Subject

Subject is a further entity that is provided by the framework. A Subject is a sort of bridge or proxy that acts both as an observer and as an Observable:

• because it is an observer, it can subscribe to one or more observables
• because it is an Observable, it can pass through the items it observes by reemitting them, and it can also emit new items

Subject has the "power" of turning a cold observable hot. Infact, when a Subject subscribes to an Observable, it will trigger that Observable to begin emitting items (and if that Observable is "cold" — that is, if it waits for a subscription before it begins to emit items). This can have the effect of making the resulting Subject a "hot" Observable variant of the original "cold" Observable.

The framework provides a wide range of subjects, each one with its own semantics. What follows is an overview of the main popular and used.

PublishSubject

PublishSubject emits to an observer only those items that are emitted by the source Observable(s) subsequent to the time of the subscription. This means that an observer will not receive the previous emitted items.

ReplaySubject

ReplaySubject emits to any observer all of the items that were emitted by the source Observable(s), regardless of when the observer subscribes. To keep the memory consuption limited, this subject also use a bounded buffer that enable to discard old items when the limit size has been reached.

AsyncSubject

An AsyncSubject caches and only remember the last value of the Observable, and only after that source Observable completes emits that value.

BehaviorSubject

When an observer subscribes to a BehaviorSubject, it begins by emitting the item most recently emitted by the source Observable (or a seed/default value if none has yet been emitted) and then continues to emit any other items emitted later by the source Observable(s).

In practice, it's really similar to the Behavior notion from Elliot's FRP.