delegateBasedConcurrencySample.swift

protocol DataFetcherDelegate {
    func fetchCompletedSuccessfully(_ things: [Thing]) -> Void
    func fetchCompletedUnsuccessfully(_ err:Error) -> Void
}

class Client {
    private let fetcher = DataFetcher()
    
    // ...

    void someFunc() {
        fetcher.delegate = self
        fetcher.fetch()
    }
}

extension Client: DataFetcherDelegate {
    func fetchCompletedSuccessfully(_ things: [Thing]) -> Void {
        // ...
    }
    func fetchCompletedUnsuccessfully(_ err:Error) -> Void {
        // ...
    }

}