控制TextField的内容长度

- (void)viewWillAppear:(BOOL)animated
{
    [super viewWillAppear:animated];
    
    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(textFieldDidChange:) name:UITextFieldTextDidChangeNotification object:nil];
}

- (void)viewDidDisappear:(BOOL)animated
{
    [super viewDidDisappear:animated];
    [[NSNotificationCenter defaultCenter] removeObserver:self name:UITextFieldTextDidChangeNotification object:nil];
}

- (void)textFieldDidChange:(NSNotification *)notification
{
    UITextField *textField = (UITextField *)notification.object;
    
    if ( textField.text.length > 6 && textField == _oldTransactionPasswordTextField)
    {
        _oldTransactionPasswordTextField.text = [textField.text substringToIndex:6];
    }
    
    if ( textField.text.length > 6 && textField == _transPasswordTextField)
    {
        _transPasswordTextField.text = [textField.text substringToIndex:6];
    }
}