explain passthroughsubject in swift

PassthroughSubject is a Subject in Combine framework, it acts both as a publisher and a subscriber.

The PassthroughSubject allows elements to be published by calling its send() method. When PassthroughSubject receives a subscriber, it keeps a reference to this subscriber and forward all published elements to it.

Here's an example of how to use PassthroughSubject:

main.swift
import Combine 

let passThroughSubject = PassthroughSubject<String, Never>()

let subscriber = passThroughSubject.sink {stringValue in
   print("Received Value: \(stringValue)") 
}

passThroughSubject.send("Hello") // will print: "Received Value: Hello"
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In the example above we created an instance of PassthroughSubject for a String, indicating that it will never encounter any errors (hence the Never). We then added a subscriber by calling .sink method on passthroughsubect. Finally, we sent a string value to the subject by calling .send() method.

Whenever we send a value to a PassthroughSubject, it will immediately forward that value to all existing subscribers.

PassthroughSubject is very useful when you want to emit events that may come from a source outside of the Combine framework, such as user interactions or data received from the network.

It is important to note that PassthroughSubject must be treated as a class, and not a struct. Its lifecycle depends on external references kept alive to it.