KVO (key-value observation)

// 1. Key-value observation
// 2. It provides a mechanism. When the attribute of the specified object is modified, the object will receive a notification.
// 3. Only objects that comply with the KVC (key-valuedcoding) mechanism can use KVO.
// 4. Implementation Process
// ① Register and specify the observer
// ② Implement the callback Method
// ③ Remove observation

- ( void )viewDidLoad {      [ super  viewDidLoad ];      // Do any additional setup after loading the view from its nib.      // Instantiate an observer object      self .stockForKVO  = [[ StockData  alloc ]  init ];      // Initialization      [ self .stockForKVO  setValue : @"searph"  forKey : @"stockName" ]; // KVC      [ self .stockForKVO  setValue : @"10.0"  forKey : @"price" ]; // KVC            // Listen and display it in lable-register the observer      [ self .stockForKVO  addObserver : self  forKeyPath : @"price"  options : NSKeyValueObservingOptionNew  context :nil ];            self .myLable .textColor  = [ UIColor  redColor ];      self .myLable .text  = [ self .stockForKVO  valueForKey : @"price" ];            // Create button      UIButton  *button = [ UIButton  buttonWithType :UIButtonTypeRoundedRect];      [button  setFrame :CGRectMake( 9 0 ,  1 5 0 ,  1 4 0 ,  4 2 )];      [button  setTitle : @ "Button"  forState :UIControlStateNormal];      [button  addTarget : self  action : @selector (buttonAction)  forControlEvents :UIControlEventTouchUpInside];      [ self .view  addSubview :button]; }  // Button RESPONSE METHOD - ( void )buttonAction {      [ self .stockForKVO  setValue : @"20.0"  forKey : @"price" ]; }  // Callback Method - ( void )observeValueForKeyPath:( NSString  *)keyPath  ofObject :( id )object  change :( NSDictionary  *)change  context :( void  *)context {      if  ([keyPath  isEqualToString : @"price" ])      {          self .myLable .text  = [ self .stockForKVO  valueForKey : @"price" ];      } }  - ( void )dealloc {      // Remove the observer      [ self .stockForKVO  removeObserver : self  forKeyPath : @"price" ]; }