typedef NS_OPTIONS(NSUInteger, NSKeyValueObservingOptions) {

    NSKeyValueObservingOptionNew = 0x01,

    NSKeyValueObservingOptionOld = 0x02,

    NSKeyValueObservingOptionInitial = 0x04,

    NSKeyValueObservingOptionPrior= 0x08

 };

内置的通知键

FOUNDATION_EXPORT NSString *const NSKeyValueChangeKindKey;

FOUNDATION_EXPORT NSString *const NSKeyValueChangeNewKey;

FOUNDATION_EXPORT NSString *const NSKeyValueChangeOldKey;

FOUNDATION_EXPORT NSString *const NSKeyValueChangeIndexesKey;

FOUNDATION_EXPORT NSString *const NSKeyValueChangeNotificationIsPriorKey ;

@interface NSObject(NSKeyValueObserving)

//接受被观察者发出变动后的通知

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context;

@end

@interface NSObject(NSKeyValueObserverRegistration)

//通过键给对应的属性添加观察者

- (void)addObserver:(NSObject *)observer forKeyPath:(NSString *)keyPath options:(NSKeyValueObservingOptions)options context:(void *)context;

//删除观察者

- (void)removeObserver:(NSObject *)observer forKeyPath:(NSString *)keyPath context:(void *)context;

- (void)removeObserver:(NSObject *)observer forKeyPath:(NSString *)keyPath;

@end

@interface NSArray(NSKeyValueObserverRegistration)————— 数组的KVO机制

NSIndexSet 是个无符号整数集合。集合中的元素不可变的、不可重复。常被用来当作索引使用

//通过键给对应的数组中指定索引的属性添加观察者

- (void)addObserver:(NSObject *)observer toObjectsAtIndexes:(NSIndexSet *)indexes forKeyPath:(NSString *)keyPath options:(NSKeyValueObservingOptions)options context:(void *)context;

//删除观察者

- (void)removeObserver:(NSObject *)observer fromObjectsAtIndexes:(NSIndexSet *)indexes forKeyPath:(NSString *)keyPath context:(void *)context;

- (void)removeObserver:(NSObject *)observer fromObjectsAtIndexes:(NSIndexSet *)indexes forKeyPath:(NSString *)keyPath;

//通过键给对应的数组添加观察者

- (void)addObserver:(NSObject *)observer forKeyPath:(NSString *)keyPath options:(NSKeyValueObservingOptions)options context:(void *)context;

//删除观察者

- (void)removeObserver:(NSObject *)observer forKeyPath:(NSString *)keyPath context:(void *)context;

- (void)removeObserver:(NSObject *)observer forKeyPath:(NSString *)keyPath;

@end

@interface NSSet(NSKeyValueObserverRegistration)—————集合的KVO机制

//通过键给对应的set集合添加观察者

- (void)addObserver:(NSObject *)observer forKeyPath:(NSString *)keyPath options:(NSKeyValueObservingOptions)options context:(void *)context;

//删除观察者

- (void)removeObserver:(NSObject *)observer forKeyPath:(NSString *)keyPath context:(void *)context;

- (void)removeObserver:(NSObject *)observer forKeyPath:(NSString *)keyPath;

@end

@interface NSObject(NSKeyValueObserverNotification)————观察者通知

//对于单个对象

- (void)willChangeValueForKey:(NSString *)key; 

- (void)didChangeValueForKey:(NSString *)key;

//对于array数组

- (void)willChange:(NSKeyValueChange)changeKind valuesAtIndexes:(NSIndexSet *)indexes forKey:(NSString *)key;

- (void)didChange:(NSKeyValueChange)changeKind valuesAtIndexes:(NSIndexSet *)indexes forKey:(NSString *)key;

//对于set集合

- (void)willChangeValueForKey:(NSString *)key withSetMutation:(NSKeyValueSetMutationKind)mutationKind usingObjects:(NSSet *)objects;

- (void)didChangeValueForKey:(NSString *)key withSetMutation:(NSKeyValueSetMutationKind)mutationKind usingObjects:(NSSet *)objects;

@end

具体演示实例如下：

要求：将人设置为股票的观察者,一旦股票价格发生改变,就立刻发送即时信息。

准备工作：

在控制器视图中添加一个按钮控件并关联事件IBAction,用来点击时改变股票的价格，需要的文件相关截图为：

以下就是代码的实现了：

Person人类：

Person.h

//声明属性

#import <Foundation/Foundation.h>

@interface Person : NSObject
{
    NSString *_name;
}
@end

Perosn.m

//接受观察者发来的即时信息

#import "Person.h"

@implementation Person

//接收被观察者发生变动后的通知
-(void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context
{
    NSLog(@"keyPath:%@,object:%@,change:%@",keyPath,object,change);
}
@end

Stock股票类：

Stock.h

//声明属性

#import <Foundation/Foundation.h>
#import <UIKit/UIKit.h>

@interface Stock : NSObject
{
    NSString *_name;
    CGFloat _price;
}
@end

ViewController控制器类

//声明属性

#import "ViewController.h"
#import "Person.h"
#import "Stock.h"

@interface ViewController ()
@property (strong,nonatomic)Person *person;
@property (strong,nonatomic)Stock *stock;
@end

@implementation ViewController

//按钮事件,点击时改变股票的价格

//按钮事件,改变股票价格
- (IBAction)buttonClicked:(UIButton *)sender
{
    CGFloat price = [[self.stock valueForKey:@"_price"] floatValue];
    
    [self.stock setValue:@(price+0.1) forKey:@"_price"];
}

//创建对象并添加观察者

- (void)viewDidLoad {
    [super viewDidLoad];
    //创建人和股票对象
    self.person = [[Person alloc]init];
    [self.person setValue:@"Tom" forKey:@"_name"];
    
    self.stock = [[Stock alloc]init];
    [self.stock setValue:@"sh0001" forKey:@"_name"];
    [self.stock setValue:@12.5 forKey:@"_price"];
    
    
    //添加观察者(观察股票的变动)
    [self.stock addObserver:self.person forKeyPath:@"_price" options:NSKeyValueObservingOptionNew | NSKeyValueObservingOptionOld context:nil];
}

//程序结束,删除观察者

-(void)dealloc
{
    //删除观察者
    [self.stock removeObserver:self.person forKeyPath:@"_price"];
}

演示结果如下：

点击第一次，输出股票价格更新即时信息

2015-10-14 22:02:56.298 02-KVO[5612:319932] keyPath:_price,object:<Stock: 0x7f9679482c40>,change:{
    kind = 1;
    new = "12.6";
    old = "12.5";
}

点击第二次，输出股票价格更新即时信息

2015-10-14 22:02:56.298 02-KVO[5612:319932] keyPath:_price,object:<Stock: 0x7f9679482c40>,change:{
    kind = 1;
    new = "12.6";
    old = "12.5";
}
2015-10-14 22:03:44.913 02-KVO[5612:319932] keyPath:_price,object:<Stock: 0x7f9679482c40>,change:{
    kind = 1;
    new = "12.70000038146973";
    old = "12.6";
}

点击第三次，输出股票价格更新即时信息

2015-10-14 22:02:56.298 02-KVO[5612:319932] keyPath:_price,object:<Stock: 0x7f9679482c40>,change:{
    kind = 1;
    new = "12.6";
    old = "12.5";
}
2015-10-14 22:03:44.913 02-KVO[5612:319932] keyPath:_price,object:<Stock: 0x7f9679482c40>,change:{
    kind = 1;
    new = "12.70000038146973";
    old = "12.6";
}
2015-10-14 22:04:09.448 02-KVO[5612:319932] keyPath:_price,object:<Stock: 0x7f9679482c40>,change:{
    kind = 1;
    new = "12.80000076293945";
    old = "12.70000038146973";
}