Swift 泛型类初始化器

Question

我发现在 swift 中设计结果类有问题

struct Result<T>{
    private let data: T?
    let error: NSError?
    var isSuccess: Bool {
        get {
            return data != nil
        }
    }

    init(data: T) {
        self.data = data
        self.error = nil
    }

    init(error: NSError) {
        self.data = nil
        self.error = error
    }
}

用法应该是这样的

Result(data: "something") // T as string

当我想传递错误时出现问题

Result(error: errorFromSomewhere) //T is not specified

下面是真实的应用用法:

class ParseRegistrationProvider: RegistrationProvider {
    func register(model: RegistrationForm) -> Promise<Result<String>> {
        return Promise { accept, reject in
            let user = PFUser()
            user.username = model.nickName
            user.password = model.password
            user.email = model.emailAdreess
            user.signUpInBackgroundWithBlock({ (isSuccess, error) -> Void in
                if isSuccess {
                    accept(Result(data: "OK"))
                } else {
                    var errorResult = Result(error: error!) //causes error
                    reject(errorResult)
                }
            })
        }
    }
}

“errorResult”导致编译器错误:

Generic parameter T could not be inferred

更新:此方法正常工作:

Result<String>(error: errorFromSomewhere)

Answer 1

我想建议像这样对 Result 使用变体类型，因为它更紧凑并且可以自然地用于模式匹配:

enum Result<T>{
    case Data(T?)
    case Error(NSError)
    var isSuccess: Bool{
        get{
            switch self{
            case .Data(_?):
                return true
            default:
                return false
            }
          }
    } }

你的代码可能会变成这样:

class ParseRegistrationProvider: RegistrationProvider {
    func register(model: RegistrationForm) -> Promise<Result<String>> {
        return Promise { accept, reject in
            let user = PFUser()
            user.username = model.nickName
            user.password = model.password
            user.email = model.emailAdreess
            user.signUpInBackgroundWithBlock({ (isSuccess, error) -> Void in
                if isSuccess {
                    accept(Result<String>.Data("OK"))
                } else {
                    var errorResult = Result<String>(error: error!) //causes error
                    reject(errorResult)
                }
            })
        }
    }