*** Terminating app due to uncaught exception 'NSUnknownKeyException', reason: '[<DataModel 0x7fe44b

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: '[<DataModel 0x7fe44bf22810> 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