Detect a Null value in NSDictionary

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.