subscription_widget_ugly.dart

  @override
  void initState() {
    textFieldController.addListener(() {
      var email = textFieldController.text;
      var isValid = EmailValidator.isEmailValid(email);
      var errorText;
      if (!isValid) {
        errorText = "Invalid email";
      }
      setState(() {
        isEmailValid = isValid;
        emailErrorText = errorText;
      });
    });
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    return Row(
      mainAxisAlignment: MainAxisAlignment.center,
      children: <Widget>[
        Expanded(
          child: MyTextField(
              controller: textFieldController,
              hintText: "Email",
              errorText: emailErrorText),
        ),
        SubmitButton(enabled: isEmailValid),
      ],
    );
  }