@interface Walker : NSObject
{
    NSInteger _age;
    NSString *_name;
}
@property (nonatomic) NSInteger age;
@property (nonatomic, retain) NSString *name;
- (id)initWithName:(NSString *)name age:(NSInteger)age;

``

import “Walker.h”

@implementation Walker

• (id)initWithName:(NSString *)name age:(NSInteger)age
  {
  if (self = [super init]) {
  _name = name;
  _age = age;
  }
  return self;
  }
  @end

import

• (void)viewDidLoad {
  [super viewDidLoad];

  _walker = [[Walker alloc] initWithName:@”LJF” age:25];
  NSLog(@”%ld”, (long)_walker.age);
  /* 关键步骤 */
  [_walker addObserver:self
  forKeyPath:@”age”
  options:NSKeyValueObservingOptionNew
  context:nil];

  UIButton *btn = [UIButton buttonWithType:UIButtonTypeCustom];
   [btn setFrame:CGRectMake(120, 200, 100, 35)];
   [btn setBackgroundColor:[UIColor lightGrayColor]];
   [btn setTitle:@"增加5岁" forState:UIControlStateNormal];
   [btn addTarget:self
                   action:@selector(buttonPressed)
         forControlEvents:UIControlEventTouchUpInside];
   [self.view addSubview:btn];
  
   _ageLabel = [[UILabel alloc] initWithFrame:CGRectMake(40, 150, 200, 35)];
   _ageLabel.text = [NSString stringWithFormat:@"%@现在的年龄是: %ld", _walker.name, (long)_walker.age];
   _ageLabel.backgroundColor = [UIColor clearColor];
   [self.view addSubview:_ageLabel];
  

  }

• (void)buttonPressed
  {
  _walker.age += 5;

  }

  /* KVO function， 只要object的keyPath属性发生变化，就会调用此函数*/

  • (void)observeValueForKeyPath:(NSString )keyPath ofObject:(id)object change:(NSDictionary )change context:(void *)context
    {
    NSLog(@”keyPath = %@ \nobject =%@ \nchange = %@ \n”, keyPath,object,change);
    if ([keyPath isEqualToString:@”age”] && object == _walker) {
    _ageLabel.text = [NSString stringWithFormat:@”%@现在的年龄是: %ld”, _walker.name, (long)_walker.age];
    }
    }
    如下图：
    “`