5.RxJava详解之操作符执行原理(五).md

RxJava详解之操作符执行原理(五)

上一篇文章介绍了RxJava的执行原理。下面继续介绍一下操作符的执行原理，但是操作符太多了，这里用map来进行说明。

Observable<String> observable = Observable.create(new Observable.OnSubscribe<String>() {
    @Override
    public void call(Subscriber<? super String> subscriber) {
        subscriber.onNext("Hello ");
        subscriber.onNext("World !");
        subscriber.onCompleted();
    }
});

observable.map(new Func1<String, String>() {
    @Override
    public String call(String s) {
        return "say" + s;
    }
});

observable.subscribe(new Subscriber<String>() {
    @Override
    public void onCompleted() {

    }

    @Override
    public void onError(Throwable e) {

    }

    @Override
    public void onNext(String s) {
        Log.i("@@@", s);
    }
});

执行结果很显然是say Hello和say World !。

我们直接进入Observable.map()方法的源码:

public final <R> Observable<R> map(Func1<? super T, ? extends R> func) {
    return create(new OnSubscribeMap<T, R>(this, func));
}

public static <T> Observable<T> create(OnSubscribe<T> f) {
    return new Observable<T>(RxJavaHooks.onCreate(f));
}   

map的内部调用了create()方法，而create()方法的源码我们再上一个版本已经介绍了，也就是说map内部会创建一个新的Observable对象，而且用一个新的OnSubscribeMap对象作为参数。 OnSubscribeMap()对象的参数分别是之前通过create()方法创建的Observable对象，以及map中传递过来的Func1类的对象。接下来就直接看OnSubscribeMap类的源码，他实现了OnSubscribe接口，并重写了call()方法:

/**
 * Applies a function of your choosing to every item emitted by an {@code Observable}, and emits the results of
 * this transformation as a new {@code Observable}.
 */
public final class OnSubscribeMap<T, R> implements OnSubscribe<R> {
    // 最初Observable.create()创建的Observable对象
    final Observable<T> source;
    // map方法传递过来的func1对象，它是一个转换功能
    final Func1<? super T, ? extends R> transformer;

    public OnSubscribeMap(Observable<T> source, Func1<? super T, ? extends R> transformer) {
        this.source = source;
        this.transformer = transformer;
    }

    @Override
    public void call(final Subscriber<? super R> o) {
        MapSubscriber<T, R> parent = new MapSubscriber<T, R>(o, transformer);
        // 把新创建的MapSubscriber添加到Observable.subscribe(subscribe)方法传递的参数subscriber中
        o.add(parent);
        // unsafeSubscribe是subscribe方法的一个安全性不高的操作，可以简单理解为subscribe方法
        source.unsafeSubscribe(parent);
    }

    static final class MapSubscriber<T, R> extends Subscriber<T> {

        final Subscriber<? super R> actual;

        final Func1<? super T, ? extends R> mapper;

        boolean done;

        public MapSubscriber(Subscriber<? super R> actual, Func1<? super T, ? extends R> mapper) {
            this.actual = actual;
            this.mapper = mapper;
        }

        @Override
        public void onNext(T t) {
            R result;

            try {
                // 先会执行以下转换函数的call方法，然后将结果再传递给Subscribe对象调用它的onNext方法
                result = mapper.call(t);
            } catch (Throwable ex) {
                Exceptions.throwIfFatal(ex);
                unsubscribe();
                onError(OnErrorThrowable.addValueAsLastCause(ex, t));
                return;
            }

            actual.onNext(result);
        }

        @Override
        public void onError(Throwable e) {
            if (done) {
                RxJavaHooks.onError(e);
                return;
            }
            done = true;

            actual.onError(e);
        }


        @Override
        public void onCompleted() {
            if (done) {
                return;
            }
            actual.onCompleted();
        }

        @Override
        public void setProducer(Producer p) {
            actual.setProducer(p);
        }
    }
}

因为在执行observable.subscribe(subscriber)方法会调用到call()方法，这里看一下call()方法的核心:

MapSubscriber<T, R> parent = new MapSubscriber<T, R>(o, transformer);
// 把新创建的MapSubscriber添加到Observable.subscribe(subscribe)方法传递的参数subscriber中
o.add(parent);
// unsafeSubscribe是subscribe方法的一个安全性不高的操作，可以简单理解为subscribe方法，注意这里传递的是parent，也就是先创建的MapSubscriber对象，而这里的source是谁呢？ 它是最初Observable.create创建的Observable对象
source.unsafeSubscribe(parent);

add()及unsasfeSubscribe()方法如下:

private final SubscriptionList subscriptions;

public final void add(Subscription s) {
    subscriptions.add(s);
}

public final Subscription unsafeSubscribe(Subscriber<? super T> subscriber) {
    try {
        // new Subscriber so onStart it
        subscriber.onStart();
        // allow the hook to intercept and/or decorate
        RxJavaHooks.onObservableStart(this, onSubscribe).call(subscriber);
        return RxJavaHooks.onObservableReturn(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(RxJavaHooks.onObservableError(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.
            RxJavaHooks.onObservableError(r);
            // TODO why aren't we throwing the hook's return value.
            throw r; // NOPMD
        }
        return Subscriptions.unsubscribed();
    }
}

这一块代码就很简单了，因为和前面一篇我们分析的subscribe()方法类似，相当于直接调用了最初Observable.create()创建的Observable对象的call(subscriber)方法，而这里的subscriber又是我们创建的MapSubscriber的子类，所以这里相当于调用了MapSubscriber类中的onNext()、onComplete()和onError()方法:

static final class MapSubscriber<T, R> extends Subscriber<T> {

    final Subscriber<? super R> actual;

    final Func1<? super T, ? extends R> mapper;

    boolean done;

    public MapSubscriber(Subscriber<? super R> actual, Func1<? super T, ? extends R> mapper) {
        this.actual = actual;
        this.mapper = mapper;
    }

    @Override
    public void onNext(T t) {
        R result;

        try {
            // 先会执行以下转换函数的call方法，这个就是我们把Hello修改为say Hello的部分
            result = mapper.call(t);
        } catch (Throwable ex) {
            Exceptions.throwIfFatal(ex);
            unsubscribe();
            onError(OnErrorThrowable.addValueAsLastCause(ex, t));
            return;
        }
        // 将转换函数的call方法的执行结果交给最初的Subscriber.onNext()方法的参数来执行
        actual.onNext(result);
    }
    ...
}

乱不乱？

梳理一下:

• 我们把不使用map操作符时正常的操作创建的Observable和Subscriber分别称为1号。
• 而map又会通过create分辨创建一个Observable2号和Subscriber2号，当我们执行map的时候，会最终执行到Subscriber2号的onNext()方法中，而该方法内部会先执行一些转换操作，然后将执行完的结果作为参数传递给并调用最初的Subscriber1号的onNext()方法。懂不？ 多看两遍，这里有点绕。

更多内容请看下一篇文章RxJava详解(六)

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• 邮箱 ：charon.chui@gmail.com
• Good Luck!