Automatic key-value observing is implemented using a technique called isa-swizzling… When an observer is registered for an attribute of an object the isa pointer of the observed object is modified, pointing to an intermediate class rather than at the true class …

-(void)setName:(NSString *)newName {
    [self willChangeValueForKey:@"name"];    //KVO在调用存取方法之前总调用
    [super setValue:newName forKey:@"name"]; //调用父类的存取方法
    [self didChangeValueForKey:@"name"];     //KVO在调用存取方法之后总调用
}

/*第一个参数observer：观察者 （这里观察self.myKVO对象的属性变化）
  第二个参数keyPath： 被观察的属性名称(这里观察self.myKVO中num属性值的改变)
  第三个参数options： 观察属性的新值、旧值等的一些配置（枚举值，可以根据需要设置，例如这里可以使用两项）
  第四个参数context： 上下文，可以为kvo的回调方法传值（例如设定为一个放置数据的字典）*/
[self.myKVO addObserver:self forKeyPath:@"num" options:NSKeyValueObservingOptionOld|NSKeyValueObservingOptionNew context:nil]; 

/*keyPath:属性名称
  object:被观察的对象
  change:变化前后的值都存储在change字典中
  context:注册观察者时，context传过来的值 */
-(void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSString *,id> *)change context:(void *)context{

}

[_myKVO removeObserver:self forKeyPath:@"num" context:nil];

@interface MyKVO : NSObject

@property (nonatomic, assign) NSInteger num;

@end

#import "ViewController.h"
#import "MyKVO.h"

@interface ViewController ()

@property (nonatomic, strong) MyKVO *myKVO;

@property (nonatomic, strong) UILabel *label;
@property (nonatomic, strong) UIButton *btn;

@end

@implementation ViewController

- (void)dealloc {
    // 移除观察者
    [_myKVO removeObserver:self forKeyPath:@"num" context:nil];
}

- (void)viewDidLoad {
    [super viewDidLoad];
    
    self.view.backgroundColor = [UIColor grayColor];
    
    _label = [[UILabel alloc] initWithFrame:CGRectMake(0, 100, self.view.bounds.size.width, 50)];
    _label.textColor = [UIColor redColor];
    _label.font = [UIFont systemFontOfSize:20];
    _label.textAlignment = NSTextAlignmentCenter;
    [self.view addSubview:_label];
    
    _label.text = @"Label";
    
    _btn = [[UIButton alloc] initWithFrame:CGRectMake((self.view.bounds.size.width - 100)/2, 220, 100, 50)];
    [_btn setTitle:@"click" forState:UIControlStateNormal];
    [_btn setTitleColor:[UIColor blackColor] forState:UIControlStateNormal];
    [_btn addTarget:self action:@selector(clickBtn) forControlEvents:UIControlEventTouchUpInside];
    [self.view addSubview:_btn];
    
    _myKVO = [[MyKVO alloc] init];
    
    // 注册观察者
    [self.myKVO addObserver:self forKeyPath:@"num" options:NSKeyValueObservingOptionNew | NSKeyValueObservingOptionOld context:nil];
}

// 重要object的keyPath的值发生变化，就会回调到这里
- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSKeyValueChangeKey,id> *)change context:(void *)context {

    if ([keyPath isEqualToString:@"num"] && object == self.myKVO) {
        // 相应变化处理，UI更新
        self.label.text = [NSString stringWithFormat:@"%@", [change valueForKey:@"new"]];
        // 注册时，枚举为2个，因此可以提取change字典中的new和old两个字端
        NSLog(@"oldNum=%@, newNum=%@", [change valueForKey:@"old"], [change valueForKey:@"new"]);
    }
}

- (void)clickBtn {

    self.myKVO.num += 1;
}

- (void)didReceiveMemoryWarning {
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}

@end