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RxJava源码阅读(一)

Observable.just("测试")
                .subscribe(new Consumer<String>() {
                    @Override
                    public void accept(String str) throws Exception {
                        System.out.println(str);
                    }
                });
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Observable.just()
subscribe
accept

2.创建被观察者

public static <T> Observable<T> just(T item) {
        ObjectHelper.requireNonNull(item, "The item is null");
        return RxJavaPlugins.onAssembly(new ObservableJust<T>(item));
}
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public static <T> Observable<T> onAssembly(@NonNull Observable<T> source) {
        Function<? super Observable, ? extends Observable> f = onObservableAssembly;
        if (f != null) {
            return apply(f, source);
        }
        return source;
}
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这里被观察者其实就是 new ObservableJust<T>(item)

3.创建观察者

这里的观察者就是一个接口,只有一个 accept 方法。被观察者发起订阅后,观察者会回调 accept 方法,将被观察者发送的数据拿到

4.发起订阅

被观察者通过 subscribe 方法订阅观察者

public final Disposable subscribe(Consumer<? super T> onNext) {
        return subscribe(onNext, Functions.ON_ERROR_MISSING, Functions.EMPTY_ACTION, Functions.emptyConsumer());
}
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public final Disposable subscribe(Consumer<? super T> onNext, Consumer<? super Throwable> onError,
            Action onComplete, Consumer<? super Disposable> onSubscribe) {
        ObjectHelper.requireNonNull(onNext, "onNext is null");
        ObjectHelper.requireNonNull(onError, "onError is null");
        ObjectHelper.requireNonNull(onComplete, "onComplete is null");
        ObjectHelper.requireNonNull(onSubscribe, "onSubscribe is null");

        LambdaObserver<T> ls = new LambdaObserver<T>(onNext, onError, onComplete, onSubscribe);

        subscribe(ls);

        return ls;
}
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public final void subscribe(Observer<? super T> observer) {
        ObjectHelper.requireNonNull(observer, "observer is null");
        try {
            observer = RxJavaPlugins.onSubscribe(this, observer);

            ObjectHelper.requireNonNull(observer, "Plugin returned null Observer");

            subscribeActual(observer);
        } catch (NullPointerException e) { // NOPMD
            throw e;
        } catch (Throwable e) {
            Exceptions.throwIfFatal(e);
            // can't call onError because no way to know if a Disposable has been set or not
            // can't call onSubscribe because the call might have set a Subscription already
            RxJavaPlugins.onError(e);

            NullPointerException npe = new NullPointerException("Actually not, but can't throw other exceptions due to RS");
            npe.initCause(e);
            throw npe;
        }
}
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最终调用抽象方法

protected abstract void subscribeActual(Observer<? super T> observer);
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也就是 Observable 子类 ObservableJust 调用了这个方法,参数是 LambdaObserver<T> ls = new LambdaObserver<T>(onNext, onError, onComplete, onSubscribe); 的 ls

protected void subscribeActual(Observer<? super T> s) {
        ScalarDisposable<T> sd = new ScalarDisposable<T>(s, value);
        s.onSubscribe(sd);
        sd.run();
}
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订阅方法最终执行了 LambdaObserver 的 onSubscribe 方法和 ScalarDisposable 的 run 方法

4.1 `LambdaObserver` 的 `onSubscribe` 方法

public void onSubscribe(Disposable s) {
        if (DisposableHelper.setOnce(this, s)) {
            try {
                onSubscribe.accept(this);
            } catch (Throwable ex) {
                Exceptions.throwIfFatal(ex);
                s.dispose();
                onError(ex);
            }
        }
}
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这里解释了为什么发起订阅后就会回调观察者的 accept 方法

4.2 `ScalarDisposable` 的 `run` 方法

public void run() {
        if (get() == START && compareAndSet(START, ON_NEXT)) {
            observer.onNext(value);
            if (get() == ON_NEXT) {
                lazySet(ON_COMPLETE);
                observer.onComplete();
            }
        }
}
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直接结束此次订阅

原文 https://juejin.im/post/5cde514551882525ef3f99bb

正文到此结束