Subjects

What are Subjects?

A Subject in Combine is both a Publisher and a Subscriber.
It acts as a bridge between imperative and reactive programming, allowing you to manually send values into a Combine pipeline.

Think of it like a news agency:

• A Publisher automatically sends news updates (values) as they happen.
• A Subject allows you to manually create and send news updates to subscribers.

Subjects let you push values into a Combine pipeline, acting as both an input and an output.

🔗 Official Docs: Apple Documentation

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How Subjects Work

• Acts as both a Publisher (emitting values) and a Subscriber (receiving values).
• Allows manually injecting values into a Combine stream.
• Multiple subscribers can listen to the same subject.

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Types of Subjects

1. PassthroughSubject → Emits values as soon as they are sent, without storing them.
2. CurrentValueSubject → Stores the most recent value and sends it to new subscribers.

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Example: Using a Subject to Broadcast Messages

import Combine
 
// 1. Create a subject
let newsSubject = PassthroughSubject<String, Never>()
 
// 2. Subscribe to the subject
let cancellable = newsSubject
    .sink { print("📢 Subscriber received: \($0)") }
 
// 3. Manually send values
newsSubject.send("Breaking News: Swift 6 Announced!")
newsSubject.send("New Combine Features Revealed!")

Example: Using CurrentValueSubject to Store Latest Value

import Combine
 
// 1. Create a CurrentValueSubject with an initial value
let latestNews = CurrentValueSubject<String, Never>("No news yet")
 
// 2. Subscribe to receive the latest news
let cancellable = latestNews
    .sink { print("📢 Latest news: \($0)") }
 
// 3. Update the stored value
latestNews.send("Breaking News: Swift 6 Announced!")
 
// 4. A new subscriber will get the latest value immediately
let newSubscriber = latestNews
    .sink { print("📢 New subscriber got: \($0)") }
 

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Key Takeaways

• Subjects act as both Publishers and Subscribers.
• PassthroughSubject → Does not store values; only sends them when available.
• CurrentValueSubject → Stores the latest value and sends it to new subscribers.
• Useful for bridging imperative and reactive code, such as user input handling.