Swift 2 - 类型转换和可选链接

Question

I am relatively new to Swift and programming. I'm developing an app which heavily relies on information downloaded from the server. So in a lot of ViewControllers, I use NSURLSession and NSJSONSerialization to download the JSON into my app.

Every time I wanted to subscript the dictionary, for example timetableDict?["timetable"]["class"]["day"]["lesson"][0]["name"] , something like Cannot subscript a value of type [String : AnyObject] with an index type of String shows up as an error.

I understand that I should avoid using AnyObject in my code, but the dictionary from the server is heavily nested with structures like this one:

"timetable": ["class": ({
                        day = ({
                            lesson = ({
                                   name = (MATHEMATICS, ENGLISH),
                                   classOrder = 0,
                                   teacher = (Someone)
                                      }),
                                     ({
                                   name = FRENCH,
                                   classOrder = 1,
                                   teacher = (Someone)
                                      )}
                               )}
                        )}]

The problem with this structure is that it is heavily nested and has different types when it gets to "name", "classOrder" and "teacher". It is very hard for me not to use AnyObject. However, this error has been annoying for me for a very long time. I would greatly appreciate it if someone could help me out on this. Thanks in advance!

Answer 1

I suggest taking a look at SwiftyJSON : https://github.com/SwiftyJSON/SwiftyJSON

It's a framework/library designed to handle JSON in a very much more elegant way than what's build into swift (especially for heavy nested structures like yours). It's easy to use and has an excellent tutorial.

EDIT: (Added sample code)

Example from the swiftyJSON tutorial :

let JSONObject: AnyObject? = NSJSONSerialization.JSONObjectWithData(data, options: nil, error: nil)

if let statusesArray = JSONObject as? [AnyObject],
   let status = statusesArray[0] as? [String: AnyObject],
   let user = status["user"] as? [String: AnyObject],
   let username = user["name"] as? String {
    // Finally we got the username
}

even with optional chaining quite messy :

let JSONObject: AnyObject? = NSJSONSerialization.JSONObjectWithData(data, options: nil, error: nil)

if let username = (((JSONObject as? [AnyObject])?[0] as? [String: AnyObject])?["user"] as? [String: AnyObject])?["name"] as? String {
    // What a disaster
}

With swiftyJSON

let json = JSON(data: dataFromNetworking)
if let userName = json[0]["user"]["name"].string {
  //Now you got your value
}

Answer 2

SwiftlyJSON that @Glenn mentions is not a bad system (though I find it over-reliant on string lookups, which is fragile). The deeper point, however, is that you want to validate and parse your JSON in one place, and turn it into a non-JSON Swift data structure for use by the rest of your program. SwiftlyJSON can be a decent tool for doing that, but I would use it to unload data into a an array of structs.

Working with JSON throughout your system is extremely error-prone and cumbersome, even wrapped up in SwiftlyJSON. If you unload the data once, then you can check for errors one time and everywhere else you know that the data is correct. If you pass around JSON structures, then you must check every single time and deal with possibly missing or incorrect data. Think through the case where the server sends you JSON in a format you didn't expect. How many times do you want to test for that?