5 + (void)observeValueForKeyPath:
 6                 (NSString*)keyPath
 7       ofObject: (id)object
 8         change: (NSDictionary*)change
 9        context: (void*)context
10 {
11   NSLog(@"%@.%@ is now %@", object, keyPath,
12     [change objectForKey: NSKeyValueChangeNewKey
            ]);
13 }
14 + (void)watchChange
15 {
16   NSMutableDictionary *dict =
17     [NSMutableDictionary new];
18   [dict addObserver: self
19          forKeyPath: @"aKey"
20             options:
21     NSKeyValueObservingOptionNew
22             context: NULL];
23
24   [dict setObject: @"set as object"
25            forKey: @"aKey"];
26
27   [dict removeObserver: self
28             forKeyPath: @"aKey"];