多泛型类继承

Question

考虑我有一个模型，并为它的基本模型：

class BaseModel: NSObject { 
    var name = "" 
} 

class Model: BaseModel { 
    var desc = "abc" 
} 

我增加了一个服务，这些模型的工作：

class AbstractService<T: NSObject>: NSObject { 
    var items = [T]() 
} 

class BaseModelService<T: BaseModel>: AbstractService<BaseModel> { 
} 

class ModelService<T: Model>: BaseModelService<Model> { 
} 

现在我要访问从ModelServiceitems变量，并期望得到的Model秒的阵列，但我得到BaseModel！而非：

我是否错过了泛型？

下面是游乐场的全码：

//: Playground - noun: a place where people can play 

import UIKit 

/// Models: 
class BaseModel: NSObject { 
    var name = "" 
} 

class Model: BaseModel { 
    var desc = "abc" 
} 

/// Services: 
class AbstractService<T: NSObject>: NSObject { 
    var items = [T]() 
} 

class BaseModelService<T: BaseModel>: AbstractService<BaseModel> { 
} 

class ModelService<T: Model>: BaseModelService<Model> { 
} 

/// ViewController: 
class ViewController: UIViewController { 

    let modelService = ModelService() 
    var items = [Model]() 

    override func viewDidLoad() { 
     super.viewDidLoad() 

     items = modelService.items // The error appears here: Cannot assign a value of type [BaseModel] to a value of type [Model] 
    } 

} 

Answer 1

在你的情况，声明应该是：

class BaseModelService<T: BaseModel>: AbstractService<T> { 
} 

class ModelService: BaseModelService<Model> { 
} 

当你这样做：

class BaseModelService<T: BaseModel>: AbstractService<BaseModel> { 
} 

T为AbstractService和T为BaseModelService是两种截然不同的类型。 前者限于混凝土BaseModel，而后者限制为BaseModel的亚型，根本不使用。

Answer 2

的问题是你逼BaseModelService是的AbstractModelService<BaseModel>子类，这意味着items键入[BaseModel]。声明BaseModelService如下：

class BaseModelService<T: BaseModel>: AbstractService<T> { 

那么你是好去。