如何在Swift中创建通用协议？

Question

我想创建一个带有通用输入并返回通用值的方法的协议.

这是我到目前为止所尝试的,但它会产生语法错误.

使用未声明的标识符T.

我究竟做错了什么？

protocol ApiMapperProtocol {
    func MapFromSource(T) -> U
}

class UserMapper: NSObject, ApiMapperProtocol {
    func MapFromSource(data: NSDictionary) -> UserModel {
        var user = UserModel() as UserModel
        var accountsData:NSArray = data["Accounts"] as NSArray     
        return user
    } 
}

Answer 1

协议有点不同.查看Apple文档中的"关联类型" .

这是您在示例中使用它的方式

protocol ApiMapperProtocol {
    associatedtype T
    associatedtype U
    func MapFromSource(_:T) -> U
}

class UserMapper: NSObject, ApiMapperProtocol {
    typealias T = NSDictionary
    typealias U = UserModel

    func MapFromSource(_ data:NSDictionary) -> UserModel {
        var user = UserModel()
        var accountsData:NSArray = data["Accounts"] as NSArray
        // For Swift 1.2, you need this line instead
        // var accountsData:NSArray = data["Accounts"] as! NSArray
        return user
    }
}

Answer 2

稍微阐述一下Lou Franco的答案,如果你想创建一个使用特定方法的方法ApiMapperProtocol,那么你就这样做了:

protocol ApiMapperProtocol {
    associatedtype T
    associatedtype U
    func mapFromSource(T) -> U
}

class UserMapper: NSObject, ApiMapperProtocol {
    // these typealiases aren't required, but I'm including them for clarity
    // Normally, you just allow swift to infer them
    typealias T = NSDictionary 
    typealias U = UserModel

    func mapFromSource(data: NSDictionary) -> UserModel {
        var user = UserModel()
        var accountsData: NSArray = data["Accounts"] as NSArray
        // For Swift 1.2, you need this line instead
        // var accountsData: NSArray = data["Accounts"] as! NSArray
        return user
    }
}

class UsesApiMapperProtocol {
    func usesApiMapperProtocol<
        SourceType,
        MappedType,
        ApiMapperProtocolType: ApiMapperProtocol where
          ApiMapperProtocolType.T == SourceType,
          ApiMapperProtocolType.U == MappedType>(
          apiMapperProtocol: ApiMapperProtocolType, 
          source: SourceType) -> MappedType {
        return apiMapperProtocol.mapFromSource(source)
    }
}

UsesApiMapperProtocol现在保证只接受SourceType与给定的兼容ApiMapperProtocol:

let dictionary: NSDictionary = ...
let uses = UsesApiMapperProtocol()
let userModel: UserModel = uses.usesApiMapperProtocol(UserMapper()
    source: dictionary)

Answer 3

为了实现拥有泛型并且让它像这样声明，let userMapper: ApiMapperProtocol = UserMapper()你必须有一个符合协议的泛型类，它返回一个泛型元素。

protocol ApiMapperProtocol {
    associatedtype I
    associatedType O
    func MapFromSource(data: I) -> O
}

class ApiMapper<I, O>: ApiMapperProtocol {
    func MapFromSource(data: I) -> O {
        fatalError() // Should be always overridden by the class
    }
}

class UserMapper: NSObject, ApiMapper<NSDictionary, UserModel> {
    override func MapFromSource(data: NSDictionary) -> UserModel {
        var user = UserModel() as UserModel
        var accountsData:NSArray = data["Accounts"] as NSArray     
        return user
    } 
}

现在，您还可以将其userMapper称为ApiMapper具有特定实现的对象UserMapper：

let userMapper: ApiMapper = UserMapper()
let userModel: UserModel = userMapper.MapFromSource(data: ...)