Rxjava2源码解析

1：用法：

 Observable<Integer> observable = Observable.create(new ObservableOnSubscribe<Integer>() {
            @Override
            public void subscribe(ObservableEmitter<Integer> emitter) throws Exception {
                //Log.d(TAG, "ObservableEmitter");
                //Log.d(TAG, "Observable thread is" + Thread.currentThread().getName());
                emitter.onNext(12);
                emitter.onNext(13);
                emitter.onNext(14);
                emitter.onNext(15);
                emitter.onComplete();
            }
        });
        Observer<Integer> observer = new Observer<Integer>() {
            private int i;
            private Disposable mDisposable;

            @Override
            public void onSubscribe(Disposable d) {
                Log.d(TAG, "onSubscribe");
                mDisposable = d;
            }

            @Override
            public void onNext(Integer integer) {
                /*i++;
                if(i == 3){
                    mDisposable.dispose();
                }*/
                Log.d(TAG, "onNext" + integer);
            }

            @Override
            public void onError(Throwable e) {
                Log.d(TAG, "onError");
            }

            @Override
            public void onComplete() {
                Log.d(TAG, "onComplete");
            }
        };

        //observable.subscribe(observer);
        observable.doOnSubscribe(disposable -> {
                    Log.d(TAG, "doOnSubscribe");
                }
        ).doOnComplete(() -> {
            Log.d(TAG, "doOnComplete");
        }).doOnNext((C) -> {
            Log.d(TAG, "doNext" + C);
        }).subscribe(observer);

2：Observable

首先看Observable:

Observable<Integer> observable = Observable.create(new ObservableOnSubscribe<Integer>() {
            @Override
            public void subscribe(ObservableEmitter<Integer> emitter) throws Exception {
                //Log.d(TAG, "ObservableEmitter");
                //Log.d(TAG, "Observable thread is" + Thread.currentThread().getName());
                emitter.onNext(12);
                emitter.onNext(13);
                emitter.onNext(14);
                emitter.onNext(15);
                emitter.onComplete();
            }
        });

看一下create：

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

参数ObservableOnSubscribe是一个接口，里面只有一个函数subscribe：

public interface ObservableOnSubscribe<T> {

    /**
     * Called for each Observer that subscribes.
     * @param e the safe emitter instance, never null
     * @throws Exception on error
     */
    void subscribe(ObservableEmitter<T> e) throws Exception;
}

create需要返回的是一个Observable：

return RxJavaPlugins.onAssembly(new ObservableCreate<T>(source));

通过在这里可知反回的是一个Observable的继承类ObservableCreate：

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

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

总结以上的几个类，可以归纳一下：

通过Observable.create返回一个Observable，具体是返回ObservableCreate，该类继承Observable，同时该类持有ObservableOnSubscribe，而ObservableOnSubscribe是一个接口，具体的实现是在：

 Observable<Integer> observable = Observable.create(new ObservableOnSubscribe<Integer>() {
            @Override
            public void subscribe(ObservableEmitter<Integer> emitter) throws Exception {
                //Log.d(TAG, "ObservableEmitter");
                //Log.d(TAG, "Observable thread is" + Thread.currentThread().getName());
                emitter.onNext(12);
                emitter.onNext(13);
                emitter.onNext(14);
                emitter.onNext(15);
                emitter.onComplete();
            }
        });

Observable的解析暂时先到这里，我们先看如何和observer关联起来：

//observable.subscribe(observer);
        observable.doOnSubscribe(disposable -> {
                    Log.d(TAG, "doOnSubscribe");
                }
        ).doOnComplete(() -> {
            Log.d(TAG, "doOnComplete");
        }).doOnNext((C) -> {
            Log.d(TAG, "doNext" + C);
        }).subscribe(observer);

进入subscribe：

 @SchedulerSupport(SchedulerSupport.NONE)
    @Override
    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;
        }
    }

可以看到这是Observable里面的一个方法，前面我们说到，create生成的是ObservableCreate，而该类继承Observable，所以我们现在就是在ObservableCreate的subscribe方法里面，看参数，传进来的是一个observer，observer也是一个interface，具体实现就是应用层的：

Observer<Integer> observer = new Observer<Integer>() {
            private int i;
            private Disposable mDisposable;

            @Override
            public void onSubscribe(Disposable d) {
                Log.d(TAG, "onSubscribe");
                mDisposable = d;
            }

            @Override
            public void onNext(Integer integer) {
                /*i++;
                if(i == 3){
                    mDisposable.dispose();
                }*/
                Log.d(TAG, "onNext" + integer);
            }

            @Override
            public void onError(Throwable e) {
                Log.d(TAG, "onError");
            }

            @Override
            public void onComplete() {
                Log.d(TAG, "onComplete");
            }
        };

到这里再来总结一下，通过subscribe，就是ObservableCreate这个类（该类继承Observable，同时该类持有ObservableOnSubscribe，而ObservableOnSubscribe是一个接口）执行subscribe，传参为observer，是一个实现为用户层定义的接口。接下来就是具体看一下subscribe这个函数是如何走的：

@SchedulerSupport(SchedulerSupport.NONE)
    @Override
    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;
        }
    }

首先看：

subscribeActual(observer);

点进这个函数看到是一个Observable里面的一个abstract函数，那实现是在哪里呢，前面一直提到的，我们现在其实是处于ObservableCreate这个类，进入该类找到subscribeActual这个函数：

 @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);
        }
    }

先看这句：

source.subscribe(parent);

先看看source是怎么来的：

 final ObservableOnSubscribe<T> source;

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

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

Observable<Integer> observable = Observable.create(new ObservableOnSubscribe<Integer>() {
            @Override
            public void subscribe(ObservableEmitter<Integer> emitter) throws Exception {
                //Log.d(TAG, "ObservableEmitter");
                //Log.d(TAG, "Observable thread is" + Thread.currentThread().getName());
                emitter.onNext(12);
                emitter.onNext(13);
                emitter.onNext(14);
                emitter.onNext(15);
                emitter.onComplete();
            }
        });

一层层往上追溯可以看到sorce其实就是用户层实现的那个接口：

public interface ObservableOnSubscribe<T> {

    /**
     * Called for each Observer that subscribes.
     * @param e the safe emitter instance, never null
     * @throws Exception on error
     */
    void subscribe(ObservableEmitter<T> e) throws Exception;
}

会到：

 source.subscribe(parent);

这里的parent是CreateEmitter，先看subscribe：

 public void subscribe(ObservableEmitter<Integer> emitter) throws Exception {
                //Log.d(TAG, "ObservableEmitter");
                //Log.d(TAG, "Observable thread is" + Thread.currentThread().getName());
                emitter.onNext(12);
                emitter.onNext(13);
                emitter.onNext(14);
                emitter.onNext(15);
                emitter.onComplete();
            }

前面看到subscribe的参数是parent：

  CreateEmitter<T> parent = new CreateEmitter<T>(observer);

这里定义了一个CreateEmitter，持有observer：

 implements ObservableEmitter<T>, Disposable {

        private static final long serialVersionUID = -3434801548987643227L;

        final Observer<? super T> observer;

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

        @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);
            }
        }

        @Override
        public void onError(Throwable t) {
            if (t == null) {
                t = new NullPointerException("onError called with null. Null values are generally not allowed in 2.x operators and sources.");
            }
            if (!isDisposed()) {
                try {
                    observer.onError(t);
                } finally {
                    dispose();
                }
            } else {
                RxJavaPlugins.onError(t);
            }
        }

        @Override
        public void onComplete() {
            if (!isDisposed()) {
                try {
                    observer.onComplete();
                } finally {
                    dispose();
                }
            }
        }

我们来看一下onNext是如何执行的，当执行到：

emitter.onNext(12);

的时候，就到了：

 @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);
            }
        }

看这句：

observer.onNext(t);

我们前面讲到了，这里的observer，就是用户层自定义的：

 Observer<Integer> observer = new Observer<Integer>() {
            private int i;
            private Disposable mDisposable;

            @Override
            public void onSubscribe(Disposable d) {
                Log.d(TAG, "onSubscribe");
                mDisposable = d;
            }

            @Override
            public void onNext(Integer integer) {
                /*i++;
                if(i == 3){
                    mDisposable.dispose();
                }*/
                Log.d(TAG, "onNext" + integer);
            }

到这里，Observable里面的onNext和Observer的onNext就联系起来了，Observavble发送一个，Observer执行一个。onError和onComplete同理。