How to show a and hide a widget when a special symbol is inputted on the textfield – Flutter

I am listening to changes of the textfield using the onChanged property of the textfield to detect when @ symbol is pressed. It’s just like a mention feature in every other social apps. I want to know when @ symbol is pressed alongside other values and then show a mention widget until space is entered, then remove the mention widget.

What i want to achieve

1. Detect when @ symbol is pressed, then show the mention widget, if another value is entered then remove the mention widget.

2. If @ is detected in the middle of texts show mention widget

What i have tried

I use regEx to detect the values with @ and print the matched value. I don’t really know how to show the mention widget at the first regEx logic. I then split the textfield values and use a different regEx pattern to check if any value matched then show the mention widget.

It doesn’t work as expected. When scrolled to the middle of the texts in the textfields and add @ to a text, the mention widget doesn’t trigger.

onChanged property code

 void onChanged(String value) { 
    final _mentionRegEx = RegExp(AppUtils.mentionPattern, caseSensitive: false);
    final _checkRegEx = RegExp(
      r"^@[a-zA-Z0-9_ ]+[a-zA-Z0-9_]*$",
      // caseSensitive: false,
      multiLine: true,
    );
    Iterable<Match> matches = _mentionRegEx.allMatches(value);

    for (var m in matches) {
      String match = m[0]!;
      final withoutMentonTag = match.substring(1);
      state = state.copyWith(matched: withoutMentonTag);
      print('MENTION => ${state.matched}');
    }

    final sentences = value.split(' ');
    for (var sentence in sentences) {
      print('SENTENCE => $sentence');
      final words = sentence.split(' ');
      final withMentionTag = words.last;

      if (_checkRegEx.hasMatch(sentence)) {
        toggleMentionWidget(true);
        String withoutMentionTag = withMentionTag.substring(1);
        print('MENTIONED USER => $withoutMentionTag');
      } else {
        toggleMentionWidget(false);
      }
    }
  }