// 注册通知监听字符的改变 
[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(textfieldEditChange:) name:@"UITextFieldTextDidChangeNotification" object:textFiel];
        // 实现通知       
- (void)textfieldEditChange:(NSNotification *)noti
{
    UITextField *textField = noti.object;
    
    if (textField.text.length > 11)
    {
        textField.text = [textField.text substringToIndex:11];
    }
    
}