RxSwift: Subjects

• Can act as both an Observable and as an Observer.
• .onNext(_:) puts a new element onto the subject.
• Need subscribers for events to happen.
• Every subject type, once terminated will re-emit its stop event to future subscribers.

4 different types of Subjects:

1. PublishSubject: Starts empty and only emits new elements to subscribers.
2. BehaviorSubject: Starts with an initial value and replays it or the latest element to new subscribers.
3. ReplaySubject: Initialised with a buffer size and will maintain a buffer of elements up to that size and replay it to new subscribers.
4. Variable: Wraps a BehaviorSubject. Preservers its current value as state and replays only the latest/initial value to new subscribers.

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