RXSwift: PublishSubject

PublishSubject is a type of observable in RxSwift that begins empty and only emits new events to its subscribers. It does not store any previous events; subscribers will only receive events that are emitted after they subscribe. This makes PublishSubject ideal for handling and broadcasting new events in real-time, without retaining any historical data.

Key Characteristics:

1. No Initial Value: PublishSubject doesn’t hold an initial value, unlike BehaviorSubject.
2. Emits Only New Events: Subscribers only receive events emitted after they subscribe.
3. Broadcasts Events: It sends events to all its subscribers simultaneously, making it suitable for situations where multiple parts of an app need to react to the same event in real-time.

Common Use Cases:

• Event Streams: For events like button taps, notifications, or any actions that need to be observed without retaining past events.
• UI Updates: To trigger UI changes across multiple views or view controllers.
• Delegation Pattern Replacement: For creating reactive callbacks, replacing traditional delegation with observable event handling.

Example Usage

import RxSwift

let disposeBag = DisposeBag()
let subject = PublishSubject<String>()

// Subscriber 1
subject.subscribe(onNext: {
    print("Subscriber 1 received: \($0)")
}).disposed(by: disposeBag)

// Emit events
subject.onNext("Hello")
subject.onNext("RxSwift")

// Subscriber 2 subscribes later
subject.subscribe(onNext: {
    print("Subscriber 2 received: \($0)")
}).disposed(by: disposeBag)

// Emit more events
subject.onNext("New Event")

Output:

Subscriber 1 received: Hello
Subscriber 1 received: RxSwift
Subscriber 1 received: New Event
Subscriber 2 received: New Event

Explanation:

• Subscriber 1 receives all events from "Hello" onward.
• Subscriber 2 only receives events starting from "New Event", as it subscribed after the previous events were emitted.

In this way, PublishSubject provides a flexible way to handle real-time event streams, ensuring that only current and future events are propagated to subscribers.

Check our Sample App