how to add undelete texts to textfield in ios

- (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string
{
    if (textField.text.length == 1 && [string isEqualToString:@""]) {//When detect backspace when have one character. 
        textField.text = @"myText";
    }
    return YES;
}