Subscribers

What are Subscribers?

A Subscriber listens to a Publisher and reacts when it receives new values.
It’s like a news reader who subscribes to a news publisher and gets updates whenever news is published.

• A subscriber only receives data if it is subscribed.
• It can process, transform, or react to the data it gets.
• If a publisher stops sending values, the subscriber also stops receiving them.

A Subscriber listens for values and reacts to them when they arrive.

🔗 Official Docs: Apple Documentation

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Types of Subscribers

• sink → A simple way to receive values and errors.
• assign(to:on:) → Assigns a value to an object’s property.
• Custom Subscribers → Implement your own subscriber by conforming to Subscriber.

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How Subscribers Work

• A subscriber must be attached to a publisher to start receiving data.
• A subscriber controls its lifecycle and can cancel at any time.
• A publisher can have multiple subscribers listening at once.

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Example: News Subscriber

This example models a news publishing system where a NewsPublisher sends news updates, and multiple subscribers receive them.

import Combine
 
// 1. Create a news publisher
class NewsPublisher {
    let news = PassthroughSubject<String, Never>() // Publishes news headlines
    
    func publish(newsHeadline: String) {
        print("📰 News Published: \(newsHeadline)")
        news.send(newsHeadline) // Send news to subscribers
    }
}
 
// 2. Create a subscriber (news reader)
class NewsSubscriber {
    private var cancellable: AnyCancellable?
    
    init(publisher: NewsPublisher) {
        // Subscribe to news updates
        cancellable = publisher.news.sink { news in
            print("📢 Subscriber received: \(news)")
        }
    }
}
 
// 3. Use the publisher and subscriber
let newsPublisher = NewsPublisher()
let subscriber1 = NewsSubscriber(publisher: newsPublisher)
let subscriber2 = NewsSubscriber(publisher: newsPublisher)
 
newsPublisher.publish(newsHeadline: "Swift 6 Announced!")
newsPublisher.publish(newsHeadline: "New Combine Features in iOS 18!")