Swift泛型无法正常工作，JSON字典来自Objective-C解析器

Question

I have a function to retrieve non optional value from JSON dictionary. It takes 3 params: dictionary, key and default value for a case where is no value in a dictionary or value have wrong type

func valueFromJSONDict<T>(dict:[NSObject: AnyObject]?, key: NSObject, defaultValue: T) -> T {
    if let value = dict?[key] as? T {
        return value
    }
    return defaultValue
}

The problem is it always returns the default value.

title = valueFromJSONDict(dict, "title", "")

but if I do same thing without generics it works fine:

title = { () -> String 
   if let value = dict?["title"] as? String {
      title = value
   } else {
      return ""
   }
}()

This parsed JSON dict is coming from Objective-C parser and if I println dynamicType of value from the dict is like __CFNSString , but T.self is Swift.String , so Swift is unable to figure out types probably

How do I make it work?

Answer 1

What does your dict object look like?

Here is what I would suggest:

typealias JSONObject = [String: AnyObject]

func valueFromJSON<T>(json: JSONObject?, key: String, defaultValue: T) -> T
{
    if let value = json?[key] as? T {
        return value
    }
    return defaultValue
}

let json = ["Name": "Sir Lancelot",
            "Quest": "To seek the Holy Grail",
            "Favorite Color": "Blue..."]

let name = valueFromJSON(json, "Name", "Sir Not Appearing In This Film")

Answer 2

I discovered, that method actually work is I assign the result to some var or let :

let value = valueFromJSONDict(dict, "title", "") // value from JSON

but didn't work if I assign the result to a property :

title = valueFromJSONDict(dict, "title", "") // always ""

So finally I've fixed this issue by replacing my property type from non-optional @NSManaged var title: String to implicitly unwrapped optional @NSManaged var title: String!