gistfile1.txt

// Playground - noun: a place where people can play

import UIKit

var str = "Hello, playground"

class Foo: NSObject {
    
    public var changingProperty: Int = 0
    public func change() {
        changingProperty++
    }
    
    init() {
//        println("some foo inited")
    }
    override func observeValueForKeyPath(keyPath: String!, ofObject object: AnyObject!, change: [NSObject : AnyObject]!, context: UnsafePointer<()>) {
        if keyPath == "changingProperty" {
            println("changed 'changingProperty' to new value: \(change[NSKeyValueChangeNewKey])")
            println("changed 'changingProperty' from old value: \(change[NSKeyValueChangeOldKey])")
            println("Change dictionary in observer \(change)")
        } else {
            println("not changed")
        }
    }
}


// sends 1 notification, with new value only
var f = Foo()
var g = Foo()
f.change()
f.addObserver(g, forKeyPath: "changingProperty", options: .New, context: nil)
f.change()

println()

// sends 1 notification with new value and old value
var h = Foo()
var i = Foo()
h.change()
h.addObserver(i, forKeyPath: "changingProperty", options: .New | .Old, context: nil)
h.change()

println()

// sends 2 notifications with new value...
// it sends a notification for the first call to change() even though that was before the observer was added
var j = Foo()
var k = Foo()
j.change()
j.addObserver(k, forKeyPath: "changingProperty", options: .New | .Initial | .Old, context: nil)
j.change()




/* 
.Initial just specifies the notification should be sent to the observer immediately, before the observer's registration method even returns. 

this in a playground yields the following output:

changed 'changingProperty' to new value: Optional(2)
changed 'changingProperty' from old value: nil
Change dictionary in observer [kind: 1, new: 2]

changed 'changingProperty' to new value: Optional(2)
changed 'changingProperty' from old value: Optional(1)
Change dictionary in observer [kind: 1, old: 1, new: 2]

changed 'changingProperty' to new value: Optional(1)
changed 'changingProperty' from old value: nil
Change dictionary in observer [kind: 1, new: 1]
changed 'changingProperty' to new value: Optional(2)
changed 'changingProperty' from old value: Optional(1)
Change dictionary in observer [kind: 1, old: 1, new: 2]

*/