Mastering Form Validation in Flutter: A Developer’s Guide

In today’s world, forms are an essential component of any real-world application. They allow users to interact with the application and provide valuable data, while the application offers a better and more secure experience in return. Whether it’s a simple login form or a complex multi-step checkout process, forms are an integral part of the user experience.

However, without proper validation, accepting user input can quickly turn into a nightmare. From bugs to security vulnerabilities, the consequences of not validating user input can be disastrous. Therefore, it is essential to validate user input to ensure that our applications function as intended and our users have a smooth and secure experience.

Luckily, Flutter provides an easy way for developers to validate user input using the Form widget. This widget acts as a container that groups and validates multiple form fields, making it a breeze to ensure that all user input is valid and secure. Plus, with properties like child and key, developers can customize the Form widget to fit their specific needs.

The key property of the Form widget is particularly useful, as it takes in a GlobalKey that uniquely identifies the form throughout the entire app. This means that developers can access and validate user input from anywhere in the application, making it easier to ensure that all input is correct and secure.

class FormValidation extends StatefulWidget {
  const FormValidation({Key? key}) : super(key: key);

  @override
  State<FormValidation> createState() => _FormValidationState();
}

class _FormValidationState extends State<FormValidation> {
  final _formKey = GlobalKey<FormState>();
  @override
  Widget build(BuildContext context) {
    return Form(
      key: _formKey,
      child: Column(),
    );
  }
}

With our Form container set up, it’s time to add form fields using the TextFormField widget. This is where users input their data, and it’s essential to validate their input. Thankfully, TextFormField comes with a built-in validator function that we can use to define our validation logic. This function not only validates the input but also provides us with access to the user input so that we can work with it in our code. With this flexibility, we can ensure that the user’s input is correct and meets our application’s requirements. So, let’s add some TextFormField widgets to our Form and start validating that user input like a pro!

class FormValidation extends StatefulWidget {
  const FormValidation({Key? key}) : super(key: key);

  @override
  State<FormValidation> createState() => _FormValidationState();
}

class _FormValidationState extends State<FormValidation> {
  final _formKey = GlobalKey<FormState>();
  @override
  Widget build(BuildContext context) {
    return Form(
      key: _formKey,
      child: Column(
        crossAxisAlignment: CrossAxisAlignment.start,
        children: [
          TextFormField(
            validator: (value) {
              if (value == null || value.isEmpty) {
                return 'Please enter some input';
              }
              return null;
            },
          ),
        ],
      ),
    );
  }
}

If the user’s input is invalid, such as being empty or null, the validator function returns a string containing an error message. This message is displayed to the user, indicating what went wrong with their input. On the other hand, if the input is valid, the validator function returns null, indicating that the input is good to go.

Now, let’s add a submit button that will send the validated user input to our server or database. When the user clicks the submit button, we can check if the form is valid in the button’s onPressed method. If the form is valid, we can proceed with submitting the data, but if it’s not valid, we can prompt the user to correct any errors before submitting the data. With this approach, we can ensure that only valid data is being sent to our server or database, improving the overall quality and security of our application.

ElevatedButton(
  onPressed: () {
    if (_formKey.currentState!.validate()) {
      ScaffoldMessenger.of(context).showSnackBar(
        const SnackBar(
          content: Text('Submission successful'),
        ),
      );
    } else {
      ScaffoldMessenger.of(context).showSnackBar(
        const SnackBar(
          content: Text('Error Invalid Input'),
        ),
      );
    }
  },
  child: const Text('Submit Form'),

To check if our form is valid or not, we can use the formKey to get the current state of the form and validate it. If the form is valid, we can show a success message to the user via a SnackBar. If it’s not valid, we can display an error message instead.

Remember that the formKey is a unique identifier that not only provides access to the form within nested elements in the tree but also provides access to the FormState class. This class contains a validate() method that runs the validator() function for each TextFormField in the form. If the input is valid, the validate() method returns true, and if it’s invalid, the method rebuilds the form to display any error messages and then returns false.

So, let’s bring everything together. With our formKey and TextFormField widgets set up, we can now validate the user’s input and submit it to our server or database like so:

class FormValidation extends StatefulWidget {
  const FormValidation({Key? key}) : super(key: key);

  @override
  State<FormValidation> createState() => _FormValidationState();
}

class _FormValidationState extends State<FormValidation> {
  final _formKey = GlobalKey<FormState>();
  @override
  Widget build(BuildContext context) {
    return Form(
      key: _formKey,
      child: Column(
        crossAxisAlignment: CrossAxisAlignment.start,
        children: [
          TextFormField(
            validator: (value) {
              if (value == null || value.isEmpty) {
                return 'Please enter some text';
              }
              return null;
            },
          ),
          Padding(
            padding: const EdgeInsets.symmetric(vertical: 16.0),
            child: ElevatedButton(
              onPressed: () {
                if (_formKey.currentState!.validate()) {
                  ScaffoldMessenger.of(context).showSnackBar(
                    const SnackBar(
                      content: Text('Submission successful'),
                    ),
                  );
                } else {
                  ScaffoldMessenger.of(context).showSnackBar(
                    const SnackBar(
                      content: Text('Error Invalid Input'),
                    ),
                  );
                }
              },
              child: const Text('Submit Form'),
            ),
          )
        ],
      ),
    );
  }
}

By taking the time to implement form validation, we are not only improving the user experience, but also protecting our users from potential security threats. Every line of code we write can make a difference, and by prioritizing validation, we are taking a crucial step towards building more robust and trustworthy applications.