Tests the interaction of KVO and resetStandardUserDefaults().

UserDefaultsKVOTest.swift

/**
 Key-Value Observing of NSUserDefaults.standardUserDefaults is unsafe.
 
 Do not KVO the standardUserDefaults because an invocation of resetStandardUserDefaults()
 anywhere in the codebase causes that KVO to silently stop observing. The comment of
 resetStandardUserDefaults() in NSUserDefaults.h (below) states this behavior, but I'd
 have found the comment more clear had it pointed out that the behavior makes 
 standardUserDefaults unfit for Key-Value Observation.
 
 > The only visible effect this has is that all KVO observers of the previous
 > standardUserDefaults will no longer be observing it.
 */

import Foundation
let key = "key"


class Observer: NSObject {
    override init() {
        super.init()
        NSUserDefaults.standardUserDefaults().addObserver(self, forKeyPath: key, options: [.New, .Old], context: nil)
    }
    
    deinit {
        NSUserDefaults.standardUserDefaults().removeObserver(self, forKeyPath: key, context: nil)
    }
    
    override func observeValueForKeyPath(keyPath: String?, ofObject object: AnyObject?, change: [String : AnyObject]?, context: UnsafeMutablePointer<Void>) {
        print("\(keyPath!) value did change: \(change!)")
    }
}


// Begin observing standardUserDefaults.
let observer = Observer()

// Confirm that `observer` is notified of changes in standardUserDefaults.
NSUserDefaults.standardUserDefaults().setObject("before resetStandardUserDefaults()", forKey: key)

// Cache pointers and reset standardUserDefaults.
let defaultsBefore = NSUserDefaults.standardUserDefaults()
NSUserDefaults.resetStandardUserDefaults()
let defaultsAfter = NSUserDefaults.standardUserDefaults()

// The pointers taken before and after differ.
print("Did NSUserDefaults.resetStandardUserDefaults(). Before and after objects equal: \(defaultsBefore == defaultsAfter).")

// Confirm that `observer` is NOT notified of changes in the new standardUserDefaults.
NSUserDefaults.standardUserDefaults().setObject("after resetStandardUserDefaults()", forKey: key)

// Clean up for next run.
defaultsBefore.removeObjectForKey(key)
defaultsAfter.removeObjectForKey(key)