Dart Flutter Textfield validate current text when textfield is unselected/loses focus

Hello I would like to validate the textfieldinput of a user when the textfield loses it's focus. I could only change the focusstate with the FocusNode but couldnt listen to it. Is there any way to listen to state changes of the FocusNode?

  TextEditingController displayNameController = TextEditingController();
  late FocusNode myFocusNode;

 void initState() {
    super.initState();
    myFocusNode = FocusNode();
    ...
  }
....
TextField(
              focusNode: myFocusNode,
              readOnly: currentview == 2 ? false : true,
              textAlign: TextAlign.center,
              textAlignVertical: TextAlignVertical.center,
              decoration: InputDecoration(
                isDense: true,
                border: InputBorder.none,
                //changeup!!
                errorText: _displayNameValid ? null : 'Bad characters'
              ),
              style: Theme.of(context).textTheme.bodyText2!.copyWith(
                    color: AppTheme.white70,
                  ),
              controller: displayNameController,
            ),