Publishes the first element of a stream, then finishes.
- iOS 13.0+
- macOS 10.15+
- Mac Catalyst 13.0+
- tvOS 13.0+
- watchOS 6.0+
- Xcode 11.0+
A publisher that only publishes the first element of a stream.
If this publisher doesn’t receive any elements, it finishes without publishing.