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final _formKey = GlobalKey<FormState>();
@override
Widget build(BuildContext context) {
// Build a Form widget using the _formKey created above.
return Form(
key: _formKey,
child: const Column(
children: <Widget>[
// Add TextFormFields and ElevatedButton here.
],
),
);
}
}<code_end>
lightbulb Tip
Using a GlobalKey is the recommended way to access a form.
However, if you have a more complex widget tree,
you can use the Form.of() method to
access the form within nested widgets.<topic_end>
<topic_start>
2. Add a TextFormField with validation logic
Although the Form is in place,
it doesn’t have a way for users to enter text.
That’s the job of a TextFormField.
The TextFormField widget renders a material design text field
and can display validation errors when they occur.Validate the input by providing a validator() function to the
TextFormField. If the user’s input isn’t valid,
the validator function returns a String containing
an error message.
If there are no errors, the validator must return null.For this example, create a validator that ensures the
TextFormField isn’t empty. If it is empty,
return a friendly error message.
<code_start>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;
},
),<code_end>
<topic_end>
<topic_start>
3. Create a button to validate and submit the form
Now that you have a form with a text field,
provide a button that the user can tap to submit the information.When the user attempts to submit the form, check if the form is valid.
If it is, display a success message.
If it isn’t (the text field has no content) display the error message.
<code_start>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>
<topic_end>
<topic_start>
How does this work?
To validate the form, use the _formKey created in
step 1. You can use the _formKey.currentState()
method to access the FormState,
which is automatically created by Flutter when building a Form.The FormState class contains the validate() method.
When the validate() method is called, it runs the validator()
function for each text field in the form.
If everything looks good, the validate() method returns true.
If any text field contains errors, the validate() method
rebuilds the form to display any error messages and returns false.<topic_end>
<topic_start>
Interactive example
<code_start>import 'package:flutter/material.dart';
void main() => runApp(const MyApp());
class MyApp extends StatelessWidget {
const MyApp({super.key});
@override
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();
}