Calls a closure with each received element and publishes any returned optional that has a value.
- iOS 13.0+
- macOS 10.15+
- Mac Catalyst 13.0+
- tvOS 13.0+
- watchOS 6.0+
- Xcode 11.0+
A closure that receives a value and returns an optional value.
A publisher that republishes all non-
nil results of calling the transform closure.