FocusSubject

Given a subject with an output of A, and functions (A)->B? and (B)->A, we provide a method that produces a new subject with output B.

    func focus<LocalOutput>(send:@escaping (LocalOutput) -> Output, receive:@escaping (Output)->LocalOutput?) -> FocusSubject<LocalOutput,Failure>

Alternatively you can provide a case path :

     func focus<LocalOutput>(_ casePath:CasePath<Output, LocalOutput>) -> FocusSubject<LocalOutput,Failure>

Motivation

Every subject has a map function because it is a publisher. If our subject has an output of type A, and we map a function (A)->B, the result is a Publisher with output of type B. However, we cannot call send(_:) on our new publisher because it is not a subject. In fact we don't provide enough information for map to achive this. We need another contravarient function (B)->A to create a Subject. With both functions, we can now focus our subject to a new subject with output B, and we can send values to our subject. This can be helpful to focus a subject defined on a broad domain onto something more specific.