11.3. Working with forms in Flutter: Field validation

Listen in audio

Mobile app development has become an essential skill for many developers, and with Flutter and Dart, building apps from scratch to advanced has become more accessible. In this context, a crucial aspect of application development is working with forms. Forms are an integral part of many applications as they allow for user interaction, be it login, registration, feedback, ordering, etc. Therefore, it is crucial that developers know how to work with forms in Flutter and how to perform field validation.

In Flutter, the 'Form' class is used to create a form. It is a widget that acts as a container for grouping and validating multiple form inputs. The 'Form' class in Flutter works together with the 'FormField' widget, which represents a single input field on the form.

To start working with forms in Flutter, you first need to create an instance of the 'Form' class. You can do this by wrapping your 'FormField' widgets in a 'Form' widget. Each 'FormField' within the 'Form' can be associated with a 'TextEditingController', which allows you to control and access the text within the input field.

Once you have created your form, the next step is field validation. Field validation is essential to ensure that the user enters correct and complete information. In Flutter, you can validate input fields using the 'validator' parameter of the 'FormField'. The 'validator' parameter accepts a function that takes the value of the input field as an argument and returns a string. If the function returns a non-null string, Flutter considers the input field as invalid and displays the string as an error message.

For example, if you want to validate an email input field, you can use a regular expression to check whether the input field value looks like an email address. If not, you can return an error message telling the user to enter a valid email address.

In addition to validating each input field individually, Flutter also allows you to validate the entire form at once using the 'validate' method of the 'Form' class. When the 'validate' method is called, it performs the validation function of each 'FormField' on the 'Form' and returns 'true' if all input fields are valid and 'false' if at least one input field is invalid .

To call the 'validate' method, you need a switch to access the 'Form' state. In Flutter, you can create a key using the 'GlobalKey' class. You can then pass this key to the 'key' parameter of the 'Form' when creating the form.

After validating the form, you can process the information entered by the user. In Flutter, you can do this using the 'save' method of the 'Form' class. The 'save' method calls the 'save' method of each 'FormField' on the 'Form', which in turn calls the 'save' method of the associated 'TextEditingController', which saves the current value of the input field.

In summary, working with forms in Flutter involves creating a 'Form' with several 'FormField', validating the input fields using the 'validator' parameter and the 'validate' method, and processing the entered information using the 'save' method. By mastering these concepts, you'll be well equipped to build robust, interactive apps with Flutter and Dart.

Now answer the exercise about the content: