Checking Optionals for nil in Swift - json

Question

How to check Optionals in Swift for nil values?
I'm trying to evaluate if parsing JSON was successful. This should print a error message when invalid json is passed, but fails with EXC_BAD_INSTRUCTION.
func parseJson(data: NSData) {
var error: NSError?
var json: NSDictionary = NSJSONSerialization.JSONObjectWithData(data, options: NSJSONReadingOptions.MutableContainers, error: &error) as NSDictionary
if let err = error {
println("error parsing json")
return
}
println("json parsed successfully")
}
Try running with valid json (works fine):
parseJson(NSData(data: "{}".dataUsingEncoding(NSUTF8StringEncoding, allowLossyConversion: true)))
Try with invalid json:
parseJson(NSData(data: "123".dataUsingEncoding(NSUTF8StringEncoding, allowLossyConversion: true)))
Did I miss the point on working with Optionals?

Answer 1

The crash is happening because JSONObjectWithData returns nil if there's an error. You then try to cast nil to NSDictionary. This works:
func parseJson(data: NSData) {
var error: NSError?
var json : AnyObject! = NSJSONSerialization.JSONObjectWithData(data, options: NSJSONReadingOptions.MutableContainers, error: &error)
if let err = error {
println("error parsing json")
return
}
println("json parsed successfully")
var jsonDict = json as NSDictionary
}
If you prefer, you can switch the type of var json to NSDictionary? so it directly handles nil. To do this you just need to add a '?' to the "as" keyword as so:
func parseJson(data: NSData) {
var error: NSError?
var json: NSDictionary? = NSJSONSerialization.JSONObjectWithData(data, options: NSJSONReadingOptions.MutableContainers, error: &error) as? NSDictionary
if let err = error {
println("error parsing json")
return
}
// unwrap the optional value
if let jsonDictionary = json {
println("json parsed successfully")
// use the dictionary
}
}

Answer 2

I restructured your code a bit:
func parseJson(data: NSData) {
var error: NSError?
var json: AnyObject? = NSJSONSerialization.JSONObjectWithData(data, options: NSJSONReadingOptions.MutableContainers, error: &error)
if let jsonDict: NSDictionary = json as? NSDictionary{
println("json parsed successfully")
//do something with the nsdictionary...
}
else{
println("error parsing json")
}
}
parseJson(NSData(data: "{}".dataUsingEncoding(NSUTF8StringEncoding, allowLossyConversion: true)))
parseJson(NSData(data: "123".dataUsingEncoding(NSUTF8StringEncoding, allowLossyConversion: true)))
The problem wasn't with your if code, but making json an NSDictionary when it isn't one when the method fails. Also, this code now checks if an NSDictionary was returned instead of checking for the error itself.

Answer 3

NSJSONSerialization.JSONObjectWithData returns an optional value that has been implicitly unwrapped, which means that the return value could be nil. In the invalid JSON case, a nil optional type was returned. So, your var json must also be an optional type since it could be nil. Further more, if var json is optional type then you must downcast it as optional by adding ? next to as. This is the final result
var json: NSDictionary? = NSJSONSerialization.JSONObjectWithData(data, options: NSJSONReadingOptions.MutableContainers, error: &error) as? NSDictionary

Answer 4

I was getting fatal error: unexpectedly found nil while unwrapping an Optional value until I made the NSDictionary type casting optional, e.g. as? NSDictionary instead of as Dictionary