为什么我不能像Swift中的接口一样使用此协议？

zpqajqem 于 2022-10-31 发布在 Swift

关注(0)|答案(2)|浏览(180)

我想使用一个协议作为接口，但是我在Swift中遇到了问题。有人能解释这个错误并给我指出正确的方向吗？
也许我对协议的理解完全不正确。
以下是示例代码：

protocol State {
    associatedtype U: State
    static func fromString(_: String) -> U
}

class ConcreteState1: State {
    static func fromString(_ value: String) -> ConcreteState1 {
        return ConcreteState1()
    }
}

protocol Loader {
    associatedtype T: State
    func load(completion: (Result<T.U, Error>) -> Void)
}

extension Loader {
    func load(completion: (Result<T.U, Error>) -> Void) {
        let value = T.fromString("")
        completion(.success(value))
    }
}

class ConcreteState1Loader: Loader {
    typealias T = ConcreteState1
}

// Abstraction to deal with various types of state
var mainState: (any State)?

// Needs to be an abstraction using the protocol because the specific loader is injected
// This is just a simplified reproduction of the issue
var loader: any Loader = ConcreteState1Loader()

// ERROR: Member 'load' cannot be used on value of type 'any Loader'; consider using a generic constraint instead
loader.load { result in
    if let state = try? result.get() {
        mainState = state
    }
}

swift

来源：https://stackoverflow.com/questions/74120130/why-cant-i-use-this-protocol-like-an-interface-in-swift

2条答案

按热度按时间

h43kikqp1#

无法使用此处的方法“load”，因为协议加载程序是通用的，它与另一个协议相关联。应在具体加载程序中定义具体状态，就像您在ConcreteState1Loader中定义的一样。
所以如果你要向下转换Loader

(oader as! ConcreteState1Loader).load { result in
    if let state = try? result.get() {
        mainState = state
    }
}

赞(0）回复(0）举报 2022-10-31

2g32fytz2#

这就是我想到的。我将State协议转换为类，并将静态方法更改为类方法，这样它就可以被覆盖。我依赖于一个新的GenericLoader，它可以使用这些类方法转换类型。
如果有更好、更聪明的方法，请告诉我。

class State {
    class func fromString(_: String) -> State {
        fatalError("must be implemented in derived class")
    }
}

class ConcreteState1: State {
    override class func fromString(_: String) -> State {
        return ConcreteState1()
    }
}

protocol Loader {
    func load(completion: (Result<State, Error>) -> Void)
}

class GenericLoader<T: State>: Loader {
    func load(completion: (Result<State, Error>) -> Void) {
        let value = T.fromString("")
        completion(.success(value))
    }
}

// Abstraction to deal with various types of state
var mainState: State?

// Needs to be an abstraction using the protocol because the specific loader is injected
// This is just a simplified reproduction of the issue
var loader: Loader = GenericLoader<ConcreteState1>()

loader.load { result in
    if let state = try? result.get() {
        mainState = state
    }
}

赞(0）回复(0）举报 2022-10-31

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为什么我不能像Swift中的接口一样使用此协议？

2条答案

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