The Form widget is used with the companion of a key of the FormState type that contains helpers to manage all of the children of its FormField instances:

final _key = GlobalKey<FormFieldState<String>>();
...
Form(
  key: _key,
  child: Column(
    children: <Widget>[
      TextFormField(),
      TextFormField(),
    ],
  ),
);

Then, we can use the key to retrieve the Form associated state and call its validation with _key.currentState.validate(). Now, let's have a look at the second option.