Swift 3 error handling and variable accessibility - ios

Question

I have a question on how to use the new error handling in Swift.
I'm reading contents of a file into a data object:
var overallData: Data?
//load file contents into data object
let dataFileURL = NSURL(string: fileName)
do {
overallData = try Data(contentsOf: dataFileURL as! URL)
} catch {
print("\(error)")
}
The problem is that I always encounter this error message:
fatal error: unexpectedly found nil while unwrapping an Optional value
The problem is that the overallData object is set as nil. But if I don't define a data variable outside the do-catch,
let dataFileURL = NSURL(string: fileName)
do {
overallData = try Data(contentsOf: dataFileURL as! URL)
} catch {
print("\(error)")
}
Later on, I can't use the overallData object because the system keeps telling me it's a variable not defined yet. So it looks like new variables defined in the do-catch loop can only be locally accessed inside the loop.
Do you know how to solve this problem? I do need to use the overallData object elsewhere.

Answer 1

The following answer assumes your error is with the line:
overallData = try Data(contentsOf: dataFileURL as! URL)
If you are getting the "fatal error" on another line, please update your question.
Your error has nothing to do with the do/catch/try.
Your problem is the force unwrapping of dataFileURL which is nil.
Your problem is this line:
let dataFileURL = NSURL(string: fileName)
This is returning nil because fileName isn't a valid URL.
Assuming fileName is a path to a local file, you need to do:
let dataFileURL = URL(fileURLWithPath: fileName)
Also note the use of URL instead of NSURL. There is no sense in using NSURL in Swift 3.