Swift optionals and their instantiation - ios

Question

I'm new to Swift. I'll explain what I'm trying to do in Java terms and hopefully someone can help me understand.
I want a class scoped array that is instantiated/set in viewDidLoad of a view controller. It sounds simple enough, but this is what I had to do to get it to work. Could someone explain to me why the _dictionary must be instantiated as an empty array and why I need to use as? when unpacking dictionary even though the componentsSeparatedByString function returns an array? Thanks.
class ViewController: UIViewController, UITextFieldDelegate
{
var _dictionary : [String] = []
override func viewDidLoad()
{
super.viewDidLoad()
let bundle = NSBundle.mainBundle()
let path = bundle.pathForResource(“TextFile”, ofType: "txt")
var err: NSError?
let dico = NSString(contentsOfFile: path!, encoding: NSUTF8StringEncoding, error: &err)
if let dictionary = dico?.componentsSeparatedByString("\n") as? [String]
{
_dictionary = dictionary
}
else
{
println("Error reading dictionary")
}
}
}

Answer 1

_dictionary must be given an initial value because all instance properties must be given an initial value. That is a Swift safety feature to make sure that all instances are well-formed at birth. You could have done this by initializing _dictionary in an actual initializer, but you didn't. You chose to give it its first "real" value in viewDidLoad which is much later. Therefore you have forced yourself to supply a "fake" initial value until such time as viewDidLoad comes along and gives you a "real" initial value.
I don't understand your other question because I don't know what "unpacking dictionary" means. But perhaps you are referring to the rest of the stuff in this code. I'll talk you through it.
dico is an Optional wrapping a String. It is an Optional because the contentsOfFile: initializer of String returns an Optional - it is a failable initializer. That is because there might be no such file, or that file might not have that encoding, in which case the initializer needs to return nil.
So now dico is an Optional wrapping a String that must be unwrapped - or nil. So you do unwrap it as dico? in the next line. dico? is a String at that point, assuming that dico was not nil.
Then you call componentsSeparatedByString. It returns an array of AnyObject. So you have chosen to cast it down to an array of String using as?. I don't know why you chose to use as? since you know it will be an array of String if we get to this point - personally, I would have used as. But in any case, as? always returns an Optional! So you unwrap that, optionally, by using an if let conditional binding.
The irony throughout is that _dictionary is not and never was or will be a dictionary - it is an array. Your name for this (and for dictionary) is very poorly chosen! dico is a String, and dictionary and _dictionary are arrays. There is not a dictionary in sight.
Assuming we keep your rather odd names, though, you could have done all this much more briefly and clearly, like this:
var err : NSError?
if let dico = NSString(contentsOfFile: path!,
encoding: NSUTF8StringEncoding, error: nil) {
_dictionary = dico.componentsSeparatedByString("\n") as [String]
} else {
println(err)
}
Moreover, if you start with the String class instead of the NSString class, then componentsSeparatedByString will be an array of String instead of an untyped NSArray (i.e. an array of AnyObject), and you can skip the cast:
var err : NSError?
if let dico = String(contentsOfFile: path!,
encoding: NSUTF8StringEncoding, error: nil) {
_dictionary = dico.componentsSeparatedByString("\n")
} else {
println(err)
}