@interface Person : NSObject
@property (nonatomic, copy) NSString *name;
@end

- (void)viewDidLoad {
    [super viewDidLoad];
   
    Person *person = [[Person alloc] init];
    person.name = @"珠珠";
    [person addObserver:self forKeyPath:@"name" options:NSKeyValueObservingOptionNew | NSKeyValueObservingOptionOld context:@"testKVO"];
    
    person.name = @"小雅";
    [person removeObserver:self forKeyPath:@"name"];
    
}
- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSKeyValueChangeKey,id> *)change context:(void *)context {
    NSLog(@"keypaht=%@, object=%@, change=%@, context=%@", keyPath,object, change, context);
    //NSKeyValueChangeOldKey  可以用"old"来代替,但是获取的话,如果系统修改old为"Old"的话,那会获取不到,所以建议用NSKeyValueChangeOldKey
    NSLog(@"获取被修改前的值=%@", change[NSKeyValueChangeOldKey]);
    //NSKeyValueChangeNewKey  可以用"new"来代替,但是获取的话,如果系统修改new为"New"的话,那会获取不到,所以建议用NSKeyValueChangeNewKey
    NSLog(@"获取修改后的值=%@", change[NSKeyValueChangeNewKey]);
}