A publisher that exposes a method for outside callers to publish elements.
- iOS 13.0+
- macOS 10.15+
- Mac Catalyst 13.0+
- tvOS 13.0+
- watchOS 6.0+
- Xcode 11.0+
A subject is a publisher that you can use to ”inject” values into a stream, by calling its
send(_:) method. This can be useful for adapting existing imperative code to the Combine model.