Flutter key 的使用

官方文档：forms validation

// Create a Form Widget
class MyCustomForm extends StatefulWidget {
  @override
  MyCustomFormState createState() {
    return MyCustomFormState();
  }
}

// Create a corresponding State class. This class will hold the data related to
// the form.
class MyCustomFormState extends State<MyCustomForm> {
  // Create a global key that will uniquely identify the Form widget and allow
  // us to validate the form
  //
  // Note: This is a GlobalKey<FormState>, not a GlobalKey<MyCustomFormState>!
  final _formKey = GlobalKey<FormState>();

  @override
  Widget build(BuildContext context) {
    // Build a Form widget using the _formKey we created above
    return Form(
      key: _formKey,
      child: Column(
        crossAxisAlignment: CrossAxisAlignment.start,
        children: <Widget>[
          TextFormField(
            validator: (value) {
              if (value.isEmpty) {
                return 'Please enter some text';
              }
            },
          ),
          Padding(
            padding: const EdgeInsets.symmetric(vertical: 16.0),
            child: RaisedButton(
              onPressed: () {
                // Validate will return true if the form is valid, or false if
                // the form is invalid.
                if (_formKey.currentState.validate()) {
                  // If the form is valid, we want to show a Snackbar
                  Scaffold.of(context)
                      .showSnackBar(SnackBar(content: Text('Processing Data')));
                }
              },
              child: Text('Submit'),
            ),
          ),
        ],
      ),
    );
  }
}

上例中，如果不使用 key 。key.currentState 直接引用到对应的 FormState 对象，并以此调用其功能。类似的还有 ScafoldState 。所以，以此例看，key 更像是一个全局变量？似乎又不是