RXSwift: BehaviorRelay

BehaviorRelay is a wrapper around BehaviorSubject in RxSwift, primarily designed for use with values that need to be observed and retained. Unlike PublishSubject, BehaviorRelay always holds a value, making it ideal for data that needs an initial state and should always provide the latest value to new subscribers.

Key Characteristics of BehaviorRelay:

1. Current Value: BehaviorRelay always has a current value, which is accessible with the .value property.
2. Last Value Replay: New subscribers immediately receive the most recent value upon subscribing, even if they subscribe after the relay has been updated.
3. No Termination Events: BehaviorRelay doesn’t support the completion or error events, making it simpler and safer for UI updates and state management, where you typically don’t want the stream to terminate.

Why Use BehaviorRelay?

• State Management: It’s ideal for storing values that represent the current state of an app or component, such as form inputs, toggle states, or user settings.
• Data Binding: Commonly used in MVVM architecture for data binding, as it continuously provides the latest value to UI elements or other observers.
• Simpler API: Without completion or error events, it’s safer and reduces complexity, making it user-friendly for many common scenarios.

Example Usage

import RxSwift
import RxCocoa

let disposeBag = DisposeBag()
let relay = BehaviorRelay(value: "Initial Value")

// Subscriber 1 subscribes and receives the initial value
relay.subscribe(onNext: {
    print("Subscriber 1 received: \($0)")
}).disposed(by: disposeBag)

// Updating the value
relay.accept("Updated Value")

// Subscriber 2 subscribes after the value is updated
relay.subscribe(onNext: {
    print("Subscriber 2 received: \($0)")
}).disposed(by: disposeBag)

// Further updates
relay.accept("Another Update")

Output:

Subscriber 1 received: Initial Value
Subscriber 1 received: Updated Value
Subscriber 2 received: Updated Value
Subscriber 1 received: Another Update
Subscriber 2 received: Another Update

Explanation:

• Initial Value: Upon creation, BehaviorRelay is initialized with "Initial Value".
• Value Update: The .accept() method updates the relay’s current value and notifies all subscribers.
• New Subscribers: When Subscriber 2 subscribes after the first update, it immediately receives the most recent value ("Updated Value") instead of starting from the initial state.
• Real-time Updates: Both subscribers receive any subsequent updates, maintaining an up-to-date view of the relay’s state.

In summary, BehaviorRelay is excellent for managing state reactively in RxSwift, offering a simple and predictable way to store and observe values that need to be accessed in real time across different components.

Check our Sample App