demo地址：https://github.com/hanhuitao/KVO.git

概念

KVO,即：Key-Value Observing樟蠕，它提供一種機(jī)制使碾，當(dāng)指定的對(duì)象的屬性被修改后悔据，則對(duì)象就會(huì)接受到通知巷疼。簡(jiǎn)單的說(shuō)就是每次指定的被觀察的對(duì)象的屬性被修改后宇整，KVO就會(huì)自動(dòng)通知相應(yīng)的觀察者了晌坤。

使用場(chǎng)景

很多情況下程奠，我們需要根據(jù)對(duì)象屬性的變化而做出相應(yīng)的事情丈牢，這就需要監(jiān)聽該屬性的變化，比如avPlayer播放器里面瞄沙，status有三種狀態(tài)：

  AVPlayerStatusUnknown,
  AVPlayerStatusReadyToPlay,
  AVPlayerStatusFailed

當(dāng)status等于AVPlayerStatusReadyToPlay時(shí)代表視頻已經(jīng)可以播放了己沛，我們就可以調(diào)用play方法播放了。
loadedTimeRange屬性代表已經(jīng)緩沖的進(jìn)度距境，監(jiān)聽此屬性可以在UI中更新緩沖進(jìn)度申尼，也是很有用的一個(gè)屬性。
具體實(shí)現(xiàn)代碼：

  [self.playerItem addObserver:self forKeyPath:@"status"  options:NSKeyValueObservingOptionNew context:nil];// 監(jiān)聽status屬性
  [self.playerItem addObserver:self forKeyPath:@"loadedTimeRanges" options:NSKeyValueObservingOptionNew context:nil];// 監(jiān)聽     loadedTimeRanges屬性
- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context {
       AVPlayerItem *playerItem = (AVPlayerItem *)object;
      if ([keyPath isEqualToString:@"status"]) {
      if ([playerItem status] == AVPlayerStatusReadyToPlay) {
          NSLog(@"AVPlayerStatusReadyToPlay");
          self.stateButton.enabled = YES;
          CMTime duration = self.playerItem.duration;// 獲取視頻總長(zhǎng)度
          CGFloat totalSecond = playerItem.duration.value / playerItem.duration.timescale;// 轉(zhuǎn)換成秒
          _totalTime = [self convertTime:totalSecond];// 轉(zhuǎn)換成播放時(shí)間
          [self customVideoSlider:duration];// 自定義UISlider外觀
          NSLog(@"movie total duration:%f",CMTimeGetSeconds(duration));
          [self monitoringPlayback:self.playerItem];// 監(jiān)聽播放狀態(tài)
      } else if ([playerItem status] == AVPlayerStatusFailed) {
          NSLog(@"AVPlayerStatusFailed");
     }
   } else if ([keyPath isEqualToString:@"loadedTimeRanges"]) {
      NSTimeInterval timeInterval = [self availableDuration];// 計(jì)算緩沖進(jìn)度
      NSLog(@"Time Interval:%f",timeInterval);
      CMTime duration = self.playerItem.duration;
      CGFloat totalDuration = CMTimeGetSeconds(duration);
      [self.videoProgress setProgress:timeInterval / totalDuration animated:YES];
  }

}

上面是iOS KVO運(yùn)用的實(shí)例垫桂，以下為大家介紹一個(gè)簡(jiǎn)單實(shí)用KVO的demo

目前很多寵物醫(yī)生會(huì)根據(jù)狗狗的眼睛顏色的變化來(lái)判斷狗狗是否患有某種疾病师幕，監(jiān)聽狗狗顏色的屬性，會(huì)相應(yīng)的做出狗狗可能患有哪寫疾病的提示诬滩。

.h文件

  @interface Dog : NSObject

  @property(nonatomic, copy)NSString*eyeColorStr;

  @end

ViewController.m文件

  #import "ViewController.h"
  #import "Dog.h"
  @interface ViewController ()
  @property(nonatomic, strong)Dog * dog;
  @property(nonatomic, strong) UILabel * dogEyeColorChangeLab;
  @property(nonatomic, strong) UILabel * dogHealthLab;
  @property(nonatomic, strong)NSMutableArray * dogEyeColorArray;



  @end

  @implementation ViewController
 -(NSMutableArray*)dogEyeColorArray
   {
     if (!_dogEyeColorArray) {
     _dogEyeColorArray=[NSMutableArray new];
   }
    return _dogEyeColorArray;
  }
 - (void)viewDidLoad {
   [super viewDidLoad];
 //黑色和深褐色正常霹粥，米黃色：肝臟有可能病變灭将，蒼白色：角膜炎或貧血，藍(lán)灰色：肝炎
  [self.dogEyeColorArray addObjectsFromArray:@[@"黑色",@"深褐色",@"米黃色",@"蒼白色",@"藍(lán)灰色"]];
  Dog *dog = [[Dog alloc]init];
  self.dog = dog;
  dog.eyeColorStr = @"黑色";

  UILabel *dogEyeColorChangeLab = [[UILabel alloc]init];
  dogEyeColorChangeLab.frame = CGRectMake(100, 100, 200, 40);
  dogEyeColorChangeLab.text = [NSString stringWithFormat:@"%@",dog.eyeColorStr];
  dogEyeColorChangeLab.textAlignment=NSTextAlignmentCenter;
  dogEyeColorChangeLab.backgroundColor = [UIColor cyanColor];
  [self.view addSubview:dogEyeColorChangeLab];
  self.dogEyeColorChangeLab = dogEyeColorChangeLab;

  [dog addObserver:self forKeyPath:@"eyeColorStr" options:NSKeyValueObservingOptionNew context:nil];


  UILabel *dogHealthLab = [[UILabel alloc]init];
  dogHealthLab.frame = CGRectMake(100, 200, 200, 40);
  dogHealthLab.text = @"狗狗眼睛健康";
  dogHealthLab.textAlignment=NSTextAlignmentCenter;
  dogHealthLab.backgroundColor = [UIColor cyanColor];
  [self.view addSubview:dogHealthLab];
  self.dogHealthLab = dogHealthLab;
 }

  -(void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event
  {
    int index = arc4random() % self.dogEyeColorArray.count;
    self.dog.eyeColorStr = self.dogEyeColorArray[index];
  }


  -(void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSString *,id> *)change context:(void *)context
  {   
NSLog(@"keyPath=%@,object=%@,change=%@,context=%@",keyPath,object,change,context);
self.dogEyeColorChangeLab.text = [change objectForKey:@"new"];

if ([[NSString stringWithFormat:@"%@",[change objectForKey:@"new"]] isEqualToString:@"黑色"]||[[NSString stringWithFormat:@"%@",[change objectForKey:@"new"]] isEqualToString:@"深褐色"]) {
     self.dogHealthLab.text = @"狗狗眼睛健康";

 }else if([[NSString stringWithFormat:@"%@",[change objectForKey:@"new"]] isEqualToString:@"米黃色"])
{
    self.dogHealthLab.text = @"肝臟病變";

}else if([[NSString stringWithFormat:@"%@",[change objectForKey:@"new"]] isEqualToString:@"蒼白色"])
{
    self.dogHealthLab.text = @"角膜炎或貧血";

}else if([[NSString stringWithFormat:@"%@",[change objectForKey:@"new"]] isEqualToString:@"藍(lán)灰色"])
{
    self.dogHealthLab.text = @"肝炎";

}

}
 @end

運(yùn)行效果如下：

KVO實(shí)用.GIF