RxJava->doOnNext()

example:

Observable
    .create(new ObservableOnSubscribe<Integer>() {
        @Override
        public void subscribe(ObservableEmitter<Integer> emitter) throws Exception {
            LogUtils.log(Test01.class, "onNext()->1");
            emitter.onNext(1);
            LogUtils.log(Test01.class, "subscribe()->2");
            emitter.onNext(2);
            LogUtils.log(Test01.class, "subscribe()->3");
            emitter.onNext(3);
            LogUtils.log(Test01.class, "subscribe()->onComplete()");
                 emitter.onComplete();
            }
        })
        .doOnNext(new Consumer<Integer>() {
            @Override
            public void accept(Integer integer) throws Exception {
                LogUtils.log(Test01.class, "accept()->integer:" + integer);
            }
        })
        .subscribe(new Observer<Integer>() {
            @Override
            public void onSubscribe(Disposable disposable) {
                sDisposable = disposable;
                LogUtils.log(Test01.class, "onSubscribe()");
            }

            @Override
            public void onNext(Integer value) {
                LogUtils.log(Test01.class, "onNext()->value:" + value);
            }

            @Override
            public void onError(Throwable e) {
                LogUtils.log(Test01.class, "onError()");
            }

           @Override
           public void onComplete() {
               LogUtils.log(Test01.class, "onComplete()");
           }
      });

doOnNext():

.doOnNext(new Consumer<Integer>() {
    @Override
    public void accept(Integer integer) throws Exception {
        LogUtils.log(Test01.class, "accept()->integer:" + integer);
    }
})

public interface Consumer<T> {
    void accept(T t) throws Exception;
}

public abstract class Observable<T> implements ObservableSource<T> {
    @SchedulerSupport(SchedulerSupport.NONE)
    public final Observable<T> doOnNext(Consumer<? super T> onNext) {
        return doOnEach(onNext, Functions.emptyConsumer(), Functions.EMPTY_ACTION, Functions.EMPTY_ACTION);
    }

    @SchedulerSupport(SchedulerSupport.NONE)
    private Observable<T> doOnEach(Consumer<? super T> onNext, Consumer<? super Throwable> onError, Action onComplete, Action onAfterTerminate) {
        return RxJavaPlugins.onAssembly(new ObservableDoOnEach<T>(this, onNext, onError, onComplete, onAfterTerminate));
    }
}

public final class RxJavaPlugins {
    return source;
}

class ObservableDoOnEach<T> extends AbstractObservableWithUpstream<T, T>;
abstract class AbstractObservableWithUpstream<T, U> extends Observable<U>;

• 1、doOnNext()返回了ObservableDoOnNext对象, 后边subcribe应当切换到ObservableDoOnNext中去.

new ObservableDoOnEach<T>(this, onNext, onError, onComplete, onAfterTerminate)

public final class ObservableDoOnEach<T> extends AbstractObservableWithUpstream<T, T> {

    public ObservableDoOnEach(ObservableSource<T> source, Consumer<? super T> onNext,
                              Consumer<? super Throwable> onError,
                              Action onComplete,
                              Action onAfterTerminate) {
        super(source);
    }
}

abstract class AbstractObservableWithUpstream<T, U> extends Observable<U> implements HasUpstreamObservableSource<T> {
    protected final ObservableSource<T> source;
    AbstractObservableWithUpstream(ObservableSource<T> source) {
        this.source = source;
    }
}

什么时候能模仿着写出这种结构的代码, 什么时候就牛逼了

• 1、subscribe被ObservableDoOnEach调用, 但是ObservableDoOnEach内部又持有ObserverCreater的引用;

subscribe():

public abstract class Observable<T> implements ObservableSource<T> {
    @SchedulerSupport(SchedulerSupport.NONE)
    @Override
    public final void subscribe(Observer<? super T> observer) {
        observer = RxJavaPlugins.onSubscribe(this, observer);
        subscribeActual(observer);
    }
    protected abstract void subscribeActual(Observer<? super T> observer);
}

• subscribeActual实际被子类ObservableDoOnEach调用;

public final class ObservableDoOnEach<T> extends AbstractObservableWithUpstream<T, T> {
    @Override
    public void subscribeActual(Observer<? super T> t) {
        source.subscribe(new DoOnEachObserver<T>(t, onNext, onError, onComplete, onAfterTerminate));
    }
}

• 最终还是走到了ObservableCreater里面的subscribeActual(), 而Observer是被DoOnEachObserver所实现;

static final class DoOnEachObserver<T> implements Observer<T>, Disposable {
    Disposable s;
    @Override
    public void onSubscribe(Disposable s) {
        if (DisposableHelper.validate(this.s, s)) {
            this.s = s;
            actual.onSubscribe(this);
        }
    }
}

public final class ObservableCreate<T> extends Observable<T> {
    final ObservableOnSubscribe<T> source;

    public ObservableCreate(ObservableOnSubscribe<T> source) {
        this.source = source;
    }

    @Override
    protected void subscribeActual(Observer<? super T> observer) {
        CreateEmitter<T> parent = new CreateEmitter<T>(observer);
        observer.onSubscribe(parent);
        source.subscribe(parent);
    }
}

这段代码应该用到的是适配器模式

• 1、ObservableCreate持有的Observer实际为DoOnEachObserver引用;
• 2、又通过observer.onSubscribe()将CreateEmitter传给了DoOnEachObserver中的Disposable s, 即s实际上指向的是CreateEmitter;
• 3、source指向的是ObservableCreate, 所以source.subscribe()将内部Observer指向了DoOnEachObserver;
• 4、actual.onSubscribe(this)将Disposable指向了DoOnEachObserver;

---

static final class CreateEmitter<T>
    extends AtomicReference<Disposable>
    implements ObservableEmitter<T>, Disposable {

        CreateEmitter(Observer<? super T> observer) {...}

        @Override
        public void onNext(T t) {...}

        @Override
        public void onError(Throwable t) {...}

        @Override
        public void onComplete() {...}

        @Override
        public void setDisposable(Disposable d) {...}

        @Override
        public void setCancellable(Cancellable c) {...}

        @Override
        public ObservableEmitter<T> serialize() {...}

        @Override
        public void dispose() {...}

        @Override
        public boolean isDisposed() {...}
}

• 1、所以每次被观察者通过发射器emitter调用onError(), onNext(), onComplete()实际上最终都会先调用CreateEmitter的对应的方法, 然后再去调用DoOnEachObserver对应的方法;
• 2、Disposable实际上指向的是DoOnEachObserver, 所以调用dispose(), isDisposed()时实际走的是DoOnEachObserver内部的方法, 而DoOnEachObserver内部的Disposable s又指向了CreateEmitter, 所以最终决定观察者能否收到消息的决定权还是在CreateEmitter手中;

总结:

• 1、通过源码可以看到, onOnNext()的accept()方法仅仅只是在Observer的onXXX()方法被调用之前调用, 方且没有与Observer的调用之间没有任何关系;
• 2、所以doOnNext()这个方法可以用来在观察者Observer:onXXX()方法被调用之前进行一些初始化操作;

试试连续调用多个doOnNext()方法:

Observable
        .create(new ObservableOnSubscribe<Integer>() {
            @Override
            public void subscribe(ObservableEmitter<Integer> emitter) throws Exception {
                LogUtils.log(Test01.class, "subscribe->onNext()->1");
                emitter.onNext(1);
                LogUtils.log(Test01.class, "subscribe()->onNext()->2");
                emitter.onNext(2);
                LogUtils.log(Test01.class, "subscribe()->onNext()->3");
                emitter.onNext(3);
                LogUtils.log(Test01.class, "subscribe()->onComplete()");
                emitter.onComplete();
            }
        })
        .doOnNext(new Consumer<Integer>() {
            @Override
            public void accept(Integer integer) throws Exception {
                LogUtils.log(Test01.class, "accept()_1->integer:" + integer);
            }
        })
        .doOnNext(new Consumer<Integer>() {
            @Override
            public void accept(Integer integer) throws Exception {
                LogUtils.log(Test01.class, "accept()_2->integer:" + integer);
            }
        })
        .subscribe(new Observer<Integer>() {
            @Override
            public void onSubscribe(Disposable disposable) {
                sDisposable = disposable;
                LogUtils.log(Test01.class, "onSubscribe()");
            }

            @Override
            public void onNext(Integer value) {
                LogUtils.log(Test01.class, "onNext()->value:" + value);
            }

            @Override
            public void onError(Throwable e) {
                LogUtils.log(Test01.class, "onError()");
            }

            @Override
            public void onComplete() {
                LogUtils.log(Test01.class, "onComplete()");
            }
        });

打印结果如下所示:

09-14 16:50:52.980 21493-21493/hb.com V/AndroidTest: Test01->onSubscribe()
09-14 16:50:52.980 21493-21493/hb.com V/AndroidTest: Test01->subscribe->onNext()->1
09-14 16:50:52.980 21493-21493/hb.com V/AndroidTest: Test01->accept()_1->integer:1
09-14 16:50:52.980 21493-21493/hb.com V/AndroidTest: Test01->accept()_2->integer:1
09-14 16:50:52.980 21493-21493/hb.com V/AndroidTest: Test01->onNext()->value:1
09-14 16:50:52.980 21493-21493/hb.com V/AndroidTest: Test01->subscribe()->onNext()->2
09-14 16:50:52.980 21493-21493/hb.com V/AndroidTest: Test01->accept()_1->integer:2
09-14 16:50:52.980 21493-21493/hb.com V/AndroidTest: Test01->accept()_2->integer:2
09-14 16:50:52.980 21493-21493/hb.com V/AndroidTest: Test01->onNext()->value:2
09-14 16:50:52.980 21493-21493/hb.com V/AndroidTest: Test01->subscribe()->onNext()->3
09-14 16:50:52.980 21493-21493/hb.com V/AndroidTest: Test01->accept()_1->integer:3
09-14 16:50:52.980 21493-21493/hb.com V/AndroidTest: Test01->accept()_2->integer:3
09-14 16:50:52.980 21493-21493/hb.com V/AndroidTest: Test01->onNext()->value:3
09-14 16:50:52.980 21493-21493/hb.com V/AndroidTest: Test01->subscribe()->onComplete()
09-14 16:50:52.980 21493-21493/hb.com V/AndroidTest: Test01->onComplete()

结合源码看看为何打印会是这种打印结果;
下面的分析可能会很绕, 也可能会让人感觉废话连篇; 这也体现了RxJava架构的复杂性;

public abstract class Observable<T> implements ObservableSource<T> {
    public static <T> Observable<T> create(ObservableOnSubscribe<T> source) {
        return RxJavaPlugins.onAssembly(new ObservableCreate<T>(source));
    }
}
public final class ObservableCreate<T> extends Observable<T> {
    final ObservableOnSubscribe<T> source;
    public ObservableCreate(ObservableOnSubscribe<T> source) {
        this.source = source;
    }
}

• 1、Observable指向ObservableCreate, ObservableCreate内部持有ObservableOnSubscribe的引用;

public abstract class Observable<T> implements ObservableSource<T> {
    public final Observable<T> doOnNext(Consumer<? super T> onNext) {
        return doOnEach(onNext, Functions.emptyConsumer(), Functions.EMPTY_ACTION, Functions.EMPTY_ACTION);
    }
    private Observable<T> doOnEach(Consumer<? super T> onNext, Consumer<? super Throwable> onError, Action onComplete, Action onAfterTerminate) {
        return RxJavaPlugins.onAssembly(new ObservableDoOnEach<T>(this, onNext, onError, onComplete, onAfterTerminate));
    }
}
public final class RxJavaPlugins {
    public static <T> Observable<T> onAssembly(Observable<T> source) {
        source;
    }
}
public final class ObservableDoOnEach<T> extends AbstractObservableWithUpstream<T, T> {
    final Consumer<? super T> onNext;
    public ObservableDoOnEach(ObservableSource<T> source, Consumer<? super T> onNext,
                              Consumer<? super Throwable> onError,
                              Action onComplete,
                              Action onAfterTerminate) {
        super(source);
        this.onNext = onNext;
    }
}
abstract class AbstractObservableWithUpstream<T, U> extends Observable<U> implements HasUpstreamObservableSource<T> {
    protected final ObservableSource<T> source;
    AbstractObservableWithUpstream(ObservableSource<T> source) {
        this.source = source;
    }
}

• 1、第一次调用doOnNext()以后, Observable指向了ObservableDoOnEach_1, 并将当前Observable的引用传给ObservableDoOnEach_1, 即ObservableDoOnEach_1持有ObservableCreate的引用, ObservableDoOnEach_1持有Consumer_1;
• 2、同理, 第二次调用doOnNext()以后, Observable指向了ObservableDoOnEach_2, 并将当前的Observable的引用传给了ObservableDoOnEach_2, 即 ObservableDoOnEach_2持有ObservableDoOnEach_1的引用. ObservableDoOnEach_2持有Consumer_2;

接下来看subscirbe(...)何如实现doOnNext()的连续调用:

public abstract class Observable<T> implements ObservableSource<T> {
    public final void subscribe(Observer<? super T> observer) {
        subscribeActual(observer);
    }
    protected abstract void subscribeActual(Observer<? super T> observer);
}

• 重点就是在subscribeActual()这个方法, 后边单线程操作符的话, 就只看这个方法了;

public final class ObservableDoOnEach<T> extends AbstractObservableWithUpstream<T, T> {
    public void subscribeActual(Observer<? super T> t) {
        source.subscribe(new DoOnEachObserver<T>(t, onNext, onError, onComplete, onAfterTerminate));
    }
}

• 1、subscribeActual实际被ObservableDoOnEach_2调用, 而此时的source为ObservableDoOnEach_1, onNext为Consumer_2;

• 2、通过subscribe(...)将DoOnEachObserver_02的引用付给ObservableDoOnEach_1, 然后递推, 将DoOnEachObserver_1的引用付给ObservableCreate;

• 3、DoOnEachObserver_2持有的Observer actual实际为我们外部通过new Observer创建的引用;

• 4、ObservableDoOnEach_1调用subscribeActual(...)时传的参数Observer实际就是ObservableDoOnEach__2调用subscribeActual(...)时所创建的DoOnEachObserver_2, 所以DoOnEachObserver_1内部Observer actual实际指向的是DoOnEachObserver_2;

• 然后切到ObservableCreate中去:

public final class ObservableCreate<T> extends Observable<T> {
    @Override
    protected void subscribeActual(Observer<? super T> observer) {
        CreateEmitter<T> parent = new CreateEmitter<T>(observer);
        observer.onSubscribe(parent);
        source.subscribe(parent);
    }
}

• 1、CreateEmitter持有的Observer为DoOnEachObserver_1, DoOnEachObserver_1持有的Disposable为CreateEmitter, ObservableOnSubscribe持有CreateEmitter的引用;

static final class CreateEmitter<T> {
    @Override
    public void onNext(T t) {
        observer.onNext(t);
    }
}
static final class DoOnEachObserver<T> {
    @Override
    public void onNext(T t) {
        onNext.accept(t);
        actual.onNext(t);
    }
}

• 当CreateEmitter调用一次onNext()时, DoOnEachObserver_1调用了自己的onNext()方法;
• 而此时onNext.accept()实际为Consumer_1.accept(t);
• actual.onNext()因为此时的acutal实际持有的是DoOnEachObserver_2的引用, 所以继续调用DoOnEachObserver_2.onNext(), DoOnEachObserver_2中的onNext()实际指向Consumer_2, 而actual实际指向我们通过new Observer创建的Observer对象;

通过几张图来对文字进行归纳总结: