KVO 处理此代码

Question

为了更好地理解 KVO，我创建了一个简单的应用程序，其中包含 1 个按钮和 2 个非常基本的模型类：Book 和 Author。我想在作者更改时触发这本书。比如一个简单的KVO例子，为什么不触发观察者？

#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

现在应该调用但不是：

#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

您在注册时混淆了观察对象和被观察对象。 nancy是home要观察的对象，所以应该是

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