Rxjava2的原理:一步一步的看得懂Rxjava源碼

時間 2019-12-26

原文原文鏈接

1 Rxjava2最簡單使用方式拆解

Observable p=Observable.create(new ObservableOnSubscribe() {
            @Override
            public void subscribe(ObservableEmitter e) throws Exception {
                e.onNext("hello world");
                e.onComplete();
            }
        });

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

調用create方法以後實際上返回了一個ObservableCreate對象.繼承了Observable,是一個被觀察者對象.java

p.subscribe(new Observer() {
            @Override
            public void onSubscribe(Disposable d) {

            }

            @Override
            public void onNext(Object value) {

            }

            @Override
            public void onError(Throwable e) {

            }

            @Override
            public void onComplete() {

            }
        });

咱們看下subscribe方法.app

public final void subscribe(Observer<? super T> observer) {
            ...
            subscribeActual(observer);
            ...
    }

其餘代碼都刪掉了,剩下最核心的 subscribeActual(observer),這個observer就是咱們建立的匿名內部類對象.subscribeActual()方法是個抽象方法,咱們看下ObservableCreate中是怎麼實現的.ide

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);

        try {
            source.subscribe(parent);
        } catch (Throwable ex) {
            Exceptions.throwIfFatal(ex);
            parent.onError(ex);
        }
    }

CreateEmitter發射器,在這裏咱們調用了 observer.onSubscribe(parent)也就是咱們建立的匿名observer類的onSubscribe方法.oop

source.subscribe(parent)最重要的方法可能沒有之一,觀察者和被觀察者順利會師,事件開始執行,ui

@Override
            public void subscribe(ObservableEmitter e) throws Exception {//這裏的ObservableEmitter就是parent,也就是CreateEmitter發射器對象
                e.onNext("hello world");
                e.onComplete();
            }

接下來看看CreateEmitter的onNext和onComplete方法.this

@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);
            }
        }

咱們看到在發射器的onNext方法中,啥也沒作,就是當了個二傳手,調用了咱們觀察者的onNext方法.code

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

onComplete方法中也就是調用了觀察者的onComplete方法.
咱們來縷縷這個過程
1 create方法傳返回了一個對象是ObservableCreate,ObservableCreate的構造方法中有一個ObservableOnSubscribe對象,也就是咱們使用create時候建立的匿名內部類對象.
2 p.subscribe(o)實際上調用了ObservableCreate的subscribeActual方法
3 subscribeActual中首先調用了 observer的onSubscribe方法,緊接着調用了source.subscribe(parent)也就是ObservableOnSubscribe的subscribe方法,事件開始執行
4 subscribe方法中調用CreateEmitter的onNext方法,這個方法調用了observer的onNext方法,觀察者對事件進行反應.
5 subscribe方法中調用CreateEmitter的onComplete方法,這個方法調用了observer的onComplete方法,整個流程結束.server

2 MAP操做符是怎麼工做的

public final <R> Observable<R> map(Function<? super T, ? extends R> mapper) {
        ObjectHelper.requireNonNull(mapper, "mapper is null");
        return RxJavaPlugins.onAssembly(new ObservableMap<T, R>(this, mapper));
    }

map操做符把咱們的observable對象變化成了具體的ObservableMap,參數是咱們以前建立好的observable和mapper function對象

public ObservableMap(ObservableSource<T> source, Function<? super T, ? extends U> function) {
        super(source);
        this.function = function;
    }

    @Override
    public void subscribeActual(Observer<? super U> t) {
        source.subscribe(new MapObserver<T, U>(t, function));
    }

注意注意:這裏造成了一個新的訂閱關係
這裏的source是咱們create建立的observable,要否則會懵,建立ObservableMap時候咱們傳進來的this是咱們生成的observable.
到這裏咱們會從新調用onSubscribe() subscribeActual(),這裏就回到了咱們最簡單模式時候的調用步驟.不一樣的是咱們真正的調用observer的方法實在MapObserver對應的方法中.
具體流程是發射器調用onNext方法-->MapObserver的onNext方法-->再到咱們定義的observer的onNext方法繼承

@Override
        public void onNext(T t) {
            if (done) {
                return;
            }

            if (sourceMode != NONE) {
                actual.onNext(null);
                return;
            }

            U v;

            try {
            //調用mapper改變數據
                **v = ObjectHelper.requireNonNull(mapper.apply(t), "The mapper function returned a null value.");**
            } catch (Throwable ex) {
                fail(ex);
                return;
            }
            //actual咱們定義的observer
            actual.onNext(v);
        }

3 進階flatMap

@SchedulerSupport(SchedulerSupport.NONE)
    public final <R> Observable<R> flatMap(Function<? super T, ? extends ObservableSource<? extends R>> mapper,
            boolean delayErrors, int maxConcurrency, int bufferSize) {
     ...
        return RxJavaPlugins.onAssembly(new ObservableFlatMap<T, R>(this, mapper, delayErrors, maxConcurrency, bufferSize));

看看ObservableFlatMap代碼

public ObservableFlatMap(ObservableSource<T> source,
            Function<? super T, ? extends ObservableSource<? extends U>> mapper,
            boolean delayErrors, int maxConcurrency, int bufferSize) {
        super(source);
        this.mapper = mapper;
        this.delayErrors = delayErrors;
        this.maxConcurrency = maxConcurrency;
        this.bufferSize = bufferSize;
    }

    @Override
    public void subscribeActual(Observer<? super U> t) {

        if (ObservableScalarXMap.tryScalarXMapSubscribe(source, t, mapper)) {
            return;
        }

        source.subscribe(new MergeObserver<T, U>(t, mapper, delayErrors, maxConcurrency, bufferSize));
    }

是否是和MAP超級像,咱們這幾看MergeObserver onNext作了什麼

@Override
        public void onNext(T t) {
             ...
               p = ObjectHelper.requireNonNull(mapper.apply(t), "The mapper returned a null ObservableSource");

            ...

            subscribeInner(p);
        }
         @SuppressWarnings("unchecked")
        void subscribeInner(ObservableSource<? extends U> p) {
            for (;;) {
                if (p instanceof Callable) {
                  
                } else {
                    InnerObserver<T, U> inner = new InnerObserver<T, U>(this, uniqueId++);
                    addInner(inner);
                    p.subscribe(inner);
                    break;
                }
            }
        }

省略了不少代碼,咱們看主要邏輯,獲取到flatMap生成的observableSource,而後 p.subscribe(inner);注意這裏的P不是observable
看innerObserver的onNext作了什麼

//這裏的onNext事件由 p.subscribe(inner)觸發
  @Override
        public void onNext(U t) {
            if (fusionMode == QueueDisposable.NONE) {
                parent.tryEmit(t, this);
            } else {
                parent.drain();
            }
        }
        
        void tryEmit(U value, InnerObserver<T, U> inner) {
            if (get() == 0 && compareAndSet(0, 1)) {
                actual.onNext(value);
                if (decrementAndGet() == 0) {
                    return;
                }
            } else {
                SimpleQueue<U> q = inner.queue;
                if (q == null) {
                    q = new SpscLinkedArrayQueue<U>(bufferSize);
                    inner.queue = q;
                }
                q.offer(value);
                if (getAndIncrement() != 0) {
                    return;
                }
            }
            drainLoop();
        }

在這裏咱們終於看到咱們定義的observer接收到了onNext事件