四. RxJava之基本原理

時間 2019-11-08

原文原文鏈接

不忘初心砥礪前行, Tomorrow Is Another Day !java

本文概要:bash

Observable的建立
subscribe訂閱過程
發送事件
線程切換過程

1. Observable的建立

對應源碼app

//Observable.java
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public static <T> Observable<T> create(ObservableOnSubscribe<T> source) {
        //參數檢查
        ObjectHelper.requireNonNull(source, "source is null");
        //裝配Observable,返回一個ObservableCreate對象
        return RxJavaPlugins.onAssembly(new ObservableCreate<T>(source));
    }

//鉤子方法
    @NonNull
    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;
    }
    
     @NonNull
    static <T, R> R apply(@NonNull Function<T, R> f, @NonNull T t) {
        try {
            return f.apply(t);
        } catch (Throwable ex) {
            throw ExceptionHelper.wrapOrThrow(ex);
        }
    }

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當咱們經過Create方法建立一個Observable對象時,框架

參數檢查,而後裝配Observable,返回一個ObservableCreate對象,它持有一個ObservableOnSubscribe對象,ObservableCreate是Observable的子類
- 裝配流程: 執行鉤子方法,對即將建立的Observale進行預處理,能夠經過onObservableAssembly進行設置.

2. subscribe訂閱過程

當咱們建立完Observable對象時,會調用subscribe去綁定觀察者Observer.因此直接看該方法.ide

對應源碼學習

//Observable.java
@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);//此方法是重中之重.
        } 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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一樣的方式,對咱們傳入的Observer觀察者對象進行檢查以及預處理,最終調用subscribeActual方法,該方法是一個抽象方法.因此咱們直接看它的實現類,也就是ObservableCreate對象下的subscribeActual方法.ui

對應源碼this

//ObservableCreate.java

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

@Override
    protected void subscribeActual(Observer<? super T> observer) {
        //1. 建立一個發射器對象
        CreateEmitter<T> parent = new CreateEmitter<T>(observer);
        //2. 回調onSubscribe,通知訂閱成功
        observer.onSubscribe(parent);

        try {
            //3. 回調subscribe,開始發送事件.
            //source對象就是建立被觀察者傳入的.
            source.subscribe(parent);
        } catch (Throwable ex) {
            Exceptions.throwIfFatal(ex);
            parent.onError(ex);
        }
    }
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上面註釋已經寫得很詳細了這裏再重複一次.整個訂閱過程就是.spa

建立一個發射器對象,綁定觀察者Observer
回調onSubscribe,通知訂閱成功
回調subscribe,開始發送事件

接着咱們來看發射器對象CreateEmitter是如何將事件發送給訂閱者的.線程

3. 發送事件

對應源碼

//ObservableCreate#CreateEmitter.java
static final class CreateEmitter<T>
    extends AtomicReference<Disposable>
    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()) {
                //回調onNext
                observer.onNext(t);
            }
        }

        @Override
        public void onError(Throwable t) {
            if (!tryOnError(t)) {
                RxJavaPlugins.onError(t);
            }
        }

        @Override
        public boolean tryOnError(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 {
                    //回調onError
                    observer.onError(t);
                } finally {
                    //最後斷開訂閱
                    dispose();
                }
                return true;
            }
            return false;
        }

        @Override
        public void onComplete() {
            if (!isDisposed()) {
                try {
                    //回調onComplete.
                    observer.onComplete();
                } finally {
                    //最後斷開訂閱
                    dispose();
                }
            }
        }

        @Override
        public void setDisposable(Disposable d) {
            DisposableHelper.set(this, d);
        }

        @Override
        public void setCancellable(Cancellable c) {
            setDisposable(new CancellableDisposable(c));
        }

        @Override
        public ObservableEmitter<T> serialize() {
            return new SerializedEmitter<T>(this);
        }

        @Override
        public void dispose() {
            DisposableHelper.dispose(this);
        }

        @Override
        public boolean isDisposed() {
            return DisposableHelper.isDisposed(get());
        }
    }
    

    //DisposableHelper.java
    //斷開訂閱方法
    public static boolean dispose(AtomicReference<Disposable> field) {
        Disposable current = field.get();
        Disposable d = DISPOSED;
        if (current != d) {
            current = field.getAndSet(d);
            if (current != d) {
                if (current != null) {
                    current.dispose();
                }
                return true;
            }
        }
        return false;
    }

    
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從上面註釋可知,發射器CreateEmitter直接回調了觀察者Observer的相關方法.當調用dispose斷開dispose訂閱時,此時和線程中斷處理同樣,僅僅只是做爲一個標識,標識當前發射器已經被中斷.

這裏最後給出一張關係圖對上面流程進行概括.

Observable的建立-訂閱-發送事件過程

4. subscribeOn過程

從上面可知,RxJava的總體流程框架仍是挺清晰的,但有時咱們須要多它進行一些附加的操做如切線程,map,filter進行轉換等.這裏以切線程爲例進行分析看如何工做的.

@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.CUSTOM)
    public final Observable<T> subscribeOn(Scheduler scheduler) {
        ObjectHelper.requireNonNull(scheduler, "scheduler is null");
        //新建一個ObservableSubscribeOn.
        return RxJavaPlugins.onAssembly(new ObservableSubscribeOn<T>(this, scheduler));
    }

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當咱們切換下游線程時,也返回了一個新建的Observable-ObservableSubscribeOn.接着咱們直接看ObservableSubscribeOn類.

對應源碼

//ObservableSubscribeOn.java
//中間的Observable.
public ObservableSubscribeOn(ObservableSource<T> source, Scheduler scheduler) {
        super(source);//上游的源Observable
        this.scheduler = scheduler;
    }

@Override
    public void subscribeActual(final Observer<? super T> s) {
        //建立一箇中間的Observer
        //建立一箇中間的Observer
        //建立一箇中間的Observer
        //重要的話說三遍.
        final SubscribeOnObserver<T> parent = new SubscribeOnObserver<T>(s);
        //回調onSubscribe,通知訂閱成功
        s.onSubscribe(parent);
        
        //切換線程
        parent.setDisposable(scheduler.scheduleDirect(new SubscribeTask(parent)));
    }
    
//線程任務類SubscribeTask    
final class SubscribeTask implements Runnable {
        private final SubscribeOnObserver<T> parent;

        SubscribeTask(SubscribeOnObserver<T> parent) {
            this.parent = parent;
        }

        @Override
        public void run() {
            //回調subscribe,進行訂閱
            //source是源Observable,parent則是中間Observer,不是最終目標的Observer.這裏必定要清楚.
            source.subscribe(parent);
        }
    }    
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從上面註釋可知,當咱們調用subscribeOn切換上游線程時

建立一箇中間的Observable,而後在這個Observable裏面會建立一箇中間的Observer
最後在線程任務的run方法裏面經過將源Observable與中間的Observer進行綁定訂閱,從而進行切換了上游線程.
中間的Observer接到相應回調時則會繼續往上回調源Observer.

這也就是解釋了最開始文章所說的subscribeOn不管調用幾回,爲何只有第一次是生效的.由於每次都建立新的Observable與Observer,線程調度器裏將源Observable與中間的Observer進行綁定訂閱時,源Observable僅僅是指上一個,已經不是第一個建立出來的.

經過上面流程分析能夠總結出RxJava操做符一些通用的流程.對於Map等操做符均可以參考.如圖

Observable附加操做通用流程

5. observeOn過程

最後咱們根據上面總結出的通用流程,去分析下切換下游線程的過程.

@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.CUSTOM)
    public final Observable<T> observeOn(Scheduler scheduler, boolean delayError, int bufferSize) {
        ObjectHelper.requireNonNull(scheduler, "scheduler is null");
        ObjectHelper.verifyPositive(bufferSize, "bufferSize");
        //返回一個新建的Observable
        return RxJavaPlugins.onAssembly(new ObservableObserveOn<T>(this, scheduler, delayError, bufferSize));
    }

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一樣當調用observeOn過程切換下游線程時,果不其然也返回了一箇中間的Observable-ObservableObserveOn.接着看ObservableObserveOn代碼.

對應源碼

//中間的Observable
//ObservableObserveOn.java
@Override
    protected void subscribeActual(Observer<? super T> observer) {
        if (scheduler instanceof TrampolineScheduler) {
            source.subscribe(observer);
        } else {
            //線程調度器
            Scheduler.Worker w = scheduler.createWorker();
            //回調subscribe進行訂閱(中間的Observer-ObserveOnObserver)
            source.subscribe(new ObserveOnObserver<T>(observer, w, delayError, bufferSize));
        }
    }
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一樣,果不其然