使用协议时，Swift Array向下转换运行时错误

Question

Here's my setup:

protocol Client {
    var identifier: NSUUID { get }
}

class PeripheralClient: NSObject, Client { 
    var identifier: NSUUID {
        get {
            return peripheral.identifier
        }
    }
}

protocol NetworkManager {
    var clients: [Client] { get set }
}

class CentralNetworkManager: NSObject, NetworkManager {
    var clients = [Client]()
    var peripheralClients: [PeripheralClient] {
        get {
            return clients as [PeripheralClient]
        }
    }
}

I get this runtime error when the peripheralClients Array is accessed for the first time: array element cannot be bridged to Objective-C .

From this answer to a question with a similar error, it looks like swift requires the items in an Array to be AnyObject compatible when converting to NSArray . So that means Swift's Array typecasting is using NSArray, thus making it impossible for me to downcast from an Array whose type is a protocol.

Anyone have a good suggestion for getting around this?

Answer 1

Did you try declaring Client as an Objective-C protocol?

@objc protocol Client {
    var identifier: NSUUID { get }
}

Answer 2

Yeah, it seems, Swift itself does not have Array<T> to Array<U> casting functionality, it uses Obj-C facility.

Instead, to avoid that, you can cast each elements, using map for example:

var clients = [Client]()
var peripheralClients: [PeripheralClient] {
    get {
        return clients.map { $0 as PeripheralClient }
    }
}