Explain the Cold concept in the context of using Combine in Swift.

In Combine in Swift, the concept of Cold Publisher means that the publisher does not start generating and sending values until a subscriber subscribes to it.

That is, each subscriber receives its own data stream from the moment of subscription, and data is not generated in advance.

Example:

let publisher = [1, 2, 3].publisher // Cold publisher

publisher.sink { print("Subscriber 1 received: \($0)") }
publisher.sink { print("Subscriber 2 received: \($0)") }

In this example, both subscribers will receive all values from the beginning because the publisher is "cold" and starts emitting data only upon subscription.