Observable is a stream (of zero, one or more values over any amount of time (may be async, but it is not mandatory)).
Everything can be a stream.
Differs in direction in which the data is being pushed:
- Array - pulling data for operation.
- Observable - performing operation on data.
Cold observables start running upon subscription and values are not shared among subscribers.
Hot observables are already producing values even before a subscription is active (such as mouse move events or stock tickers) and sequence is shared among all subscribers (each subscriber is pushed the next value in the sequence).
When an observer subscribes to a hot observable sequence, it will get all values in the stream that are emitted after it subscribes.
A Subject is like an Observable, but can multicast to many Observers. At a high-level Subjects can be thought of as messengers, or proxies.
Every Subject is an Observable. Every Subject is an Observer.
| Subject | Description |
|---|---|
| BehaviorSubject | "The current value". It stores the latest value emitted to its consumers, and whenever a new Observer subscribes, it will immediately receive the "current value". |
| ReplaySubject | Records multiple values from the Observable execution and replays them to new subscribers. When creating a ReplaySubject, you can specify how many values to replay. |
| AsyncSubject | Only the last value of the Observable execution is sent to its observers, and only when the execution completes. |