Idiomatic way of handling optional values in Swift

Question

I'm new to iOS development and also Swift. I'm working on a project consisting of a menu that leads to my various experimentations with the iOS SDK.

I've written the menu titles and segueIdentifiers in a plist and I'm populating an array of NavigationItem in viewDidLoad().

Is the following code idiomatic, particularly in the way it handles optional values? What might be better?

override func viewDidLoad() {
 super.viewDidLoad()
 let path = NSBundle.mainBundle().pathForResource("NavigationMenu", ofType: "plist")
 if path == nil{
 NSLog("Unable to locate file : NavigationMenu.plist")
 return
 }
 let items = NSArray(contentsOfFile: path!)
 if items == nil {
 NSLog("Navigation menu items could not be read from plist.")
 return
 }
 for var i = 0 ; i < items!.count ; i++ {
 let item : NSDictionary = items!.objectAtIndex(i) as! NSDictionary
 navigationItems.append(NavigationItem(
 name:(item.objectForKey("name") as! String),
 andSegueIdentifier: (item.objectForKey("segueIdentifier") as! String)
 ))
 }
 }

• nhgrif: Does pathForResource return NSURL! or NSURL? and same question for everything else here that might return nil?
• W.K.S: pathForResource returns String?. NSArray(contentsOfFile) returns [AnyObject]?

Answer 1

A better way to handle optional values is "optional binding":

if let path = NSBundle.mainBundle().pathForResource("NavigationMenu", ofType: "plist") {
 // do something with `path`
} else {
 NSLog("Unable to locate file : NavigationMenu.plist")
}

which tests and unwraps the optional result as a single action. Inside the if-block, path is the unwrapped String.

You can do the same with the items:

if let path = NSBundle.mainBundle().pathForResource("NavigationMenu", ofType: "plist") {
 if let items = NSArray(contentsOfFile: path) {
 // do something with `items` ...
 } else {
 NSLog("Navigation menu items could not be read from plist.")
 }
} else {
 NSLog("Unable to locate file : NavigationMenu.plist")
}

As of Swift 1.2, multiple optional bindings can be combined in a single if-statement, which reduces the number of nested if-levels:

if
 let path = NSBundle.mainBundle().pathForResource("NavigationMenu", ofType: "plist"),
 let items = NSArray(contentsOfFile: path) {
 // do something with `items` ...
} else {
 NSLog("Unable to load NavigationMenu")
}

The downside is that you have only a common else-block for the error condition and cannot distinguish which of the optional bindings failed.

Instead of objectAtIndex() and objectForKey() you can use subscripting for NSArray and NSDictionary:

for var i = 0 ; i < items!.count ; i++ {
 let item = items![i] as! NSDictionary
 let navItem = NavigationItem(name: item["name"] as! String,
 andSegueIdentifier: item["segueIdentifier"] as! String)
 navigationItems.append(navItem)
}

But a better way is to cast the NSArray to a Swift array of Swift dictionaries in the first step. Then you can use array enumeration and don't need any casts later:

if
 let path = NSBundle.mainBundle().pathForResource("NavigationMenu", ofType: "plist"),
 let items = NSArray(contentsOfFile: path) as? [[String : String]] {
 for item in items {
 let navItem = NavigationItem(name: item["name"]!, andSegueIdentifier: item["segueIdentifier"]!)
 navigationItems.append(navItem)
 }
} else {
 NSLog("Unable to load NavigationMenu")
}

This assumes that values for the "name" and "segueIdentifier" are present (and will crash at runtime otherwise). Alternatively, use optional binding again:

for item in items {
 if let name = item["name"], ident = item["segueIdentifier"] {
 let navItem = NavigationItem(name: name, andSegueIdentifier: ident)
 navigationItems.append(navItem)
 } else {
 NSLog("Invalid navigation item")
 }
}

Finally, you could replace the inner loop by a map() operation:

if
 let path = NSBundle.mainBundle().pathForResource("NavigationMenu", ofType: "plist"),
 let items = NSArray(contentsOfFile: path) as? [[String : String]] {
 navitationItems = map(items) {
 NavigationItem(name: 0ドル["name"]!, andSegueIdentifier: 0ドル["segueIdentifier"]!)
 }
} else {
 NSLog("Unable to load NavigationMenu")
}

• W.K.S: This is a terrific answer! I didn't know you could do multiple optional bindings with a single if else statement. The large number of nested statement is what put me off