Easy form building in Flutter with a fluent schema API

July 30, 2021 API

former – Easy form building in Flutter

Motivation

Formik is one of my favorite React libraries. It is a form library that drastically reduces boilerplate for keeping track of field values, validation, and form submission.

Form building in Flutter suffers from similar issues:

Developers have to manually keep track of field values, for example using TextEditingControllers.
Validation and error handling requires imperative logic.

This is where former comes in.

Installation

Latest version: 0.2.0-rc.1

NOTE: THIS PACKAGE IS AT A PRE-RELEASE STAGE – API CAN CHANGE DRASTICALLY IN THE FUTURE.

Add former to the dependencies section of your pubspec.yaml:

dependencies:
  # ...your other dependencies
  former: # optionally lock-in a version

and add former_gen to the dev_dependencies section:

dev_dependencies:
  # ...your other dependencies
  former_gen: # optionally lock-in a version

Finally, run flutter pub get.

Features

former provides the following features:

Enabling/disabling form globally
Declarative form validation
Automatic value tracking via Former widgets
Easy error handling with FormerError widget.
Type-safe access of form.

Usage

Creating the form

former works by inspecting your form class and generating the corresponding code that makes it work with the former API.

First, lets create our form class in my_form.dart:

import 'package:former_gen/former_gen.dart';

@Formable()
abstract class _MyForm extends FormerForm {
}

A couple of things to note:

The form class is abstract and private. This is because some logic has to be mixed in before it is usable by former.
The form class extends FormerForm. It interfaces our form class with former so that it can be used by former internals.

The FormerForm requires subclasses to implement the bracket operators. This is not needed in our abstract class because that burden will be handled by former‘s code generation. We only need to implement the submit method. For example, it can include submitting your form to some API for further processing.

For simplicity’s sake, our implementation of submit only returns an empty future value.

Notice that the submit method accepts a BuildContext. This is the same BuildContext used by the Former widget that we will use later to provide this form. It can come in handy when you want to access other Providers in the context. Just make sure that the Providers you want to access are parents of the Former widget that is providing the form.

@Formable()
abstract class _MyForm extends FormerForm {
  @override
  Future<void> submit(BuildContext context) {
    // TODO: implement submit()
    return Future.value();
  }
}

Let’s also add some fields to our form:

@Formable()
abstract class _MyForm extends FormerForm {
  String username = '';
  String email = '';

  @override
  Future<void> submit(BuildContext context) {
    // TODO: implement submit()
    return Future.value();
  }
}

Our form class is not usable until we mix in the generated mixin which makes the form “indexable” with the bracket operator, and also contain type information of the fields in the form. Add the following before the class declaration:

class MyForm = _MyForm with _$MyForm;

…and add this:

part 'my_form.g.dart';

to import the generated code.

The Dart analyzer will complain about unrecognized symbols and imports. To fix it, start the code generation via build_runner:

flutter pub run build_runner build

Specifying the requirements

Imagine that our form has the following requirements:

the username should be at least 10 characters long, but not longer than 50 characters.
the email field, well, should contain a valid email.

Without former, this has to be done in an imperative way by, for example, checking the length of the string. former‘s super declarative API for specifying requirements makes everything easy and readable. All you have to do is to create the schema class that is generated for you. In my_form.dart,

final schema = MyFormSchema(
  username: StringMust()
    ..hasMinLength(10)
    ..hasMaxLength(50),
  email: StringMust()
    ..beAnEmail(),
);

As you can see, the API is very self-explanatory. Note the use of the cascade operator .. – in Dart, instead of returning this for method chaining, the cascade operator .. is preferred.

Building form controls

former exports various widgets that interacts with the given form. To start, let’s first create our form widget:

import 'package:flutter/material.dart';
import 'package:former/former.dart';

import 'my_form.dart';

class Form extends StatelessWidget {
  @override
  Widget build() {
    return Column(
      children: [
        FormerTextField<MyForm>(field: MyFormField.username),
        FormerTextField<MyForm>(field: MyFormField.email),
        ElevatedButton(
            onPressed: () {
              Former.of<MyForm>(context, listen: false).submit();
            },
            child: Text('Submit form')
        )
      ],
    );
  }
}

Our form contains two text fields that control the username and the email field respectively. The MyFormField class is automatically generated for you, so you don’t have to create one yourself.

When the button is clicked, MyForm‘s submit method is called to submit the form. Beside submitting the form, Former.of(context) gives you access to:

the current form with .form. For example, you can access the current value of the username field with Former.of<MyForm>(context).form.username
enabling/disabling the form with .isFormEnabled getter/setter. When a form is disabled, all the former controls controlling the form is automatically disabled as well.
the error of a given field with .errorOf(field) which returns the error message as a result of a failed validation. It returns an empty string when the field is valid, or when no validation is performed yet.

This is an extremely simplified version of a form to showcase the widgets. Realistically, each Former control should have a label describing what they do. In the future, there may be a widget that attaches a label to a Former control. For now, it has to be done manually.

Wrapping it all up

Finally, all we have to do is to wrap our form widget with the Former widget:

import 'package:flutter/material.dart';
import 'package:former/former.dart';

import 'my_form.dart';

class MyApp extends StatelessWidget {
  @override
  Widget build() {
    return MaterialApp(
      home: Scaffold(
        body: Former(
          form: () => MyForm(),
          schema: () => schema, // exported from my_form.dart
          child: _MyForm(),
        ),
      ),
    );
  }
}

Source code

The full source code is available in the example folder.

API

Available widgets

The following widgets are available for use with former:

FormerTextField
FormerCheckbox
FormerSwitch
FormerSlider

In development:

FormerRadio
FormerDropdownButton

Schema

The following validators can be used to validate form fields. Each validator has various methods that imposes extra requirements on a given value (called requirement methods).

Every requirement method accepts an optional error message param that is returned when the value does not meet that requirement.