Rxjava2 Completable第二节ambArray

简言

参数传入多个CompletableSource,哪个CompletableObserver中的OnSubscribe执行的快就执行就执行哪个，而其他CompletableObserver都被忽略。
多个异步操作，你也不知道哪个会先执行完毕，你只想在某一个操作执行完毕后，执行一次某个方法，那么这个操作符就刚好适用。

例子

 CompletableSource source1 = Completable.create(new CompletableOnSubscribe() {
            @Override
            public void subscribe(CompletableEmitter emitter) throws Exception {
                emitter.onComplete();
            }
        });

        CompletableSource source2 = Completable.create(new CompletableOnSubscribe() {
            @Override
            public void subscribe(CompletableEmitter emitter) throws Exception {
                emitter.onComplete();
            }
        });

        Completable.ambArray(source1,source2).subscribe(new CompletableObserver() {
            @Override
            public void onSubscribe(Disposable d) {
                System.out.println(d.isDisposed());
            }

            @Override
            public void onComplete() {
                System.out.println("onComplete");
            }

            @Override
            public void onError(Throwable e) {
                System.out.println(e.getMessage());
            }
        });

源码分析

   public static Completable ambArray(final CompletableSource... sources) {
        ...

        return RxJavaPlugins.onAssembly(new CompletableAmb(sources, null));
    }

构建的是CompletableAmb实例,

public final class CompletableAmb extends Completable {    
    //这里的sources的来源就是上面的source1,source2组成的数组
    private final CompletableSource[] sources;
    // 这里可以使用Iterable传递
    private final Iterable<? extends CompletableSource> sourcesIterable;

    public CompletableAmb(CompletableSource[] sources, Iterable<? extends CompletableSource> sourcesIterable) {
        this.sources = sources;
        this.sourcesIterable = sourcesIterable;
    }

    @Override
    public void subscribeActual(final CompletableObserver observer) {
         // 开始构建CompletableSource
        CompletableSource[] sources = this.sources;
       // 开始默认数量0
        int count = 0;
        if (sources == null) {
            // 默认数量8
            sources = new CompletableSource[8];
            try {
                //遍历Iterable，将数据添加sources 里
                for (CompletableSource element : sourcesIterable) {
                    if (element == null) {
                        EmptyDisposable.error(new NullPointerException("One of the sources is null"), observer);
                        return;
                    }
                    if (count == sources.length) {
                        CompletableSource[] b = new CompletableSource[count + (count >> 2)];
                        System.arraycopy(sources, 0, b, 0, count);
                        sources = b;
                    }
                    sources[count++] = element;
                }
            } catch (Throwable e) {
                Exceptions.throwIfFatal(e);
                EmptyDisposable.error(e, observer);
                return;
            }
        } else {
            count = sources.length;
        }
        //这里的set控制sources
        final CompositeDisposable set = new CompositeDisposable();
        
        
        observer.onSubscribe(set);
        
        final AtomicBoolean once = new AtomicBoolean();

        CompletableObserver inner = new Amb(once, set, observer);

        for (int i = 0; i < count; i++) {
            CompletableSource c = sources[i];
            if (set.isDisposed()) {
                return;
            }
            if (c == null) {
                NullPointerException npe = new NullPointerException("One of the sources is null");
                if (once.compareAndSet(false, true)) { 
                    //修改成功
                    set.dispose();
                    observer.onError(npe);
                } else {
                    RxJavaPlugins.onError(npe);
                }
                return;
            }

            // 开始执行，就看谁先跑到最后
            c.subscribe(inner);
        }

        if (count == 0) {
            observer.onComplete();
        }
    }
...
}

总结上面方法，将ambArray里面的参数收集到sources数组，遍历数组，执行每一个
subscribe()方法

谁先执行到onComplete或者onError，就会忽略后面的

static final class Amb implements CompletableObserver {
        private final AtomicBoolean once;
        private final CompositeDisposable set;
        private final CompletableObserver downstream;

        Amb(AtomicBoolean once, CompositeDisposable set, CompletableObserver observer) {
            this.once = once;
            this.set = set;
            this.downstream = observer;
        }

        @Override
        public void onComplete() {
            if (once.compareAndSet(false, true)) {
                //只会执行一次，就会忽略后面的
                set.dispose();
                downstream.onComplete();
            }
        }

        @Override
        public void onError(Throwable e) {
            if (once.compareAndSet(false, true)) {
                set.dispose();
                downstream.onError(e);
            } else {
                RxJavaPlugins.onError(e);
            }
        }

        @Override
        public void onSubscribe(Disposable d) {
            set.add(d);
        }

   }