专用通用协议只能用作通用约束

Question

创建了一个通用协议，并创建了一个专用协议，它解析了所有通用参数。 尝试将专用协议用作变量类型时，Xcode给出以下错误：Protocol 'MY_SPECIALIZED_PROTOCOL_NAME' can only be used as a generic constraint because it has Self or associated type requirements

尝试以下代码。

protocol GenericFetching {
    associatedtype FetchEntity
    associatedtype FetchOptions
    associatedtype FetchResult = Result<FetchEntity, Swift.Error>
}

protocol BananaService: class, GenericFetching where
    FetchEntity == [Banana],
    FetchOptions == Int {
}

class FruitStore {
    var bananaService: BananaService

    // ... more fruity code for our steamy company ...
}

预期：Swift意识到BananaService不需要任何其他通用参数，并允许开发人员使用该类型进行变量定义。

经验丰富：var bananaService: BananaService的定义出错。

❌Protocol 'BananaService' can only be used as a generic constraint because it has Self or associated type requirements

Answer 1

当您在协议（associatedtype）中使用泛型类型时，您不能将协议用作参考，您需要使class从协议中继承并设置typealias associatedtype。

示例：

protocol BananaServiceProtocol: class, GenericFetching {

}

class BananaService: BananaServiceProtocol {
    typealias FetchEntity = [Banana]

    typealias FetchOptions = Int

}

class FruitStore {
    var bananaService: BananaService

    // ... more fruity code for our steamy company ...
}