What does conforming to _ObjectiveCBridgeable really do ?

Hi everyone.

I realy wonder what the purpose of the _ObjectiveCBridgeable protocol is.

Say that you make a Swift struct that could bridge to NSNumber. A thing simple enough to look like that:

struct IntHolder {
     var holdInt: Int
     init(value: Int) {
          self.holdInt = value
     }

     mutating func makeItDouble() {
          holdInt *= 2
     }
}

Now say that you want to bridge it to ObjectiveC to bind an IntHolder property to another for exemple. All you'll (normaly) need is to conform IntHolder to _ObjectiveCBridgeable protocol:

extension IntHolder: _ObjectiveCBridgeable {
    typealias _ObjectiveCType = NSNumber

    init(fromObjectiveC source: _ObjectiveCType) {
        self.holdInt = source.integerValue
    }

    static func _getObjectiveCType() -> Any.Type {
        return _ObjectiveCType.self
    }
    static func _isBridgedToObjectiveC() -> Bool {
        return true
    }
    func _bridgeToObjectiveC() -> IntHolder._ObjectiveCType {
        return NSNumber(integer: self.holdInt)
    }
    static func _forceBridgeFromObjectiveC(source: IntHolder._ObjectiveCType, inout result: IntHolder?) {
        result = IntHolder(fromObjectiveC: source)
    }
    static func _conditionallyBridgeFromObjectiveC(source: IntHolder._ObjectiveCType, inout result: IntHolder?) -> Bool {
        _forceBridgeFromObjectiveC(source, result: &result)
        return true
    }
}

Now you can use IntHolder as any objc class without forgeting to make anotate the property as dynamic (best with binding):

class Dummy: NSObject {
    dynamic var holder: IntHolder
    override init() {
        self.holder = IntHolder(value: 22)
    }
}

And..... :

error: property cannot be marked dynamic because its type cannot be represented in Objective-C dynamic var holder: IntHolder
note: Swift structs cannot be represented in Objective-C dynamic var holder: IntHolder

So is _ObjectiveCBridgeable useful for things other than :

let arr = [IntHolder(value: 1), IntHolder(value: 2), IntHolder(value: 3)]
let bridgedArr = arr as NSArray

?

If yes how can I do ?