[置顶] Rxjava分析—Subject

Subject在ReactiveX是作为observer和observerable的一个bridge或者proxy。因为它是一个观察者，所以它可以订阅一个或多个可观察对象，同时因为他是一个可观测对象，所以它可以传递和释放它观测到的数据对象，并且能释放新的对象。

1. Subject的类型

一共有四种为不同用途而设计的Subject，分别为AsyncSubject、BehaviorSubject、PublishSubject和ReplaySubject。

1. AsyncSubject

AsyncSubject仅释放Observable释放的最后一个数据，并且仅在Observable完成之后。然而如果当Observable因为异常而终止，AsyncSubject将不会释放任何数据，但是会向Observer传递一个异常通知。

2. BehaviorSubject

当Observer订阅了一个BehaviorSubject，它一开始就会释放Observable最近释放的一个数据对象，当还没有任何数据释放时，它则是一个默认值。接下来就会释放Observable释放的所有数据。如果Observable因异常终止，BehaviorSubject将不会向后续的Observer释放数据，但是会向Observer传递一个异常通知。

3. PublishSubject

PublishSubject仅会向Observer释放在订阅之后Observable释放的数据。

4. ReplaySubject

不管Observer何时订阅ReplaySubject，ReplaySubject会向所有Observer释放Observable释放过的数据。

有不同类型的ReplaySubject，它们是用来限定Replay的范围，例如设定Buffer的具体大小，或者设定具体的时间范围。

如果使用ReplaySubject作为Observer，注意不要在多个线程中调用onNext、onComplete和onError方法，因为这会导致顺序错乱，这个是违反了Observer规则的。

2. RxJava的Subject源码分析

1. Subject

Subject表示一个同时是Observable和Observer的对象。类Subject的代码如下：

package rx.subjects;

import rx.Observable;
import rx.Observer;
import rx.Subscriber;

/**
 * Represents an object that is both an Observable and an Observer.
 */
public abstract class Subject<T, R> extends Observable<R> implements Observer<T> {
    protected Subject(OnSubscribe<R> onSubscribe) {
        super(onSubscribe);
    }

    public abstract boolean hasObservers();
    
    public final SerializedSubject<T, R> toSerialized() {
        return new SerializedSubject<T, R>(this);
    }
}

2. BehaviorSubject

Subject有四个主要的子类，分别为AsyncSubject、BehaviorSubject、PublishSubject和ReplaySubject。接下来将以BehaviorSubject为例进行源码分析。

2.1 BehaviorSubject订阅subscribe过程

在需要使用subject时，调用Subject的subscribe(..)方法，该方法实际会调用下面这个subscribe(Subscriber<? super T> subscriber)方法，所以其他的subscribe方法都要将输入参数转化为一个Subscriber对象。

public final Subscription subscribe(Subscriber<? super T> subscriber) {
        ...  
        // new Subscriber so onStart it
        subscriber.onStart();
        
        ...

        // The code below is exactly the same an unsafeSubscribe but not used because it would add a sigificent depth to alreay huge call stacks.
        try {
            // allow the hook to intercept and/or decorate
            hook.onSubscribeStart(this, onSubscribe).call(subscriber);
            return hook.onSubscribeReturn(subscriber);
        } catch (Throwable e) {
            // special handling for certain Throwable/Error/Exception types
            Exceptions.throwIfFatal(e);
            // if an unhandled error occurs executing the onSubscribe we will propagate it
            try {
                subscriber.onError(hook.onSubscribeError(e));
            } catch (OnErrorNotImplementedException e2) {
                // special handling when onError is not implemented ... we just rethrow
                throw e2;
            } catch (Throwable 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 RuntimeException("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();
        }
    }

方法中hook.onSubsribeStart(this, onSubscribe).call(subscriber)默认情况下等价于onSubscribe.call(subscriber)。onSubscriber是什么呢？这个就需要了解BehaviorSubject的构造方法

protected BehaviorSubject(OnSubscribe<T> onSubscribe, SubjectSubscriptionManager<T> state) {
        super(onSubscribe);
        this.state = state;
    }

其中调用了父类Subject的构造方法

protected Subject(OnSubscribe<R> onSubscribe) {
        super(onSubscribe);
    }

其中调用了父类Observer的构造方法

protected Observable(OnSubscribe<T> f) {
        this.onSubscribe = f;
    }

obSubscribe即是BehaviorSubject构造方法中传入的第一个参数。

BehaviorSubject有3个静态工厂方法用来生产BehaviorSubject对象。

public final class BehaviorSubject<T> extends Subject<T, T> {

    public static <T> BehaviorSubject<T> create() {
        return create(null, false);
    }

    public static <T> BehaviorSubject<T> create(T defaultValue) {
        return create(defaultValue, true);
    }

    private static <T> BehaviorSubject<T> create(T defaultValue, boolean hasDefault) {
        final SubjectSubscriptionManager<T> state = new SubjectSubscriptionManager<T>();
        if (hasDefault) {
            state.set(NotificationLite.instance().next(defaultValue));
        }
        state.onAdded = new Action1<SubjectObserver<T>>() {

            @Override
            public void call(SubjectObserver<T> o) {
                o.emitFirst(state.get(), state.nl);
            }
            
        };
        state.onTerminated = state.onAdded;
        return new BehaviorSubject<T>(state, state); 
    }
    ....
}

前两个Public的静态构造方法实际上调用的是第三个private方法。

最后return new BehaviorSubject<T>(state, state)，所以onSubscribe实际为一个SubjectSubscriptionManager的对象，onSubscribe.call(subscriber)实际调用的是SubjectSubscriptionManager的call方法。

/* package */final class SubjectSubscriptionManager<T> implements OnSubscribe<T> {
    ...
    @Override
    public void call(final Subscriber<? super T> child) {
        SubjectObserver<T> bo = new SubjectObserver<T>(child);
        addUnsubscriber(child, bo);
        onStart.call(bo);
        if (!child.isUnsubscribed()) {
            if (add(bo) && child.isUnsubscribed()) {
                remove(bo);
            }
        }
    }
}

1. 调用addUnsubscriber方法，注册一个在取消订阅时执行的一个动作，即将该观擦者Observer移除掉。

/** Registers the unsubscribe action for the given subscriber. */
    void addUnsubscriber(Subscriber<? super T> child, final SubjectObserver<T> bo) {
        child.add(Subscriptions.create(new Action0() {
            @Override
            public void call() {
                remove(bo);
            }
        }));
    } 

2. 调用add(SubjectObserver<T> o)方法，将该Observer加入已经注册的Observer[]数组当中。

boolean add(SubjectObserver<T> o) {
        do {
            State oldState = state;
            if (oldState.terminated) {
                onTerminated.call(o);
                return false;
            }
            State newState = oldState.add(o);
            if (STATE_UPDATER.compareAndSet(this, oldState, newState)) {
                onAdded.call(o);
                return true;
            }
        } while (true);
    }

该方法会调用onAdd.call(o)。BehaviorSubject的onAdd对象如下，state.get()得到的是最近的数据对象，o.emitFirst即会释放最近的数据对象，这正体现了BehaviorSubject的特点。

state.onAdded = new Action1<SubjectObserver<T>>() {

            @Override
            public void call(SubjectObserver<T> o) {
                o.emitFirst(state.get(), state.nl);
            }
            
        };

在这个过程中使用了SubjectSubscriptionManager的两个内部类。

1. State<T>

该类用来管理已经注册的Observer数组，以及他们的状态。

/** State-machine representing the termination state and active SubjectObservers. */
    protected static final class State<T> {
        final boolean terminated;
        final SubjectObserver[] observers;
        static final SubjectObserver[] NO_OBSERVERS = new SubjectObserver[0];
        static final State TERMINATED = new State(true, NO_OBSERVERS);
        static final State EMPTY = new State(false, NO_OBSERVERS);
        
        public State(boolean terminated, SubjectObserver[] observers) {
            this.terminated = terminated;
            this.observers = observers;
        }
        public State add(SubjectObserver o) {
            ...
        }
        public State remove(SubjectObserver o) {
            ...
        }
    }

2. SubjectObserver<T>

该类时Observer的一个装饰类，运用了装饰模式给Observer类添加新的功能。

以上就是Subject对象订阅Observer时的流程。

2.2 BehaviorSubject的onNext

Behavior的onNext(T v)方法如下

@Override
    public void onNext(T v) {
        Object last = state.get();
        if (last == null || state.active) {
            Object n = nl.next(v);
            for (SubjectObserver<T> bo : state.next(n)) {
                bo.emitNext(n, state.nl);
            }
        }
    }

state是SubjectSubscriptionManager类的对象，是这个对象来维护最近释放的数据对象，state.get()获取最近释放的数据对象，state.next(Object n)方法重新设置最近释放的数据对象，并返回已经注册的Observer数组。

SubjectObserver<T>[] next(Object n) {
        set(n);
        return state.observers;
    }

bo.emitNext(Object n, final NotificationLite<T> nl)释放给定的数据对象。

2.3 BehaviorSubject的onCompleted和onError

onCompleted和onError会调用SubjectSubscriptionManager的terminate(Object n)方法，该方法会重新设置最近释放的数据对象，设置Subject状态为TERMINATED，表示终结了，最后返回已注册的Observer数组。

SubjectObserver<T>[] terminate(Object n) {
        set(n);
        active = false;

        State<T> oldState = state;
        if (oldState.terminated) {
            return State.NO_OBSERVERS;
        }
        return STATE_UPDATER.getAndSet(this, State.TERMINATED).observers;
    }