handleException(unsubscribeException);
                    throw new RuntimeException("Observer.onError not implemented and error while unsubscribing.", new CompositeException(Arrays.asList(e, unsubscribeException)));
                }
                throw (OnErrorNotImplementedException) e2;
            } else {
                /*
                 * throw since the Rx contract is broken if onError failed
                 *
                 * https://github.com/ReactiveX/RxJava/issues/198
                 */
                RxJavaPluginUtils.handleException(e2);
                try {
                    unsubscribe();
                } catch (Throwable unsubscribeException) {
                    RxJavaPluginUtils.handleException(unsubscribeException);
                    throw new OnErrorFailedException("Error occurred when trying to propagate error to Observer.onError and during unsubscription.", new CompositeException(Arrays.asList(e, e2, unsubscribeException)));
                }

                throw new OnErrorFailedException("Error occurred when trying to propagate error to Observer.onError", new CompositeException(Arrays.asList(e, e2)));
            }
        }
        // if we did not throw above we will unsubscribe here, if onError failed then unsubscribe happens in the catch
        try {
            unsubscribe();
        } catch (RuntimeException unsubscribeException) {
            RxJavaPluginUtils.handleException(unsubscribeException);
            throw new OnErrorFailedException(unsubscribeException);
        }
    }
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本文参考了http://www.jianshu.com/p/c26f96c0ab81

困扰我了好久的问题终于解决了，呜呼！

可以结合下面文章来看：
http://blog.csdn.net/jdsjlzx/article/details/51534504

再分享一下我老师大神的人工智能教程吧。零基础！通俗易懂！风趣幽默！还带黄段子！希望你也加入到我们人工智能的队伍中来！http://www.captainbed.net

原文地址：https://www.cnblogs.com/skiwnchhw/p/10471638.html

时间： 2024-10-09 11:02:38

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