定期接收可观测发射值

Question

我必须定期轮询一些RESTfulendpoint以刷新android应用程序的数据。我还必须根据连接情况暂停并恢复（如果手机处于脱机状态，甚至无需尝试）。我当前的解决方案正在运行，但它使用标准Java的ScheduledExecutorService来执行定期任务，但我想继续使用Rx范式。

这是我当前的代码，为了简洁起见，跳过了部分代码。

userProfileObservable = Observable.create(new Observable.OnSubscribe<UserProfile>() {
    @Override
    public void call(final Subscriber<? super UserProfile> subscriber) {
        final ScheduledExecutorService scheduledExecutorService = Executors.newSingleThreadScheduledExecutor();
        final Runnable runnable = new Runnable() {
            @Override
            public void run() {
                // making http request here
            }
        };
        final List<ScheduledFuture<?>> futures = new ArrayList<ScheduledFuture<?>>(1);
        networkStatusObservable.subscribe(new Action1<Boolean>() {
            @Override
            public void call(Boolean networkAvailable) {
                if (!networkAvailable) {
                    pause();
                } else {
                    pause();                        
                    futures.add(scheduledExecutorService.scheduleWithFixedDelay(runnable, 0, SECOND_IN_MILLIS * SECONDS_TO_EXPIRE, TimeUnit.MILLISECONDS));
                }
            }

            private void pause() {
                for (ScheduledFuture<?> future : futures) {
                    future.cancel(true);
                }
                futures.clear();
            }
        });

        final Subscription subscription = new Subscription() {
            private boolean isUnsubscribed = false;

            @Override
            public void unsubscribe() {
                scheduledExecutorService.shutdownNow();
                isUnsubscribed = true;
            }

            @Override
            public boolean isUnsubscribed() {
                return isUnsubscribed;
            }
        };
        subscriber.add(subscription);
    }
}).multicast(BehaviorSubject.create()).refCount();

网络状态可观察基本上是一个包裹在可观察中的广播接收器

正如我所说，这个解决方案是有效的，但我想使用Rx方法进行定期轮询并发出新的UserProfiles，因为手动调度有许多问题，我想避免。我知道Observable.timer和Observable.interval，但不知道如何将它们应用于此任务（我不确定是否需要使用它们）。

Answer 1

简短的回答。RxJava2：

Observable.interval(initialDelay, unitAmount, timeUnit)
            .subscribe(value -> {
                // code for periodic execution
            });

根据需要选择initialDelay、unitAmount和TimeUnit。

示例：0,1， TimeUnit. MINUTES。

Answer 2

选择之一是使用Observable。间隔并在发出间隔时检查用户状态：

     Observable<Long> interval = Observable.interval(1, TimeUnit.SECONDS);

    //pulling the user data
    Observable<Observable<String>> userObservable = interval.map(new Func1<Long, Observable<String>>() {
        Random random = new Random();
        @Override
        public Observable<String> call(Long tick) {
            //here you are pulling user data; you should do it asynchronously - rx way - because the interval is using Schedulers.computation which is not best suited for doing io operations
            switch(random.nextInt(10)){
                case 0://suppose this is for cases when network in  not available or exception happens
                    return Observable.<String>just(null);
                case 1:
                case 2:
                    return Observable.just("Alice");
                default:
                    return Observable.just("Bob");
            }
        }
    });

    Observable<String> flatUsers = userObservable.flatMap(new Func1<Observable<String>, Observable<? extends String>>() {
        @Override
        public Observable<? extends String> call(Observable<String> stringObservable) {
            return stringObservable;
        }
    });

    //filter valid data
    Observable<String> usersWithoutErrors = flatUsers.filter(new Func1<String, Boolean>() {
        @Override
        public Boolean call(String s) {
            return s != null;
        }
    });

    //publish only changes
    Observable<String> uniqueUsers = usersWithoutErrors.distinctUntilChanged();

如果networkStatusObservable发出事件的频率至少与检查用户数据所需的频率相同，则可以更简单地执行此操作

 networkStatusObservable.sample(1,TimeUnit.Seconds).filter(/*the best is to filter only connected state */).map(/*now start pulling the user data*/)

最后，您可以创建使用调度程序定期发出用户状态的可观察-请参阅调度程序留档以了解哪个调度程序最适合您的需求：

public abstract class ScheduledOnSubscribe<T> implements Observable.OnSubscribe<T>{
    private final Scheduler scheduler;
    private final long initialDelay;
    private final long period;
    private final TimeUnit unit;

    public ScheduledOnSubscribe(Scheduler scheduler, long initialDelay, long period, TimeUnit unit) {
        this.scheduler = scheduler;
        this.initialDelay = initialDelay;
        this.period = period;
        this.unit = unit;
    }

    abstract T next() throws Exception;


    @Override
    public void call(final Subscriber<? super T> subscriber) {
        final Scheduler.Worker worker = scheduler.createWorker();
        subscriber.add(worker);
        worker.schedulePeriodically(new Action0() {
            @Override
            public void call() {
                try {
                    subscriber.onNext(next());
                } catch (Throwable e) {
                    try {
                        subscriber.onError(e);
                    } finally {
                        worker.unsubscribe();
                    }
                }
            }

        }, initialDelay, period, unit);
    }
}

//And here is the sample usage
 Observable<String> usersObservable = Observable.create(new ScheduledOnSubscribe(Schedulers.io(), 1, 1, TimeUnit.SECONDS ){
        Random random = new Random();
        @Override
        String next() throws Exception {
            //if you use Schedulers.io, you can call the remote service synchronously
            switch(random.nextInt(10)){
                case 0:
                    return null;
                case 1:
                case 2:
                    return "Alice";
                default:
                    return "Bob";
            }
        }
    });

Answer 3

关于这个GitHub问题，您可能会发现一些有用的方法。

https://github.com/ReactiveX/RxJava/issues/448

这三个实现是：

Observable.interval

Observable.interval(delay, TimeUnit.SECONDS).timeInterval()
        .flatMap(new Func1<Long, Observable<Notification<AppState>>>() {
            public Observable<Notification<AppState>> call(Long seconds) {
                return lyftApi.updateAppState(params).materialize(); } });

调度程序。定期安排

Observable.create({ observer ->
        Schedulers.newThread().schedulePeriodically({
            observer.onNext("application-state-from-network");
        }, 0, 1000, TimeUnit.MILLISECONDS);
    }).take(10).subscribe({ v -> println(v) });

手动递归

Observable.create(new OnSubscribeFunc<String>() {
        @Override
        public Subscription onSubscribe(final Observer<? super String> o) {
            return Schedulers.newThread().schedule(0L, new Func2<Scheduler, Long, Subscription>() {
                @Override
                public Subscription call(Scheduler inner, Long t2) {
                    o.onNext("data-from-polling");
                    return inner.schedule(t2, this, 1000, TimeUnit.MILLISECONDS);
                }
            });
        }
    }).toBlockingObservable().forEach(new Action1<String>() {
        @Override
        public void call(String v) {
            System.out.println("output: " + v);
        }
    });

结论是手动递归是可行的，因为它会等到操作完成后再安排下一次执行。