CODEWITHSUNDEEP
iosswiftnsarraynsdictionary
user5273262
user5273262

Reputation:

Intricate access to dictionary key

From a server I receive a JSON string, then I try to convert it to an NSDictionary this way:

let JSON = try NSJSONSerialization.JSONObjectWithData(rToData!, options:[])
guard let JSONDictionary:NSDictionary = (JSON as! NSDictionary) else {
print("My grandma is way more NSDictionary than this")
return
}

Once converted, I try to get some data contained in the dictionary: in particular I need an array I can access this way:

let myArray = JSONDictionary["data1"][0]["data2"];

XCode really doesn't like this idea, it puts an arrow under the first bracket and says Value of optional type "AnyObject?" not unwrapped, did you mean to use "!" or "?" ?. I follow its suggestion and I insert a "!", converting my preceding code to this:

let myArray = JSONDictionary["data1"]![0]["data2"];

At this point, the following line (where I count the number of elements in data2) shows an error, stating AnyObject has no member count.

The only thing that seems to work fine is this solution but, apart from being ugly and unreadable, I really don't understand it:

let myArray = (JSONDictionary["data1"]?[0]["data2"])!;

Can you help me understand why this basic access to a key in a dictionary must be so intricate?

I must say I like Swift but I spend a lot of time dealing with optionals and bizarre XCode alerts.

Upvotes: 0

Views: 47

Answers (1)

Paulw11
Paulw11

Reputation: 114846

There is no guarantee that your JSON dictionary will contain a value for the key data1 (OK, you know it will, but Swift doesn't) so JSONDictionary["data1"] returns an optional. You need to unwrap the optional with ? or !

Also, since you have an NSDictionary, not a Swift dictionary, Swift doesn't know the type of the values, so they are AnyObject. Now again, you know it is an array, but Swift doesn't so you get an error stating that AnyObject doesn't have a count method.

While it is more verbose, it is cleaer for both the compiler and anyone else looking at your code if you split the line into multiple lines. It also lets you downcast the various objects so that Swift knows what is going on and handle any malformed JSON;

if let array1 = JSONDictionary["data1"] as? NSArray {
   if let dictionary1 = array1[0] as? NSDictionary {
       if let data2Array = dictionary1["data2"] as? NSArray {
          let count=data2Array.count
       }
   }
}

You could implement appropriate else statements to handle errors

Optionals are one of Swift's most powerful features. They help avoid a whole family of bugs associated with uninitialised variables and special sentinnel values for boundary conditions. It is important that you learn how they can help you and not just throw ? or ! at your code until it compiles.

Upvotes: 1

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