Rxjava2.0 超时处理 -- Observable.timeout() 操作符的源码解析

在开发的过程中，有一个需求要求在进行网络请求的时候，响应速度超过500毫秒就取消请求并沿用本地缓存数据。

这时候就想起Rxjava的TimeOut操作符：

* Returns an Observable that mirrors the source ObservableSource but applies a timeout policy for each emitted
* item. If the next item isn't emitted within the specified timeout duration starting from its predecessor,
* the resulting ObservableSource begins instead to mirror a fallback ObservableSource.
（渣渣翻译：返回对每次发射数据都执行超时策略的原始Observable镜像。在指定的timeout时间中，下一个item并没有发射的话，则最后返回的ObservableSource会替代会fallback ObservableSource的镜像）

@CheckReturnValue
@SchedulerSupport(SchedulerSupport.COMPUTATION)
public final Observable timeout(long timeout, TimeUnit timeUnit, ObservableSource other) {
    ObjectHelper.requireNonNull(other, "other is null");
    return timeout0(timeout, timeUnit, other, Schedulers.computation());
}

• long timeout --> 设定的超时范围
• TimeUnit timeUnit --> 时间类型
• ObservableSource other --> 超时后切换的Observable
• Schedulers.computation() --> 调度器

TimeOut操作符的实现有三种：

1. timeout(long timeout, TimeUnit timeUnit)：每当原始Observable发射了一项数据，computation调度器就启动一个计时器，如果计时器超过了指定指定的时长而原始Observable没有发射另一项数据，timeout就抛出 TimeoutException，以一个错误通知终止Observable。
2. timeout(long timeout, TimeUnit timeUnit, ObservableSource other)：每当原始Observable发射了一项数据，computation调度器就启动一个计时器，如果计时器超过了指定指定的时长而原始Observable没有发射另一项数据，timeout 在超时时会切换到使用一个你指定的备用的 Observable。
3. timeout(Function<> itemTimeoutIndicator)：timeout使用一个Function对原始Observable发射的每一项进行观察，如果当这个Function执行完但原始Observable还没有发射下一个数据时，系统就会认为是超时了，timeout 就抛出 TimeoutException，以一个错误通知终止原始Observable。

本次解析的第二种方法，当超时后就切换指定的Observable：

1. 首先我们先看到对应的timeOut中return的time0()函数：

[--> Observable.java]

private Observable timeout0(long timeout, TimeUnit timeUnit, ObservableSource other, Scheduler scheduler) {
        ObjectHelper.requireNonNull(timeUnit, "timeUnit is null");
        ObjectHelper.requireNonNull(scheduler, "scheduler is null");
        return RxJavaPlugins.onAssembly(new ObservableTimeoutTimed(this, timeout, timeUnit, scheduler, other));
    }

 2. 见最后一行，传入各种参数来创建一个ObservableTimeoutTimed<>()对象，进来看下这个类：

[>> ObservableTimeoutTimed.java]

public final class ObservableTimeoutTimed extends AbstractObservableWithUpstream {
    ....

    public ObservableTimeoutTimed(Observable source,
            long timeout, TimeUnit unit, Scheduler scheduler, ObservableSource other) {
        super(source);
        this.timeout = timeout;
        this.unit = unit;
        this.scheduler = scheduler;
        this.other = other;
    }

    @Override
    protected void subscribeActual(Observer observer) {
        if (other == null) {
            TimeoutObserver parent = new TimeoutObserver(observer, timeout, unit, scheduler.createWorker());
            observer.onSubscribe(parent);
            parent.startTimeout(0L);
            source.subscribe(parent);
        } else {
            TimeoutFallbackObserver parent = new TimeoutFallbackObserver(observer, timeout, unit, scheduler.createWorker(), other);
            observer.onSubscribe(parent);
            parent.startTimeout(0L);
            source.subscribe(parent);
        }
    }

 

• 因为该类继承AbstractObservableWithUpstream，在构造函数中调用父类，是observable拥有原始上游。
• 在subscribeActual()方法中，假如有指定的observable的话执行下面的语句--> 创建一个TimeoutFallbackObserver
  类的对象，设置初始timeout为0，其他的两步都为常规的订阅操作（绑定生命周期，让用户执行parent中的函数）。

3. 按照国际惯例，我们继续了解TimeoutFallbackObserver<>()这个内部类：

[>> ObservableTimeoutTimed.java]

static final class TimeoutFallbackObserver extends AtomicReference
    implements Observer, Disposable, TimeoutSupport {

 ...

        TimeoutFallbackObserver(Observer actual, long timeout, TimeUnit unit,
                Scheduler.Worker worker, ObservableSource fallback) {
            this.downstream = actual;
            this.timeout = timeout;
            this.unit = unit;
            this.worker = worker;
            this.fallback = fallback;
            this.task = new SequentialDisposable();
            this.index = new AtomicLong();
            this.upstream = new AtomicReference();
        }

        @Override
        public void onSubscribe(Disposable d) {
            DisposableHelper.setOnce(upstream, d);
        }

        @Override
        public void onNext(T t) {
            long idx = index.get();
            if (idx == Long.MAX_VALUE || !index.compareAndSet(idx, idx + 1)) {
                return;
            }

            task.get().dispose();

            downstream.onNext(t);

            startTimeout(idx + 1);
        }

        void startTimeout(long nextIndex) {
            task.replace(worker.schedule(new TimeoutTask(nextIndex, this), timeout, unit));
        }

        @Override
        public void onError(Throwable t) {
            if (index.getAndSet(Long.MAX_VALUE) != Long.MAX_VALUE) {
                task.dispose();

                downstream.onError(t);

                worker.dispose();
            } else {
                RxJavaPlugins.onError(t);
            }
        }

        @Override
        public void onComplete() {
            if (index.getAndSet(Long.MAX_VALUE) != Long.MAX_VALUE) {
                task.dispose();

                downstream.onComplete();

                worker.dispose();
            }
        }

        @Override
        public void onTimeout(long idx) {
            if (index.compareAndSet(idx, Long.MAX_VALUE)) {
                DisposableHelper.dispose(upstream);

                ObservableSource f = fallback;
                fallback = null;

                f.subscribe(new FallbackObserver(downstream, this));

                worker.dispose();
            }
        }

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

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

•  从timeout方法的定义来看，timeout的操作都是从原始Observable执行第一次onNext()方法后才进行的
• 在onNext()方法中，看到第二的if语句，有两个功能 >> 判断是否继续执行该方法 、进行index自加1
• 在onNext()方法中，看到最后一行“startTimeout(idx + 1)”，看了下startTimeOut()函数，就会发现实现方法模式worker.schedule(new Task())跟rxjava的observable线程切换原理的实现很相似！！！
• 这时候我们马上想起在timeOut()函数中传入的“Schedulers.computation()”~~

 4. 到这里，我们大概猜到timeOut操作符是利用子线程来执行Timeout的计时工作(*-*好像说了跟没说一样..)。那我们一起来分析这个scheduler从初始化到被调用的整个流程：

[ComputationScheduler.java]

public final class ComputationScheduler extends Scheduler implements SchedulerMultiWorkerSupport {
   ...
   static {
        MAX_THREADS = cap(Runtime.getRuntime().availableProcessors(), Integer.getInteger(KEY_MAX_THREADS, 0));

        SHUTDOWN_WORKER = new PoolWorker(new RxThreadFactory("RxComputationShutdown"));
        SHUTDOWN_WORKER.dispose();

        int priority = Math.max(Thread.MIN_PRIORITY, Math.min(Thread.MAX_PRIORITY,
                Integer.getInteger(KEY_COMPUTATION_PRIORITY, Thread.NORM_PRIORITY)));

        THREAD_FACTORY = new RxThreadFactory(THREAD_NAME_PREFIX, priority, true);

        NONE = new FixedSchedulerPool(0, THREAD_FACTORY);
        NONE.shutdown();
    }

    static int cap(int cpuCount, int paramThreads) {
        return paramThreads <= 0 || paramThreads > cpuCount ? cpuCount : paramThreads;
    }

    static final class FixedSchedulerPool implements SchedulerMultiWorkerSupport {
        final int cores;

        final PoolWorker[] eventLoops;
        long n;

        FixedSchedulerPool(int maxThreads, ThreadFactory threadFactory) {
            // initialize event loops
            this.cores = maxThreads;
            this.eventLoops = new PoolWorker[maxThreads];
            for (int i = 0; i < maxThreads; i++) {
                this.eventLoops[i] = new PoolWorker(threadFactory);
            }
        }

        public PoolWorker getEventLoop() {
            int c = cores;
            if (c == 0) {
                return SHUTDOWN_WORKER;
            }
            // simple round robin, improvements to come
            return eventLoops[(int)(n++ % c)];
        }

        public void shutdown() {
            for (PoolWorker w : eventLoops) {
                w.dispose();
            }
        }

        @Override
        public void createWorkers(int number, WorkerCallback callback) {
            int c = cores;
            if (c == 0) {
                for (int i = 0; i < number; i++) {
                    callback.onWorker(i, SHUTDOWN_WORKER);
                }
            } else {
                int index = (int)n % c;
                for (int i = 0; i < number; i++) {
                    callback.onWorker(i, new EventLoopWorker(eventLoops[index]));
                    if (++index == c) {
                        index = 0;
                    }
                }
                n = index;
            }
        }
    }

    public ComputationScheduler() {
        this(THREAD_FACTORY);
    }

    public ComputationScheduler(ThreadFactory threadFactory) {
        this.threadFactory = threadFactory;
        this.pool = new AtomicReference(NONE);
        start();
    }

    @NonNull
    @Override
    public Worker createWorker() {
        return new EventLoopWorker(pool.get().getEventLoop());
    }

    @Override
    public void createWorkers(int number, WorkerCallback callback) {
        ObjectHelper.verifyPositive(number, "number > 0 required");
        pool.get().createWorkers(number, callback);
    }

    @NonNull
    @Override
    public Disposable scheduleDirect(@NonNull Runnable run, long delay, TimeUnit unit) {
        PoolWorker w = pool.get().getEventLoop();
        return w.scheduleDirect(run, delay, unit);
    }

    @NonNull
    @Override
    public Disposable schedulePeriodicallyDirect(@NonNull Runnable run, long initialDelay, long period, TimeUnit unit) {
        PoolWorker w = pool.get().getEventLoop();
        return w.schedulePeriodicallyDirect(run, initialDelay, period, unit);
    }

    @Override
    public void start() {
        FixedSchedulerPool update = new FixedSchedulerPool(MAX_THREADS, threadFactory);
        if (!pool.compareAndSet(NONE, update)) {
            update.shutdown();
        }
    }

    @Override
    public void shutdown() {
        for (;;) {
            FixedSchedulerPool curr = pool.get();
            if (curr == NONE) {
                return;
            }
            if (pool.compareAndSet(curr, NONE)) {
                curr.shutdown();
                return;
            }
        }
    }

    static final class EventLoopWorker extends Scheduler.Worker {
        ...

        EventLoopWorker(PoolWorker poolWorker) {
            this.poolWorker = poolWorker;
            this.serial = new ListCompositeDisposable();
            this.timed = new CompositeDisposable();
            this.both = new ListCompositeDisposable();
            this.both.add(serial);
            this.both.add(timed);
        }

        @Override
        public void dispose() {
            if (!disposed) {
                disposed = true;
                both.dispose();
            }
        }

        @Override
        public boolean isDisposed() {
            return disposed;
        }

        @NonNull
        @Override
        public Disposable schedule(@NonNull Runnable action) {
            if (disposed) {
                return EmptyDisposable.INSTANCE;
            }

            return poolWorker.scheduleActual(action, 0, TimeUnit.MILLISECONDS, serial);
        }

        @NonNull
        @Override
        public Disposable schedule(@NonNull Runnable action, long delayTime, @NonNull TimeUnit unit) {
            if (disposed) {
                return EmptyDisposable.INSTANCE;
            }

            return poolWorker.scheduleActual(action, delayTime, unit, timed);
        }
    }

    static final class PoolWorker extends NewThreadWorker {
        PoolWorker(ThreadFactory threadFactory) {
            super(threadFactory);
        }
    }
}

• 首先，在静态模块内创建优先级为5的线程工厂(RxThreadFactory)
• 在构造函数中执行start()函数 >> 创建FixedSchedulerPool对象、判断当前FixedSchedulerPool不为None值的话就将其shutdown
• 在FixedSchedulerPool内部类的构造函数中，主要工作是创建PoolWoker对象
• 再来看看PoolWoker这个内部类，继承newThreadWoker，并传入ThreadFactory通过super()调用父类构造函数 >>> emm..显然易见的操作就是通过这个线程工厂和newThreadWoker()构造函数中的SchedulerPoolFactory.create()方法，利用Executors.newScheduledThreadPool()方法创建核心线程为1的子线程：

public class NewThreadWorker extends Scheduler.Worker implements Disposable {
    ...

    public NewThreadWorker(ThreadFactory threadFactory) {
        executor = SchedulerPoolFactory.create(threadFactory);
    }
}

>>>

public final class SchedulerPoolFactory {
   ...

   public static ScheduledExecutorService create(ThreadFactory factory) {
        final ScheduledExecutorService exec = Executors.newScheduledThreadPool(1, factory);
        tryPutIntoPool(PURGE_ENABLED, exec);
        return exec;
    }
}

• 还记得创建TimeOutFallBackObserver<>()对象时传入的schduler.createWoker()参数吗，看下调用的createWorker()函数是怎么实现的 >> 也是返回一个EventLoopWoker()内部类的对象
• 在EventLoopWorker()内部类还有一个重要的schedule()函数，返回一个Disposable >> poolWorker.scheduleActual(action, delayTime, unit, timed)，该方法实际上执行了NewThreadWorker()类的scheduleActual()函数，通过submit()或者schedule()方法执行子线程

public class NewThreadWorker extends Scheduler.Worker implements Disposable {
   ...

   @NonNull
    public ScheduledRunnable scheduleActual(final Runnable run, long delayTime, @NonNull TimeUnit unit, @Nullable DisposableContainer parent) {
        Runnable decoratedRun = RxJavaPlugins.onSchedule(run);

        ScheduledRunnable sr = new ScheduledRunnable(decoratedRun, parent);

        if (parent != null) {
            if (!parent.add(sr)) {
                return sr;
            }
        }

        Future f;
        try {
            if (delayTime <= 0) {
                f = executor.submit((Callable