@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：

    “`

    

    

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