Relays are RxJava types which are both an Observable and a Consumer.

Basically: A Subject except without the ability to call onComplete or onError.

Subjects are useful to bridge the gap between non-Rx APIs. However, they are stateful in a damaging way: when they receive an onComplete or onError they no longer become usable for moving data. This is the observable contract and sometimes it is the desired behavior. Most times it is not.

Relays are simply Subjects without the aforementioned property. They allow you to bridge non-Rx APIs into Rx easily, and without the worry of accidentally triggering a terminal state.

As more of your code moves to reactive, the need for Subjects and Relays should diminish. In the transitional period, or for quickly adapting a non-Rx API, Relays provide the convenience of Subjects without the worry of the statefulness of terminal event behavior.

• BehaviorRelay

  Relay that emits the most recent item it has observed and all subsequent observed items to each subscribed Observer.

  // observer will receive all events.
  BehaviorRelay<Object> relay = BehaviorRelay.createDefault("default");
  relay.subscribe(observer);
  relay.accept("one");
  relay.accept("two");
  relay.accept("three");

  // observer will receive the "one", "two" and "three" events, but not "zero"
  BehaviorRelay<Object> relay = BehaviorRelay.createDefault("default");
  relay.accept("zero");
  relay.accept("one");
  relay.subscribe(observer);
  relay.accept("two");
  relay.accept("three");

• PublishRelay

  Relay that, once an Observer has subscribed, emits all subsequently observed items to the subscriber.

  PublishRelay<Object> relay = PublishRelay.create();
  // observer1 will receive all events
  relay.subscribe(observer1);
  relay.accept("one");
  relay.accept("two");
  // observer2 will only receive "three"
  relay.subscribe(observer2);
  relay.accept("three");

• ReplayRelay

  Relay that buffers all items it observes and replays them to any Observer that subscribes.

  ReplayRelay<Object> relay = ReplayRelay.create();
  relay.accept("one");
  relay.accept("two");
  relay.accept("three");
  // both of the following will get the events from above
  relay.subscribe(observer1);
  relay.subscribe(observer2);

All relays use the Relay base class which also allows custom implementations.

See the Javadoc for more information.

(There is no AsyncRelay since relays have no terminal events to support its behavior.)

Maven:

<dependency>
  <groupId>com.jakewharton.rxrelay2</groupId>
  <artifactId>rxrelay</artifactId>
  <version>2.1.0</version>
</dependency>

Gradle:

compile 'com.jakewharton.rxrelay2:rxrelay:2.1.0'

Snapshots of the development version are available in Sonatype's snapshots repository.

Copyright 2014 Netflix, Inc.
Copyright 2015 Jake Wharton

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.