*** Terminating app due to uncaught exception 'NSUnknownKeyException', reason: '[

self.model=[[DataModel alloc]init];
    [self.model setValue:@"searph" forKey:@"stockName"];
    [self.model setValue:@"10.0" forKey:@"price"];
    [self.model addObserver:self forKeyPath:@"price" options:NSKeyValueObservingOptionNew|NSKeyValueObservingOptionOld context:nil];
    
    
    self.myLabel=[[UILabel alloc]initWithFrame:CGRectMake(100, 100, 100, 30)];
    self.myLabel.layer.masksToBounds=YES;
    self.myLabel.layer.cornerRadius=50;
    self.myLabel.textColor=[UIColor cyanColor];
    self.myLabel.text=[self.model valueForKey:@"price"];
    [self.view addSubview:self.myLabel];
    
    self.myButton=[UIButton buttonWithType:UIButtonTypeRoundedRect];
    self.myButton.frame=CGRectMake(200,200 , 100, 30);
    [self.myButton addTarget:self action:@selector(changeValue) forControlEvents:UIControlEventTouchUpInside];
    [self.myButton setBackgroundColor:[UIColor lightGrayColor]];
    [self.view addSubview:self.myButton];
}
- (void)changeValue{
    [self.model setValue:@"20.0" forKey:@"price"];
}
- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context{
    if ([keyPath isEqualToString:@"price"]) {
        self.myLabel.text=[self.model valueForKey:@"price"];
    }
}

程序运行之后就报错

*** Terminating app due to uncaught exception 'NSUnknownKeyException', reason: '[ setValue:forUndefinedKey:]: this class is not key value coding-compliant for the key price.'

检查之后发现self.myLabel.text=[self.model valueForKey:@"price"];改为self.myLabel.text=[NSString stringWithFormat:@"%@",[self.model valueForKey:@"price"]];即可