src/main/java/rx/Scheduler.java
/**
* Copyright 2014 Netflix, Inc.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package rx;
import java.util.concurrent.TimeUnit;
import rx.functions.*;
import rx.internal.schedulers.*;
import rx.schedulers.Schedulers;
/**
* A {@code Scheduler} is an object that schedules units of work. You can find common implementations of this
* class in {@link Schedulers}.
*/
public abstract class Scheduler {
/*
* Why is this an abstract class instead of an interface?
*
* : Java doesn't support extension methods and there are many overload methods needing default
* implementations.
*
* : Virtual extension methods aren't available until Java8 which RxJava will not set as a minimum target for
* a long time.
*
* : If only an interface were used Scheduler implementations would then need to extend from an
* AbstractScheduler pair that gives all of the functionality unless they intend on copy/pasting the
* functionality.
*
* : Without virtual extension methods even additive changes are breaking and thus severely impede library
* maintenance.
*/
/**
* Retrieves or creates a new {@link Scheduler.Worker} that represents serial execution of actions.
* <p>
* When work is completed it should be unsubscribed using {@link Scheduler.Worker#unsubscribe()}.
* <p>
* Work on a {@link Scheduler.Worker} is guaranteed to be sequential.
*
* @return a Worker representing a serial queue of actions to be executed
*/
public abstract Worker createWorker();
/**
* Sequential Scheduler for executing actions on a single thread or event loop.
* <p>
* Unsubscribing the {@link Worker} cancels all outstanding work and allows resources cleanup.
*/
public abstract static class Worker implements Subscription {
/**
* Schedules an Action for execution.
*
* @param action
* Action to schedule
* @return a subscription to be able to prevent or cancel the execution of the action
*/
public abstract Subscription schedule(Action0 action);
/**
* Schedules an Action for execution at some point in the future.
* <p>
* Note to implementors: non-positive {@code delayTime} should be regarded as non-delayed schedule, i.e.,
* as if the {@link #schedule(rx.functions.Action0)} was called.
*
* @param action
* the Action to schedule
* @param delayTime
* time to wait before executing the action; non-positive values indicate an non-delayed
* schedule
* @param unit
* the time unit of {@code delayTime}
* @return a subscription to be able to prevent or cancel the execution of the action
*/
public abstract Subscription schedule(final Action0 action, final long delayTime, final TimeUnit unit);
/**
* Schedules a cancelable action to be executed periodically. This default implementation schedules
* recursively and waits for actions to complete (instead of potentially executing long-running actions
* concurrently). Each scheduler that can do periodic scheduling in a better way should override this.
* <p>
* Note to implementors: non-positive {@code initialTime} and {@code period} should be regarded as
* non-delayed scheduling of the first and any subsequent executions.
*
* @param action
* the Action to execute periodically
* @param initialDelay
* time to wait before executing the action for the first time; non-positive values indicate
* an non-delayed schedule
* @param period
* the time interval to wait each time in between executing the action; non-positive values
* indicate no delay between repeated schedules
* @param unit
* the time unit of {@code period}
* @return a subscription to be able to prevent or cancel the execution of the action
*/
public Subscription schedulePeriodically(final Action0 action, long initialDelay, long period, TimeUnit unit) {
return SchedulePeriodicHelper.schedulePeriodically(this, action,
initialDelay, period, unit, null);
}
/**
* Gets the current time, in milliseconds, according to this Scheduler.
*
* @return the scheduler's notion of current absolute time in milliseconds
*/
public long now() {
return System.currentTimeMillis();
}
}
/**
* Gets the current time, in milliseconds, according to this Scheduler.
*
* @return the scheduler's notion of current absolute time in milliseconds
*/
public long now() {
return System.currentTimeMillis();
}
/**
* Allows the use of operators for controlling the timing around when
* actions scheduled on workers are actually done. This makes it possible to
* layer additional behavior on this {@link Scheduler}. The only parameter
* is a function that flattens an {@link Observable} of {@link Observable}
* of {@link Completable}s into just one {@link Completable}. There must be
* a chain of operators connecting the returned value to the source
* {@link Observable} otherwise any work scheduled on the returned
* {@link Scheduler} will not be executed.
* <p>
* When {@link Scheduler#createWorker()} is invoked a {@link Observable} of
* {@link Completable}s is onNext'd to the combinator to be flattened. If
* the inner {@link Observable} is not immediately subscribed to an calls to
* {@link Worker#schedule} are buffered. Once the {@link Observable} is
* subscribed to actions are then onNext'd as {@link Completable}s.
* <p>
* Finally the actions scheduled on the parent {@link Scheduler} when the
* inner most {@link Completable}s are subscribed to.
* <p>
* When the {@link Worker} is unsubscribed the {@link Completable} emits an
* onComplete and triggers any behavior in the flattening operator. The
* {@link Observable} and all {@link Completable}s give to the flattening
* function never onError.
* <p>
* Limit the amount concurrency two at a time without creating a new fix
* size thread pool:
*
* <pre>
* Scheduler limitScheduler = Schedulers.computation().when(workers -> {
* // use merge max concurrent to limit the number of concurrent
* // callbacks two at a time
* return Completable.merge(Observable.merge(workers), 2);
* });
* </pre>
* <p>
* This is a slightly different way to limit the concurrency but it has some
* interesting benefits and drawbacks to the method above. It works by
* limited the number of concurrent {@link Worker}s rather than individual
* actions. Generally each {@link Observable} uses its own {@link Worker}.
* This means that this will essentially limit the number of concurrent
* subscribes. The danger comes from using operators like
* {@link Observable#zip(Observable, Observable, rx.functions.Func2)} where
* subscribing to the first {@link Observable} could deadlock the
* subscription to the second.
*
* <pre>
* Scheduler limitScheduler = Schedulers.computation().when(workers -> {
* // use merge max concurrent to limit the number of concurrent
* // Observables two at a time
* return Completable.merge(Observable.merge(workers, 2));
* });
* </pre>
*
* Slowing down the rate to no more than than 1 a second. This suffers from
* the same problem as the one above I could find an {@link Observable}
* operator that limits the rate without dropping the values (aka leaky
* bucket algorithm).
*
* <pre>
* Scheduler slowScheduler = Schedulers.computation().when(workers -> {
* // use concatenate to make each worker happen one at a time.
* return Completable.concat(workers.map(actions -> {
* // delay the starting of the next worker by 1 second.
* return Completable.merge(actions.delaySubscription(1, TimeUnit.SECONDS));
* }));
* });
* </pre>
*
* @param <S> a Scheduler and a Subscription
* @param combine the function that takes a two-level nested Observable sequence of a Completable and returns
* the Completable that will be subscribed to and should trigger the execution of the scheduled Actions.
* @return the Scheduler with the customized execution behavior
* @since 1.3
*/
@SuppressWarnings("unchecked")
public <S extends Scheduler & Subscription> S when(Func1<Observable<Observable<Completable>>, Completable> combine) {
return (S) new SchedulerWhen(combine, this);
}
}