title: how to use kvo to observer textfiled's text change?
body: <p>the demo is very simple </p>
  
  <pre><code>// add a textField to viewController's view 
  UITextField *textField = [[UITextField alloc] initWithFrame:CGRectMake(0, 30, 375, 40)];
  
  self.textField = textField;
  
  textField.placeholder = @"Please input text";
  
  // add observer for textField's attribute "text"
  [textField addObserver:self forKeyPath:@"text" options:NSKeyValueObservingOptionNew | NSKeyValueObservingOptionOld context:nil];
  
  [self.view addSubview:textField];
  </code></pre>
  
  <p>and then implement the method:</p>
  
  <pre><code>- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary&lt;NSString *,id&gt; *)change context:(void *)context {
  
  
  NSLog(@"----  text changed"); 
  }
  </code></pre>
  
  <p>at the dealloc method :</p>
  
  <pre><code>- (void)dealloc {
  
  [_textField removeObserver:self forKeyPath:@"text"];
  }
  </code></pre>
  
  <p>but when i input text , the method  observeValueForKeyPath:ofObject:change:context: do not execute</p>
  
  <p>i don't know why</p>