大概有以下幾種方法：

UITextField的代理方法：shouldChangeChractersInRange

- (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string {
    //Replace the string manually in the textbox
    textField.text = [textField.text stringByReplacingCharactersInRange:range withString:string];
    //perform any logic here now that you are sure the textbox text has changed
    [self didChangeTextInTextField:textField];
    return NO; //this make iOS not to perform any action
}
 

[textField addTarget:self action:@selector(textFieldDidChange:) forControlEvents:UIControlEventEditingChanged];

使用kVO

[myUITextField addObserver:self forKeyPath:@"text"
    options:NSKeyValueObservingOptionNew|NSKeyValueObservingOptionOld 
    context:myContext];
 

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

  if(context == myContext) {
    //Here you get notified every time myUITextField's "text" property is updated
    NSLog(@"New value: %@ - Old value: %@",
      [change objectForKey:NSKeyValueChangeNewKey],
      [change objectForKey:NSKeyValueChangeOldKey]);
  }
  else 
    [super observeValueForKeyPath:keyPath ofObject:object 
      change:change context:context];

}