How do I validate IBOutlet/var for nil? - iphone

Question

I've written a simple application using Swift and Xcode 6.1.1. The program, which is a simple calculator, works well but I haven't been able to validate non-nil input for the three text fields. As a result, if the user leaves the field blank and then hits "calculate," the application crashes.
The app takes three inputs, initially as strings. I wrote an if statement to check for nil but it doesn't work - it will just pass to else regardless. Here's the code block that's relevant to my question:
...
#IBOutlet var calcButton: UIBarButtonItem!
#IBOutlet var yearOneField: UITextField!
#IBOutlet var yearTwoField: UITextField!
#IBOutlet var yearThreeField: UITextField!
#IBOutlet var displayWindow: UILabel!
#IBAction func calcButtonTapped(sender: AnyObject) {
if (yearOneField == nil) {
displayWindow.text = ("Non-zero entries are not permitted. Please enter new values.")
} else {
let yearOne = yearOneField.text.toInt()
let yearTwo = yearTwoField.text.toInt()
let yearThree = yearThreeField.text.toInt()
...
I was thinking I could evaluate the IBOutlet for nil but that didn't work. I'm new to Swift and Xcode so I hope this is a n00b question to the more experienced developers out there. Thank you.

Answer 1

The only way the #IBOutlets could be nil is if you forgot to wire them up in Interface Builder. Usually you don't need to check that because the crash will tell you to fix that problem.
The toInt() function returns an Optional Int (aka Int?) that must be unwrapped before being used. toInt() will return nil if the value in the text field does not represent a valid Int. "2.1", "seven", and "" would all return nil if converted with toInt(). I recommend you use optional binding (if let) syntax to check the conversion for nil and unwrap the result if it is not nil:
if let yearOne = yearOneField.text.toInt() {
if let yearTwo = yearTwoField.text.toInt() {
if let yearThree = yearThreeField.text.toInt() {
// yearOne, yearTwo, and yearThree are all valid Ints
// so do the calculations
}
}
}
Alternatively, if you know you want to use a default value (like 0) when the field can't be converted to an Int, you can unwrap the result using the nil coalescing operator ?? like so:
let yearOne = yearOneField.text.toInt() ?? 0
let yearTwo = yearTwoField.text.toInt() ?? 0
let yearThree = yearThreeField.text.toInt() ?? 0

Answer 2

The text fields themselves will never be nil. They are created and assigned during initialization, and you're never removing them.
I think you want to check if their text properties contain any text, which you can do like this:
Updated for Swift 2:
if let text = yearOneField.text where !text.isEmpty {
// perform the conversions
} else {
// the text field is empty
}
You can avoid nesting using guard:
guard let text = yearOneField.text where !text.isEmpty else {
// the text field is empty
return
}
// perform the conversions
I prefer the guard syntax because it's clearer about what the ideal result is.

Answer 3

You can just check as you do with normal optionals.
guard let unwrapped = myLabel else {return}
Or like this
if myLabel == nil {
//do stuff
}
Or like this:
if let unwrappedLabel = myLabel {
}