Property observers on Dictionaries in Swift

Question

I want to call a delegate method when a new item is added to my dictionary. For single value properties the didSet(newValue) observer works fine. But for my dictionary it seems that the newUser argument below returns the full dictionary.

var userIdKeyedDict:Dictionary<String, User> = [:] {
    didSet(newUser) {
    println("Updating userIdKeyedDict")
    self.delegate.didAddUser(newUser)
} 
willSet(newUser) {
  println("Will set \(newUser)")
}

}

The output from willSet(newUser) is:

Will set [3E33BD4D-6F48-47FC-8612-565250126E51: User: userId: Optional("3E33BD4D-6F48-47FC-8612-565250126E51"), listenerUUID: Optional("77D01D6F-D017-EF9E-55A9-78570E455C51"), beaconUUID: nil]

As more items are added the newUser simply contains the entire dictionary.

How can I get a handle on just the new item added?

Answer 1

The easiest way is to use subscripts, these provide a flexible way to assign keys subscript values, allowing code process before and after updates to a dictionary or internal object structure. For more information regarding subscripts checkout the following link:

https://developer.apple.com/library/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Subscripts.html

I've included some example code to help.

class User {
    var userId:String = NSUUID.init().UUIDString
    var name:String = "Your Users Name"
}

class UserDictionary {
    var users = [String:User]()
    subscript(userId:String) -> User? {
        get {
            return users[userId]
        } set {
            // if newValue is nil then you are about to remove user
            users[userId] = newValue
            // do after value processing.
            print("[\(userId)]=\(newValue)")
        }
    }
}

var userdict = UserDictionary()
let user = User()
userdict[user.userId] = user