swift - 使用从另一个协议(protocol)继承的协议(protocol)作为关联类型

Question

我正在尝试创建一个具有两种关联类型的协议(protocol)。这些关联类型之一适用于委托(delegate)。当我尝试使用另一个协议(protocol)作为关联类型时，收到错误“类型‘HostConnectionService’不符合协议(protocol)‘ConnectionService’”。我的代码写在下面:

protocol ConnectionService: class {
    associatedtype Peer: Sharelist.Peer
    associatedtype Delegate: ConnectionServiceDelegate
    var connectedPeers: [Peer] { get }
    var delegate: Delegate? { get }
}

protocol ConnectionServiceDelegate { }

// Error
class HostConnectionService: NSObject, ConnectionService {
    typealias Peer = GuestPeer
    typealias Delegate = HostConnectionServiceDelegate

    var delegate: HostConnectionServiceDelegate?
    var connectedPeers: [GuestPeer] = []
}

protocol HostConnectionServiceDelegate: ConnectionServiceDelegate { }

当我改变线路时

typealias Delegate = HostConnectionServiceDelegate

成为非协议(protocol)类型，我不再收到错误:

struct NonProtocolConnectionServiceDelegate: ConnectionServiceDelegate { }

// No Error
class HostConnectionSertice: NSObject, ConnectionService {
    ...
    typealias Delegate = NonProtocolConnectionServiceDelegate
    ...
}

这是一个基本的 Swift 限制，还是我做错了什么？

Answer 1

你的例子太复杂了，难以理解。我试图简化它。

编译没有错误:

protocol ProtocolA {}

protocol ProtocolB {
    associatedtype SomeType
}

class SomeClass: ProtocolB {
    typealias SomeType = ProtocolA
}

let object = SomeClass()

但是下面的例子不再编译:

protocol ProtocolA {}

protocol ProtocolB {
    associatedtype SomeType: ProtocolA
}

class SomeClass: ProtocolB {
    typealias SomeType = ProtocolA
}

错误如下:

error: type 'SomeClass' does not conform to protocol 'ProtocolB'

note: possibly intended match 'SomeType' (aka 'ProtocolA') does not conform to 'ProtocolA'

这是因为protocols don't conform to themselves

在您的情况下，很可能需要制作类模板:

protocol ProtocolA {}

protocol ProtocolB {
    associatedtype SomeType: ProtocolA
}

class SomeClass<T: ProtocolA>: ProtocolB {
    typealias SomeType = T
}

extension Int: ProtocolA {}
extension Double: ProtocolA {}

let object1 = SomeClass<Int>()
let object2 = SomeClass<Double>()