KVO

import "ViewController.h"

import "tempView.h"

@interface ViewController ()

@property (nonatomic,strong) tempView *tempView;

@end

@implementation ViewController

• (tempView *)tempView
  {
  if (!_tempView) {
  _tempView = [[tempView alloc] initWithFrame:CGRectMake(100, 100, 100, 100)];

    [_tempView addObserver:self forKeyPath:@"name" options:NSKeyValueObservingOptionNew context:nil];
    
    [self.view addSubview:_tempView];
  

  }
  return _tempView;
  }

• (void)viewDidLoad {
  [super viewDidLoad];

  [self tempView];
  }

• (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event
  {
  self.tempView.name = @"2345";
  }

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

{
//第一个参数就是观察的属性名
//第二个参数是观察对象
//第三个参数是改变的值
//第四个参数是说明
NSLog(@"str 变了");
NSLog(@"%@",keyPath);
if([keyPath isEqualToString:@"name"])//观察的str属性改变了 记准了哈！！！是属性名字！！！不是属性的值，，所以会进来

{
    NSLog(@"%@",change[@"new"]);
    self.view.backgroundColor = [UIColor colorWithRed:arc4random()%255/255.0 green:arc4random()%255/255.0 blue:arc4random()%255/255.0 alpha:arc4random()%255/255.0];
    
}
else{
    //该干嘛干嘛
}

}

tempView

import

@interface tempView : UIView

@property (nonatomic,copy) NSString *name;

@end

import "tempView.h"

@interface tempView ()

@property (nonatomic,weak) UIView *tempView;

@end

@implementation tempView

• (instancetype)initWithFrame:(CGRect)frame
  {
  if (self = [super initWithFrame:frame]) {

    UIView *view = [[UIView alloc] initWithFrame:self.bounds];
    view.backgroundColor = [UIColor redColor];
    [self addSubview:view];
    self.name = @"123";
    self.tempView = view;
  

  }
  return self;
  }

@end