BehaviorSubject is often used to hold the most recent state as it always provides the latest
value it receives.
This is often utilized with MVVM architecture in Android where ViewModel define its state as
a BehaviorSubject to which View subscribes to observe the latest changes in state. This is
done mainly because ViewModel's lifecycle is longer than that of View and we need to be able to
unsubscribe and resubscribe whenever there is a configuration change.
But not only the state needs to be communicated to View. Often times there are situations where we need to send a signal to View, such as a command to open a confirmation dialog. We can call such signals events. There are notable differences between the two:
- A state has a timespan — it is valid for a period of time. Usually we are interested only in the latest state.
- An event does not have a timespan — it occurs and immediately is over. Usually we are interested in all events.
BehaviorSubject is a great fit for states but not so much for events because it emits
a value at least once. We don't want to process an event again when we resubscribe to the
subject.
PublishSubject is more suitable for events in a sense that it does not emit any values upon
resubscription. But it emits the value at most once. An event is lost if it occurs when there
is no subscriber.
EventSubject is specifically designed to emit each value exactly once. Whenever no Observer is
observing, events are being queued up. That ensures no events are lost. Whenever a new
Observer subscribes, all queued up events are replayed and removed from the queue. That ensures
no event is processed twice. While an Observer is subscribed all events are relayed to the
Observer directly.
Internally, EventSubject is similar to
UnicastSubject.
Just like UnicastSubject, events are queued up whenever no Observer is subscribed. Whenever a new
Observer subscribes to this, the queued up events are replayed and the queue subsequently emptied.
While an Observer is subscribed all emission are relayed to the Observer directly.
UnicastSubject allows only a single Subscriber during its lifespan. EventSubject relaxes this
in a sense that it allows only one Observer to be subscribed at a time but when it unsubscribes,
another one is allowed to resubscribe to this subject again.
If an Observer attempts to subscribe to an EventSubject that already has another Observer
subscribed already, the Observer attempting to subscribe will receive an IllegalStateException.
All other properties of this EventSubject are the same as of UnicastSubject.
Library can be added as a dependency from mavenCentral repository:
dependencies {
implementation 'io.github.antimonit:eventsubject:1.1.0'
}