URLSchemeConvertible.swift

protocol URLSchemeConvertible {
    var url: URL? { get }
    var urlString: String { get }
}

extension URLSchemeConvertible {
    var queryItems: [URLQueryItem]? {
        return URLComponents(string: urlString)?.queryItems
    }

    var host: String? {
        return url?.host
    }

    func queryValue(for key: String) -> String? {
        let lowcasedKey = key.lowercased()
        return queryItems?.first { $0.name.lowercased() == lowcasedKey }?.value
    }
}

extension String: URLSchemeConvertible {
    var url: URL? {
        if let url = URL(string: self) {
            return url
        }
        var set = CharacterSet()
        set.formUnion(.urlHostAllowed)
        set.formUnion(.urlPathAllowed)
        set.formUnion(.urlQueryAllowed)
        set.formUnion(.urlFragmentAllowed)
        return self.addingPercentEncoding(withAllowedCharacters: set).flatMap { URL(string: $0) }
    }

    var urlString: String {
        return self
    }
}

extension URL: URLSchemeConvertible {
    var url: URL? {
        return self
    }

    var urlString: String {
        return self.absoluteString
    }
}