有无法识别的选择器-replacementObjectForKeyedArchiver：在Swift中实现NSCoding时崩溃

我创建了一个符合NSCoding的Swift类。（Xcode 6 GM，Swift 1.0）

import Foundation

private var nextNonce = 1000

class Command: NSCoding {

    let nonce: Int
    let string: String!

    init(string: String) {
        self.nonce = nextNonce++
        self.string = string
    }

    required init(coder aDecoder: NSCoder) {
        nonce = aDecoder.decodeIntegerForKey("nonce")
        string = aDecoder.decodeObjectForKey("string") as String
    }

    func encodeWithCoder(aCoder: NSCoder) {
        aCoder.encodeInteger(nonce, forKey: "nonce")
        aCoder.encodeObject(string, forKey: "string")
    }
}

但是当我打电话...

let data = NSKeyedArchiver.archivedDataWithRootObject(cmd);

它崩溃给了我这个错误。

2014-09-12 16:30:00.463 MyApp[30078:60b] *** NSForwarding: warning: object 0x7a04ac70 of class '_TtC8MyApp7Command' does not implement methodSignatureForSelector: -- trouble ahead
Unrecognized selector -[MyApp.Command replacementObjectForKeyedArchiver:]

我该怎么办？

尽管Swift类无需继承即可工作，但是要使用它，NSCoding您必须从继承NSObject。

class Command: NSObject, NSCoding {
    ...
}

太糟糕了，编译器错误不是很有用:(