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widget build(BuildContext context) {
const appTitle = 'form validation demo';
return MaterialApp(
title: appTitle,
home: scaffold(
appBar: AppBar(
title: const Text(appTitle),
),
body: const MyCustomForm(),
),
);
}
}
// create a form widget.
class MyCustomForm extends StatefulWidget {
const MyCustomForm({super.key});
@override
MyCustomFormState createState() {
return MyCustomFormState();
}
}
// create 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(
crossAxisAlignment: CrossAxisAlignment.start,
children: [
TextFormField(
// the validator receives the text that the user has entered.
validator: (value) {
if (value == null || value.isEmpty) {
return 'please enter some text';
}
return null;
},
),
padding(
padding: const EdgeInsets.symmetric(vertical: 16),
child: ElevatedButton(
onPressed: () {
// validate returns true if the form is valid, or false otherwise.
if (_formkey.currentstate!.validate()) {
// if the form is valid, display a snackbar. in the real world,
// you'd often call a server or save the information in a database.
ScaffoldMessenger.of(context).showSnackBar(
const SnackBar(content: Text('Processing data')),
);
}
},
child: const Text('Submit'),
),
),
],
),
);
}
}
<code_end>
to learn how to retrieve these values, check out the
retrieve the value of a text field recipe.
<topic_end>
<topic_start>
display a snackbar
it can be useful to briefly inform your users when certain actions
take place. for example, when a user swipes away a message in a list,
you might want to inform them that the message has been deleted.
you might even want to give them an option to undo the action.
in material design, this is the job of a SnackBar.
this recipe implements a snackbar using the following steps:
<topic_end>
<topic_start>
1. create a scaffold
when creating apps that follow the material design guidelines,
give your apps a consistent visual structure.
in this example, display the SnackBar at the bottom of the screen,
without overlapping other important
widgets, such as the FloatingActionButton.
the scaffold widget, from the material library,
creates this visual structure and ensures that important
widgets don’t overlap.
<code_start>
return MaterialApp(
title: 'snackbar demo',
home: scaffold(
appBar: AppBar(
title: const Text('SnackBar demo'),
),
body: const SnackBarPage(),
),