In the context of app development, gathering accurate user input is crucial. Whether it's for logging in or signing up, ensuring the data entered meets certain criteria is essential for app security and usability. Let's explore how to implement validation in a form with a single text field:

• Create a Form with a GlobalKe: Establish a form with a unique identifier to manage its state and perform validation.

• Add Validation Logic to the TextFormField : Implement validation logic within the text field to verify the entered data meets specific requirements.

• Provide Feedback to the User: Display informative error messages to guide users when they input invalid data, ensuring a seamless and user-friendly experience.

1. Create a Form with a GlobalKey

Create a Form with a GlobalKey to organize and validate form fields efficiently. Using a StatefulWidget ensures optimal resource usage by storing a single GlobalKey<FormState>() once.

If you made this a StatelessWidget, you'd need to store this key somewhere. As it is resource-expensive, you wouldn't want to generate a new GlobalKey each time you run the build method.

import 'package:flutter/material.dart';

// Define a custom Form widget.
class MyCustomForm extends StatefulWidget {
  const MyCustomForm({super.key});

  @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: const Column(
        children: <Widget>[
          // Add TextFormFields and ElevatedButton here.
        ],
      ),
    );
  }
}

2. Add a TextFormField with validation logic

Let's integrate text input fields into our form using TextFormField. This widget is essential for user interaction and validation.

We'll ensure the input's validity by adding a validator() function to the TextFormField. If the input doesn't meet the required criteria, the validator function returns a String containing an error message. Otherwise, it returns null.

In this example, we'll validate if the TextFormField is empty. If so, we'll prompt the user to fill in the field.

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;
  },
),

3. Create a button to validate and submit the form

Finally, we'll include a button to trigger form validation and submission. Upon pressing the button, the validator function executes, validating the input. If the input is valid, the form can be submitted for further processing.

ElevatedButton(
  onPressed: () {
    if (_formKey.currentState!.validate()) {
      // Form is valid, submit data
    }
  },
  child: Text('Submit'),
)

How does this work?

To validate the form, we utilize the _formKey created earlier. With this key, we access the FormState using _formKey.currentState(). The FormState class provides the validate() method, which triggers the validation process for each field in the form.

If the validation succeeds for all fields, validate() returns true, indicating that the form is valid. However, if any field fails validation, the method rebuilds the form, displaying error messages accordingly, and returns false.

Flutter FormX Package -

Flutter FormX is an extensible package to make it easy to build, react to and validate forms using MobX and Bloc.

Interactive example

• Demonstrates building a form with validation in Flutter.

• Includes input fields for email and password.

• Features a checkbox for accepting terms and conditions.

• Form validation ensures both fields are filled and terms are accepted before submission.

• Upon successful submission, the form data is processed and a success message is displayed.