Swift. How can I deal with null values into json files?

Question

I'm reading json from a URL and, again (I had the same issue with ObjectiveC) the values crash my app. I don't have any problems with Strings and Numbers. I can println(value) but when I assign the value into a UILabel, it crashes.

I use this method to read the JSON:

func jsonFromURL(jsonURL: String) -> Dictionary<String, AnyObject> {
    var jsonNSURL: NSURL = NSURL(string: jsonURL)
    let jsonSource: NSData = NSData(contentsOfURL: jsonNSURL)
    var json = NSJSONSerialization.JSONObjectWithData(jsonSource, options:NSJSONReadingOptions.MutableContainers, error: nil) as Dictionary<String, AnyObject>
    return json
}

...and this code to assign values into a UILabel inside a custom Cell

func tableView(tableView: UITableView!, cellForRowAtIndexPath indexPath: NSIndexPath!) -> UITableViewCell? {

    var regularTextCell:celda = celda(style: UITableViewCellStyle.Default, reuseIdentifier: "cell")
    var cell:celda = NSBundle.mainBundle().loadNibNamed("celda", owner: self, options: nil)[0] as celda

    cell.name.text = myJson["list"]![indexPath.row]!["name"] as String
    cell.id.text = "id: " + String(myJson["list"]![indexPath.row]!["id"] as Int)

    // THIS line crash because some values of adress are <null>
    cell.address.text = myJson["list"]![indexPath.row]!["address"] as String

    return cell

}

You can view an example of the JSON at: https://dl.dropboxusercontent.com/u/787784/example.json

Thanks!

possible duplicate of [Detect a Null value in NSDictionary](http://stackoverflow.com/questions/24026609/detect-a-null-value-in-nsdictionary) — Jeff, Jun 09 '14 at 22:15

GoZoner · Accepted Answer · 2017-04-07T11:35:01.777

17

You'll get an exception from syntax like object as String if object is not a String. You can avoid the exception by using object as? String which may result in a nil being assigned into your text.

In your specific case you could use:

cell.address.text = (myJson["list"]![indexPath.row]!["address"] as? String) ?? "default"

where I've replaced your as with as? and exploited the nil-coalescing operator ??.

edited Apr 07 '17 at 11:35

answered Jun 09 '14 at 20:45

GoZoner

67,920
20
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145

if I do: let text = object as? String, "text" I can't call String methods on text as it is not recognized as a String – Van Du Tran Jul 28 '14 at 03:08
1

If `object` can't be coerced to a `String` then `text` will be `nil`. – GoZoner Jul 28 '14 at 05:07

score 9 · Answer 2 · answered Sep 09 '14 at 15:16

9

I found this solution in online Swift tutorial. It's very nice solution how to deal with null value in Swift.

This is the concept:

 let finalVariable = possiblyNilVariable ?? "Definitely Not Nil Variable"

In tutorial example :

  let thumbnailURL = result["artworkUrl60"] as? String ?? ""

Here is the link for tutorial: http://jamesonquave.com/blog/developing-ios-8-apps-using-swift-interaction-with-multiple-views/

answered Sep 09 '14 at 15:16

Marijan V.

403
5
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I tried to do this but i wanted to unwrap the values by doing let thumbnailURL = (result["artworkUrl60"] as? String ?? "")! but didn't work is there a good way that while the assignment can i unwrap it? @Marijan V. – Cesar Mtz Aug 16 '16 at 03:29

score 6 · Answer 3 · answered Jun 09 '14 at 20:45

You can change the offending line to:

if let address = myJson["list"]![indexPath.row]!["address"] as? String {
  cell.address.text = address
}
else {
  cell.address.text = ""
}

The as? operator will cast the value the same way as would, but if the casting fails, it will instead assign nil

The if let … syntax combines this with a check and when the casting fails (and nil is returned) the else block runs instead

score 1 · Answer 4 · answered Jul 28 '14 at 22:23

If you use:

https://github.com/dankogai/swift-json/

You'll be able to go like this:

let yourJSON = JSON.fromURL("https://dl.dropboxusercontent.com/u/787784/example.json")
for (i, node) in yourJSON["list"] {
    let isnull = node["address"].isNull
    println("//[\"list\"][\(i)]:\t\(isnull)")
}

//["list"][0]:  false
//["list"][1]:  false
//["list"][2]:  true
//["list"][3]:  false
//["list"][4]:  false
//["list"][5]:  true
//["list"][6]:  false
//["list"][7]:  false
//["list"][8]:  false
//["list"][9]:  false
//["list"][10]: false
//["list"][11]: false
//["list"][12]: true
//["list"][13]: false
//["list"][14]: false
//["list"][15]: false
//["list"][16]: false
//["list"][17]: false
//["list"][18]: false
//["list"][19]: false

No ? or ! required and it does not crash on nonexistent nodes.

Swift. How can I deal with null values into json files?

4 Answers4