swift NSNumber 转 Sting 遇到的问题

编译器报错：Could not cast value of type ‘__NSCFNumber‘ (0x...) to ‘NSString‘ (0x...).

解决方案：

The value is an NSNumber,
not an NSString.
You can use stringValue to
convert it:

if let a = d["a"] as? NSNumber {
    let aString = a.stringValue
    println(aString) // -1
}

If you‘re sure it‘s there, you can use forced unwrapping and string interpolation:

let a = d["a"]! as! NSNumber
let aString = "\(a)"