Observable创建和订阅流程

这里的创建和订阅不考虑操作符操作。

 Observable<String> observable = Observable
                .create(new ObservableOnSubscribe<String>() {
                    @Override
                    public void subscribe(@NonNull ObservableEmitter<String> emitter) throws Exception {
                        emitter.onNext("hello");
                    }
                });

查看Observable的create方法：

 public static <T> Observable<T> create(ObservableOnSubscribe<T> source) {
        ObjectHelper.requireNonNull(source, "source is null");
        return RxJavaPlugins.onAssembly(new ObservableCreate<T>(source));
    }

返回 ObservableCreate对象，ObservableCreate的构造函数的参数为create方法传递的参数，参数类型为ObservableOnSubscribe。ObservableCreate保存该参数为source。
ObservableCreate的构造方法如下：

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

这样observable对象实际上就是一个ObservableCreate对象。

subscribe方法调用如下：

 observable.subscribe(new Observer<String>() {
            @Override
            public void onSubscribe(Disposable d) {
                comDisposable.add(d);
            }

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

            @Override
            public void onError(Throwable e) {

            }

            @Override
            public void onComplete() {

            }
        });

subscribe方法的参数是一个Observer。执行了Observable的subscribe方法，subscribe方法代码如下：

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

            ObjectHelper.requireNonNull(observer, "The RxJavaPlugins.onSubscribe hook returned a null Observer. Please change the handler provided to RxJavaPlugins.setOnObservableSubscribe for invalid null returns. Further reading: https://github.com/ReactiveX/RxJava/wiki/Plugins");
            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;
        }
    }

真正起作用的是subscribeActual方法，由于Observable实际上是ObservableCreate对象，因此subscribeActual也就是调用ObservableCreate的subscribeActual方法。subscribeActual方法如下：

    @Override
    protected void subscribeActual(Observer<? super T> observer) {
        CreateEmitter<T> parent = new CreateEmitter<T>(observer);
        observer.onSubscribe(parent);
        try {
            source.subscribe(parent);
        } catch (Throwable ex) {
            Exceptions.throwIfFatal(ex);
            parent.onError(ex);
        }
    }

该方法中首先以observer为参数生成CreateEmitter对象，再调用obsever的onSubscribe方法，然后调用source的subscribe方法，而该source为Observable的create方法的参数。因此 source.subscribe(parent);实际上执行的是创建时参数的subscribe代码：

  public void subscribe(@NonNull ObservableEmitter<String> emitter) throws Exception {
                        emitter.onNext("hello");
           }

CreateEmitter的onNext方法定义如下：

 @Override
        public void onNext(T t) {
            if (t == null) {
                onError(new NullPointerException("onNext called with null. Null values are generally not allowed in 2.x operators and sources."));
                return;
            }
            if (!isDisposed()) {
                observer.onNext(t);
            }
        }

实际上调用了observer的onNext方法，这里的observer就是Observable的subscribe方法传入的参数。

subscribeOn,observeOn方法影响

subscribeOn() : 影响的是最开始的被观察者(第一个被观察者)所在的线程。当使用多个subscribeOn() 的时候，只有第一个 subscribeOn() 起作用；

observeOn() : 影响的是跟在后面的操作（指定观察者运行的线程）。所以如果想要多次改变线程，可以多次使用 observeOn；

参考:
https://www.jianshu.com/p/d9da64774f7b