使用关联类型在Swift中使用协议一致性的问题
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例子来说明这个问题:生产者生产ItemType
兼容物品和消费者。它遵循:使用关联类型在Swift中使用协议一致性的问题
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)")
}
}
的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) {
^
任何建议,关于解决方案(如果存在)这个问题的?
答
你应该定义你的类DummyProducer
和DummyConsumer
这样的:
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) {
}
}
既然你严格规定associatedType
T
要在协议定义ItemType
,你不能仅仅因为EmptyItem
是EmptyItem
在使用你的类不是唯一可以采用ItemType
协议的结构。