spot.swift

extension URLRequest {

    public enum HTTPMethod: String {
        case get = "GET"
        case put = "PUT"
        case post = "POST"
        case delete = "DELETE"
        case head = "HEAD"
        case options = "OPTIONS"
        case trace = "TRACE"
        case connect = "CONNECT"
    }

}
typealias SpotifyQueryParameter = [String: String]

enum APIPath: String {
    case search = "search"
    case ...
    ...and so on
}

struct APIResponse {
    let error: Error?
    let result: [String: AnyObject]?
}

func searchArtist(queryParams: [SpotifyQueryParameter], completionHandler: ((APIResponse) -> Void)) {
    let request = createSpotifyRequest(path: .search, parameters: queryParams, method: .get, completionHandler: completionHandler)
    request.resume()
}

func createSpotifyRequest(path: String, method: URLRequest.HTTPMethod, completionHandler: ((APIResponse) -> Void)) {
    ...
}

// Somewhere in your app:

searchArtist(queryParams: ["artist": "Katy Perry"], completionHandler: { response in
    if let error = response.error { print(error); return }
    // deal with response.result
})