一，概述

KVO,即：Key-Value Observing，它提供一种机制，当指定的对象的属性被修改后，则对象就会接受到通知。简单的说就是每次指定的被观察的对象的属性被修改后，KVO就会自动通知相应的观察者了。

二，使用方法

系统框架已经支持KVO，所以程序员在使用的时候非常简单。

1. 注册，指定被观察者的属性，

2. 实现回调方法

3. 移除观察

三，实例:

假设一个场景,股票的价格显示在当前屏幕上，当股票价格更改的时候，实时显示更新其价格。

1.定义数据模型(为了使用KVC)

#import <Foundation/Foundation.h>

NS_ASSUME_NONNULL_BEGIN

@interface StockData : NSObject{
    NSString *stockName;
    float price;
}
@end

NS_ASSUME_NONNULL_END

2.viewcontroller详细实现

#import "StockViewController.h"
#import "StockData.h"

@interface StockViewController ()
@property(nonatomic,strong) StockData *stockData;
@property (weak, nonatomic) IBOutlet UILabel *myLabel;

@end

@implementation StockViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    self.stockData = [StockData new];
    [self.stockData setValue:@"searph" forKeyPath:@"stockName"];
    [self.stockData setValue:@"10.0" forKeyPath:@"price"];
    //1.注册
    [self.stockData addObserver:self forKeyPath:@"price" options:NSKeyValueObservingOptionNew|NSKeyValueObservingOptionOld context:NULL];
    
}
//2.实现回调
/**
 (lldb) po keyPath
 price

 (lldb) po object
 <StockData: 0x600003a9bee0>

 (lldb) po change
 {
     kind = 1;
     new = 20;
     old = 10;
 }

 (lldb) po context
 <nil>

 (lldb) po [change valueForKey:@"new"]
 20

 (lldb) po [change valueForKey:@"old"]
 10

 (lldb)
 */
-(void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSKeyValueChangeKey,id> *)change context:(void *)context{
    if (object == self.stockData && [keyPath isEqualToString:@"price"]) {
        self.myLabel.text = [NSString stringWithFormat:@"%.2f",[[self.stockData valueForKey:@"price"] floatValue]];
    }
}


static float price;
- (IBAction)btnAction:(id)sender {
    price+=20.00;
    [self.stockData setValue:[NSString stringWithFormat:@"%f",price] forKey:@"price"];
}

//3.移除观察者
-(void)dealloc{
    [self.stockData removeObserver:self forKeyPath:@"price"];
}

/*
#pragma mark - Navigation

// In a storyboard-based application, you will often want to do a little preparation before navigation
- (void)prepareForSegue:(UIStoryboardSegue *)segue sender:(id)sender {
    // Get the new view controller using [segue destinationViewController].
    // Pass the selected object to the new view controller.
}
*/

@end

四，小结 KVO这种编码方式使用起来很简单，很适用与datamodel修改后，引发的UIVIew的变化这种情况，就像上边的例子那样，当更改属性的值后，监听对象会立即得到通知。

源码地址:https://github.com/wvqusrtg/NIiOS