KVO working on this code

Question

To better understand KVO, I created a simple application with 1 button and 2 very basic model classes: Book and Author. I want to trigger the Book when the Author changes. For example, A simple KVO example, why doesn't this trigger the observer?

#import "AppDelegate.h"
#import "Book.h"
#import "Author.h"

@implementation AppDelegate {
    Book *home;
    Author *nancy;
}

- (void)applicationDidFinishLaunching:(NSNotification *)aNotification
{
    // Insert code here to initialize your application

    NSLog(@"FunWithKVO");

    nancy = [[Author alloc] init];

    [nancy setFirstName:@"Nancy"];
    [nancy setLastName:@"Drew"];

    home = [[Book alloc] init];

    [home addObserver:nancy forKeyPath:@"lastName" options:NSKeyValueObservingOptionNew | NSKeyValueObservingOptionOld context:NULL];

    [home setAuthor:@"Nancy Drew"];

}

- (IBAction)changeName:(id)sender {
    NSLog(@"%@",[home author]);

    [nancy setLastName:@"Martin"];
}

@end

NOW THIS SHOULD BE CALLED BUT ISN'T:

#import "Book.h"

@implementation Book

@synthesize author;

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context {

    [author setValue:[NSString stringWithFormat:@"Nancy %@",[change value]]];

    [super observeValueForKeyPath:keyPath ofObject:object change:change context:context];

    NSLog(@"name is now: %@",author);
}

@end

Answer 1

You mixed up the observing and the observed object in the registration. nancy is the object to be observed by home, therefore it should be

[nancy addObserver:home forKeyPath:@"lastName" options:NSKeyValueObservingOptionNew | NSKeyValueObservingOptionOld context:NULL];