Publishers

What are Publishers?

A Publisher is a source of data in Combine that emits values over time.
It produces a sequence of values that can be observed by Subscribers.

• Think of it like a news publisher that broadcasts news updates.
• Subscribers (readers) receive the updates and react to them.
• Publishers do not send values unless there is an active Subscriber.

A Publisher emits values to a Subscriber, which then processes those values.

🔗 Official Docs: Apple Documentation

---

Types of Publishers

• Built-in Publishers
  → Just(value) → Emits a single value and finishes.
  → Future { promise in } → Performs work and emits one value asynchronously.
  → Empty() → Emits no values and immediately finishes.
  → Fail(error) → Emits an error immediately.
  → PassthroughSubject → A manually triggered publisher.
  → CurrentValueSubject → Stores a value and emits new ones when updated.
  → URLSession.shared.dataTaskPublisher → Emits network request results.

---

How Publishers Work

• Publishers emit values asynchronously.
• They need a Subscriber to start sending data.
• Can be transformed or combined using Operators before reaching a Subscriber.

---

Example: News Publisher

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 (PassthroughSubject)
class NewsPublisher {
    let news = PassthroughSubject<String, Never>() // Publishes strings (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: "Breaking: Swift 6 Announced!")
newsPublisher.publish(newsHeadline: "New Combine Features in iOS 18!")