ViewController

class ViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()
        
        let ani = Animal()
        ani.name = "Dog"
        ani.weight = 20
        ani.age = 3
        
        //to JSON String -> File
        //字典/数组
        let dict = ani.toDict()

        //NSData
        let data = try! NSJSONSerialization.dataWithJSONObject(dict, options: [])
        let s = NSString(data: data, encoding: NSUTF8StringEncoding)
        print(s!)
        
        let path1 = NSHomeDirectory() + "/Documents/1.json"
        try! data.writeToFile(path1, options: .AtomicWrite)
        
        let path2 = NSHomeDirectory() + "/Documents/2.json"
        try! s?.writeToFile(path2, atomically: true, encoding: NSUTF8StringEncoding)
        
        print(path1)
        
        let data2 = NSData(contentsOfFile: path1)
        let dict2 = try! NSJSONSerialization.JSONObjectWithData(data2!, options: .AllowFragments) as! NSDictionary
        let ani2 = Animal(dict: dict2)

        print(ani2.name, ani2.age, ani2.weight)
    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }


}

Animal类

class Animal: NSObject {
    var weight: Double = 0
    var name: String!
    var age: Int = 0
    
    override init() {
        super.init()
    }
    
    init(dict: NSDictionary) {
        super.init()
        
        //一定要检查安全性
        if let n = dict["name"] as? String {
            name = n
        }
        
        age = dict["age"] as! Int
        weight = dict["weight"] as! Double
    }
    
    func toDict() -> Dictionary<String, AnyObject> {
        var dict = Dictionary<String, AnyObject>()
        dict["name"] = "Dog"
        dict["weight"] = 20
        dict["age"] = 3
        
        return dict
    }
}