Swift:为符合通用协议(protocol)的通用类创建包装类时出错

Question

我想为符合通用协议(protocol)的通用类创建一个包装器类，但由于某些原因我无法使其正常工作。

背后的想法是在应用程序中使用包装器 AnyNetworkRequest 作为 Erased-Type，这样就不需要像 _NetworkRequest 那样定义通用类型。

我看不到 AnyNetworkRequest 上有什么遗漏/错误。如果有人能指出我遗漏或错误的地方，我将不胜感激。

// Protocol with associatedtypes

public protocol NetworkRequest {

    associatedtype RequestSerializationType: RequestSerializationProtocol
    associatedtype RequestResponseType: NetworkResponseProtocol

    var requestSerializer: RequestSerializationType { get }
    var requestResponse: RequestResponseType? { get }
}


// Generic Request

public class _NetworkRequest<RequestSerializationType: RequestSerializationProtocol, RequestResponseType: NetworkResponseProtocol>: NetworkRequest {

    fileprivate init() {}

    public lazy var requestSerializer: RequestSerializationType = { RequestSerializationType.init() }()
    public var requestResponse: RequestResponseType?
}

// Concrete Request

public class DataNetworkRequest: _NetworkRequest<ConcreteHTTPRequestSerializer, ConcreteDataNetworkResponse> {}

// Concrete Request

public class JSONDataNetworkRequest: _NetworkRequest<ConcreteJSONRequestSerializer, ConcreteJSONDataNetworkResponse> {}


// Type Erased Wrapper
// Cannot make this wrapper work
// Error 1: Type 'AnyNetworkRequest' does not conform to protocol 'NetworkRequest'
// Error 2: Reference to invalid associated type 'RequestSerializationType' of type 'AnyNetworkRequest'

public class AnyNetworkRequest : NetworkRequest { //E1

    private let request : _NetworkRequest<RequestSerializationType, RequestResponseType> //E2

    init<T: NetworkRequest>(_ networkRequest: T) where T.RequestSerializationType == RequestSerializationType, T.RequestResponseType == RequestResponseType {
        request = networkRequest
    }
}

编辑:第一次修改

// Protocol with associatedtypes

public protocol NetworkRequest {

    associatedtype RequestSerializationType: RequestSerializationProtocol
    associatedtype RequestResponseType: NetworkResponseProtocol

    var requestSerializer: RequestSerializationType { get }
    var requestResponse: RequestResponseType? { get }
}


// Generic Request

public class _NetworkRequest<RST: RequestSerializationProtocol, RRT: NetworkResponseProtocol>: NetworkRequest {

    public typealias RequestSerializationType = RST
    public typealias RequestResponseType = RRT


    fileprivate init() {}

    public lazy var requestSerializer: RequestSerializationType = { RequestSerializationType.init() }()
    public var requestResponse: RequestResponseType?
}

// Concrete Request

public class DataNetworkRequest: _NetworkRequest<ConcreteHTTPRequestSerializer, ConcreteDataNetworkResponse> {}

// Concrete Request

public class JSONDataNetworkRequest: _NetworkRequest<ConcreteJSONRequestSerializer, ConcreteJSONDataNetworkResponse> {}


// Type Erased Wrapper
// Cannot make this wrapper work
// Error 1: Type 'AnyNetworkRequest' does not conform to protocol 'NetworkRequest'
// Error 2: Reference to invalid associated type 'RequestSerializationType' of type 'AnyNetworkRequest'

public class AnyNetworkRequest : NetworkRequest { //E1

    /*  // E1 forces me to include typealiases
        public typealias RequestSerializationType = <#type#>
        public typealias RequestResponseType = <#type#>
    */

    private let request : _NetworkRequest<RequestSerializationType, RequestResponseType>

    var requestSerializer: RequestSerializationType { //E2
        return request.requestSerializer
    }

    var requestResponse: RequestResponseType? {
        return request.requestResponse
    }

    init<T: NetworkRequest>(_ networkRequest: T) where T.RST == RequestSerializationType, T.RRT == RequestResponseType {
        request = networkRequest
    }
}

Answer 1

错误很容易解释。

1) 您的AnyNetworkRequest 类确实不符合NetworkRequest 协议(protocol)。顺便说一句，我不明白为什么它应该这样做。未满足的要求是 requestSerializer 和 requestResponse 属性以及所需的类型别名。通常，您可以将其重写如下:

public class AnyNetworkRequest: NetworkRequest {

    var requestSerializer: RequestSerializationType { 
       return request.requestSerializer
    }

    var requestResponse: RequestResponseType? { 
       return request.requestResponse
    }

    private let request : _NetworkRequest<RequestSerializationType, RequestResponseType>

   // ...
}

但它需要我们去

2) 需要为关联类型指定内容的地方。您不能在泛型声明中使用 RequestSerializationType 和 RequestResponseType，因为它们不是具体类型。

所以你不能以这种方式执行类型删除。

除了输入更少的字母之外，我不知道为什么你需要在这里摆脱泛型，但我可以提供的是使用类型别名，比如:

typealias DataNetworkRequest = _NetworkRequest<ConcreteHTTPRequestSerializer, ConcreteDataNetworkResponse>

typealias JSONDataNetworkRequest = _NetworkRequest<ConcreteJSONRequestSerializer, ConcreteJSONDataNetworkResponse>

这样您就可以避免冗余继承并拥有更清晰表达的类型，以防这是您的目标。

AnyNetworkRequest 在这种情况下根本不需要。