Detect a Null value in NSDictionary - ios

Question

I have an NSDictionary that's populated from a JSON response from an API server. Sometimes the values for a key in this dictionary are Null
I am trying to take the given value and drop it into the detail text of a table cell for display.
The problem is that when I try to coerce the value into an NSString I get a crash, which I think is because I'm trying to coerce Null into a string.
What's the right way to do this?
What I want to do is something like this:
cell.detailTextLabel.text = sensor.objectForKey( "latestValue" ) as NSString
Here's an example of the Dictionary:
Printing description of sensor:
{
"created_at" = "2012-10-10T22:19:50.501-07:00";
desc = "<null>";
id = 2;
"latest_value" = "<null>";
name = "AC Vent Temp";
"sensor_type" = temp;
slug = "ac-vent-temp";
"updated_at" = "2013-11-17T15:34:27.495-07:00";
}
If I just need to wrap all of this in a conditional, that's fine. I just haven't been able to figure out what that conditional is. Back in the Objective-C world I would compare against [NSNull null] but that doesn't seem to be working in Swift.

Answer 1

You can use the as? operator, which returns an optional value (nil if the downcast fails)
if let latestValue = sensor["latestValue"] as? String {
cell.detailTextLabel.text = latestValue
}
I tested this example in a swift application
let x: AnyObject = NSNull()
if let y = x as? String {
println("I should never be printed: \(y)")
} else {
println("Yay")
}
and it correctly prints "Yay", whereas
let x: AnyObject = "hello!"
if let y = x as? String {
println(y)
} else {
println("I should never be printed")
}
prints "hello!" as expected.

Answer 2

You could also use is to check for the presence of a null:
if sensor["latestValue"] is NSNull {
// do something with null JSON value here
}

Answer 3

I'm using this combination and it also checks if object is not "null".
func isNotNull(object: AnyObject?) -> Bool {
guard let object = object else { return false }
return isNotNSNull(object) && isNotStringNull(object)
}
func isNotNSNull(object: AnyObject) -> Bool {
object.classForCoder != NSNull.classForCoder()
}
func isNotStringNull(object: AnyObject) -> Bool {
guard let object = object as? String where object.uppercaseString == "NULL" else {
return true
}
return false
}
It's not that pretty as extension but work as charm :)

Answer 4

NSNull is a class like any other. Thus you can use is or as to test an AnyObject reference against it.
Thus, here in one of my apps I have an NSArray where every entry is either a Card or NSNull (because you can't put nil in an NSArray). I fetch the NSArray as an Array and cycle through it, switching on which kind of object I get:
for card:AnyObject in arr {
switch card { // how to test for different possible types
case let card as NSNull:
// do one thing
case let card as Card:
// do a different thing
default:
fatalError("unexpected object in card array") // should never happen!
}
}
That is not identical to your scenario, but it is from a working app converted to Swift, and illustrates the full general technique.

Answer 5

my solution for now:
func isNull(someObject: AnyObject?) -> Bool {
guard let someObject = someObject else {
return true
}
return (someObject is NSNull)
}
tests look good so far...

Answer 6

I had a very similar problem and solved it with casting to the correct type of the original NSDictionary value. If your service returns a mixed type JSON object like this
{"id":2, "name":"AC Vent Temp", ...}
you'll have to fetch it's values like that.
var id:int = sensor.valueForKey("id") as Int;
var name:String? = sensor.valueForKey("name") as String;
This did solve my problem. See BAD_INSTRUCTION within swift closure