Publishers and Subscribers
The Publisher/Subscriber contract, sink and assign built-in subscribers.
Welcome
Combine is Apple's reactive framework. At its core are Publishers (produce values) and Subscribers (consume them).
Publisher Protocol
A `Publisher` emits values over time and eventually completes or fails:
```swift
publisher
.sink { print("Received: \($0)") }
```
The `Output` type is the value type; `Failure` is the error type.