how to add undelete texts to textfield in ios

1- (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string
2{
3    if (textField.text.length == 1 && [string isEqualToString:@""]) {//When detect backspace when have one character. 
4        textField.text = @"myText";
5    }
6    return YES;
7}