多泛型類繼承

Question

考慮我有一個模型，併爲它的基本模型：

class BaseModel: NSObject { 
    var name = "" 
} 

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

我增加了一個服務，這些模型的工作：

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

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

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

現在我要訪問從ModelServiceitems變量，並期望得到的Model秒的陣列，但我得到BaseModel！而非：

我是否錯過了泛型？

下面是遊樂場的全碼：

//: 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

在你的情況，聲明應該是：

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

class ModelService: BaseModelService<Model> { 
} 

當你這樣做：

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

T爲AbstractService和T爲BaseModelService是兩種截然不同的類型。 前者限於混凝土BaseModel，而後者限制爲BaseModel的亞型，根本不使用。

Answer 2

的問題是你逼BaseModelService是的AbstractModelService<BaseModel>子類，這意味着items鍵入[BaseModel]。聲明BaseModelService如下：

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

那麼你是好去。