An example that shows how to map a publisher with an asynchronous function, using DispatchGroup

CombineAsyncMapping.swift

func makeUppercase(_ string: String, completion: (String) -> Void) {
    sleep(2)
    completion(string.uppercased())
}

let subject = CurrentValueSubject<String, Never>("foo")
let publisher = subject.eraseToAnyPublisher()

let group = DispatchGroup()

let mappedPublisher = publisher.map { string -> String in
    var uppercase: String = string
    group.enter()
    makeUppercase(string) { uppercaseString in
        uppercase = uppercaseString
        group.leave()
    }
    group.wait()
    return uppercase
}
mappedPublisher.sink { value in
    print(value)
}

subject.send("bar")
print("done")

/// The program prints:
/// FOO
/// BAR
/// done