Enable / Disable a confirm button based on validation

[aidanmack]

Is it possible to setup a listener with FormBuilder to enable/disable other widgets?
I've used the original form prop controller to do this . Which works, but is less elegant and I guess kind of defeats the purpose of adding FormBuilder.

Would be nice to have a listener on formKey changes.

final TextEditingController _controllerFirstName = TextEditingController();
final TextEditingController _controllerLastName = TextEditingController();
var _isSendButtonEnabled = false;

@override
void initState() {
  super.initState();
  _controllerFirstName.addListener(() => enableSendCheckBtn(_controllerFirstName.text.trim(), _controllerLastName.text.trim()));
  _controllerLastName.addListener(() => enableSendCheckBtn(_controllerFirstName.text.trim(), _controllerLastName.text.trim()));
}

void enableSendCheckBtn(String firstName, String lastName) {
  (firstName != "" && lastName != "")
      ? setState(() => _isSendButtonEnabled = true)
      : setState(() => _isSendButtonEnabled = false);
}

[deandreamatias]

Related to #1155