add form key and validation in flutter code example

Example 1: create a validator in flutter

// Define a custom Form widget.
class MyCustomForm extends StatefulWidget {
  @override
  MyCustomFormState createState() {
    return MyCustomFormState();
  }
}

// Define a corresponding State class.
// This class holds data related to the form.
class MyCustomFormState extends State<MyCustomForm> {
  // Create a global key that uniquely identifies the Form widget
  // and allows validation of 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 created above.
    return Form(
      key: _formKey,
      child: Column(
        children: <Widget>[
              // Add TextFormFields and ElevatedButton here.
        ]
     )
    );
  }
}

Example 2: validator flutter

final passwordValidator = MultiValidator([  
    RequiredValidator(errorText: 'password is required'),  
    MinLengthValidator(8, errorText: 'password must be at least 8 digits long'),  
    PatternValidator(r'(?=.*?[#?!@$%^&*-])', errorText: 'passwords must have at least one special character')  
 ]);  
  
  String password;  
  
  Form(  
    key: _formKey,  
    child: Column(children: [  
      TextFormField(  
        obscureText: true,  
        onChanged: (val) => password = val,  
        // assign the the multi validator to the TextFormField validator  
        validator: passwordValidator,  
      ),  
  
      // using the match validator to confirm password  
      TextFormField(  
        validator: (val) => MatchValidator(errorText: 'passwords do not match').validateMatch(val, password),  
      )  
    ]),  
  );