如何在Swift中使用嵌套方便的可用初始值设定项检查nil？

class OAuthToken: NSObject,NSCoding {

var refreshToken: String?
var accessToken: String?
var scope: String?

convenience init?(refreshToken: String?,accessToken: String?,scope:String) {
    self.init()

    if let acutalRefreshToken = refreshToken as String? {
        self.refreshToken = acutalRefreshToken
    } else {
        return nil
    }
    if let actualAccessToken = accessToken as String? {
        self.accessToken = actualAccessToken
    }else {
        return nil
    }
    self.scope = scope
}

convenience init?(attributes: Dictionary<String,AnyObject>,scope: String) {
    var aRefreshToken: String!
    var anAccessToken: String?
    aRefreshToken = attributes["refresh_token"] as String?
    anAccessToken = attributes["access_token"] as String?
    let token = self.init(refreshToken: aRefreshToken,accessToken: anAccessToken,scope: scope) as OAuthToken // () is not convertible to OAuthToken
    if token != nil {
        storeInKeyChain()
    } else {
        return nil
    }
}
}

当你在另一个内部调用嵌套的可用初始化程序时,如何检查nil的可用初始化程序？

let token = self.init(refreshToken：aRefreshToken,accessToken：anAccessToken,scope：scope)想要返回一个type()对象,而不是可转换为我的类.我如何使用此模式并仅将对象存储到钥匙串(如果它实际上已成功创建)？