Swift 中使用关联类型的协议一致性问题答案

【问题标题】：Issue with protocol conformance in Swift using associatedtypeSwift 中使用关联类型的协议一致性问题
【发布时间】：2016-09-07 12:12:19
【问题描述】：

我无法使类符合使用associatedtype 的协议。在 Playground 中，我输入了一个简短而简单的示例来说明该问题：一个生产符合 ItemType 的项目的生产者和一个消费它们的消费者。如下：

protocol ItemType { }

protocol Producer: class {
    associatedtype T: ItemType
    func registerConsumer<C: Consumer where C.T == T>(consumer: C)
}

protocol Consumer: class {
    associatedtype T: ItemType
    func consume<P: Producer where P.T == T>(producer: P, item: T)
}

struct EmptyItem: ItemType { }

class DummyProducer: Producer {
    var consumer: DummyConsumer?

    func registerConsumer(consumer: DummyConsumer) {
        self.consumer = consumer
    }
}

class DummyConsumer: Consumer {
    func consume(producer: DummyProducer, item: EmptyItem) {
        print("Received \(item) from producer \(producer)")
    }
}

Xcode 警告我以下错误：

Playground execution failed: MyPlaygroundYeYe.playground:14:7: error: type 'DummyProducer' does not conform to protocol 'Producer'
class DummyProducer: Producer {
      ^
MyPlaygroundYeYe.playground:3:20: note: protocol requires nested type 'T'
    associatedtype T: ItemType
                   ^
MyPlaygroundYeYe.playground:22:7: error: type 'DummyConsumer' does not conform to protocol 'Consumer'
class DummyConsumer: Consumer {
      ^
MyPlaygroundYeYe.playground:9:10: note: protocol requires function 'consume(_:item:)' with type '<P> (P, item: EmptyItem) -> ()' (aka '<τ_1_0> (τ_1_0, item: EmptyItem) -> ()')
    func consume<P: Producer where P.T == T>(producer: P, item: T)
         ^
MyPlaygroundYeYe.playground:23:10: note: candidate has non-matching type '(DummyProducer, item: EmptyItem) -> ()' [with T = EmptyItem]
    func consume(producer: DummyProducer, item: EmptyItem) {
         ^

关于该问题的解决方案（如果存在）有什么建议吗？

【问题讨论】：

标签： swift generics protocols

【解决方案1】：

您应该像这样定义您的类DummyProducer 和DummyConsumer：

class DummyProducer: Producer {
    typealias T = EmptyItem

    func registerConsumer<C: Consumer where C.T == T>(consumer: C) {

    }
}

class DummyConsumer: Consumer {
    typealias T = EmptyItem

    func consume<P: Producer where P.T == T>(producer: P, item: T) {

    }
}

由于您在协议定义中严格指定associatedType T 为ItemType，因此您不能在您的类中仅使用EmptyItem，因为EmptyItem 不是唯一可以采用@987654329 的结构@协议。

【讨论】：