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Why use Reactive Forms in Flutter?

joanpablo profile image Joan Pablo ・3 min read

Haven't you ever felt that getting data from a Form, validating a field or even disabling the submit button is tedious and full of boilerplate code?

Fortunly that's why Reactive Forms exists.

1. Ease and clean way of collecting data from your Forms.

No more onSave, onChanges, onEditionCompleted callbacks needed or TextEditionControllers to collect data from inputs

Reactive Forms get a clean separation between views an model and maintains data synchronization in a transparent two-way binding mechanism. Just declare your model and define your widgets. Then layback and relax, data will flow smoothly.

// group and controls
final form = FormGroup({
  'name': FormControl(defaultValue: 'John Doe'),
  'email': FormControl(),
});
// two-way bindings with widgets
@override
Widget build(BuildContext context) {
  return ReactiveForm(
    formGroup: this.form,
    child: Column(
      children: <Widget>[
        ReactiveTextField(
          formControlName: 'name',
        ),
        ReactiveTextField(
          formControlName: 'email',
        ),
      ],
    ),
  );
}

2. Transparent Validations of inputs fields.

No more StatefulWidget defining a key for the Form widget, goodbye to Form.of(context) and Goodbye to:

// boilerplate code...
if (form.validate()) {
  form.save();
} else {
  setState(() {
    _autoValidate = true;
  });
}

Reactive Forms Validations occurs transparently. Just declare your model and define the validators you need. It will also handle validity of the entire Form and will shows error messages when needed. All of that without you needing to write a single line of imperative code.

// controls and validators
final form = FormGroup({
  'email': FormControl(validators: [
    Validators.required,
    Validators.email,
  ]),
});
// customize validation messages
@override
Widget build(BuildContext context) {
  return ReactiveForm(
    formGroup: this.form,
    child: Column(
      children: <Widget>[
        ReactiveTextField(
          formControlName: 'email',
          validationMessages: {
            ValidationMessage.required: 'The email must not be empty',
            ValidationMessage.email: 'The email value must be a valid email'
          },
        ),
      ],
    ),
  );
}

3. Enable/Disable buttons depending of Form validity.

Getting this behavior, even in such a great framework as Flutter, can sometimes be hard and full of boilerplate code.

In Reactive Forms is as simple as asking to the Form if it is valid or not:

@override
  Widget build(BuildContext context) {
    final form = ReactiveForm.of(context);
    return RaisedButton(
      child: Text('Submit'),
      // just ask to the form
      onPressed: form.valid ? _onPressed : null,
    );
  }

4. Perfect integration with Provider plugin and other state management plugins.

The use of a state management library (MVC, MVVM or just BLoc) clean your code and perfectly separate responsibilities between the UI and business logic.

// your bloc/controller/viewmodel/etc
class SignInViewModel {
  final form = FormGroup({
    'email': FormControl<String>(validators: [Validators.required, Validators.email]),
    'password': FormControl<String>(validators: [Validators.required, Validators.minLength(8)])
  });

  void signIn() {
    final credentials = this.form.value;
    // ... make some business logic
    // ...
  }

}
// simple sign-in view
class SignInScreen extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    final viewModel = Provider.of<SignInViewModel>(context, listen: false);
    return ReactiveForm(
      formGroup: viewModel.form,
      child: Column(
        children: <Widget>[
          ReactiveTextField(
            formControlName: 'email',
          ),
          ReactiveTextField(
            formControlName: 'password',
            obscureText: true,
          ),
          ReactiveFormConsumer(
            builder: (context, form, child) {
              return RaisedButton(
                child: Text('Submit'),
                // if form valid, sign-in
                onPressed: form.valid ? viewModel.signIn : null,
              );
            },
          ),
        ],
      ),
    );
  }
}

5. Focus/Unfocus text fields.

To add or remove focus to a control use FormControl.focus() or FormControl.unfocus(). No more StatefulWidgets and FocusNode declarations:

// declare a form and controls
final form = FormGroup({
  'name': FormControl(defaultValue: 'John Doe'),
});

// the name control
final name = form.control('name');

// UI text field get focus and the device keyboard pop up
name.focus();

// UI text field lose focus
name.unfocus();

What is Reactive Forms?

  • Reactive Forms provides a model-driven approach to handling form inputs whose values change over time. It's heavily inspired in Angular Reactive Form.

  • It lets you focus on business logic and save you time from collect, validate and maintain synchronization data between models and widgets.

  • Remove boilerplate code and brings you the posibility to write clean code with minimal efforts.

  • Integrates perfectly with common state management libraries like Provider, Bloc and many others.

What is not Reactive Forms?

  • Reactive Forms is not a fancy widgets package. It's not a widget's library with new shapes, colors or animations. It frees you from the responsibility of gathering and validating the data. And keeps the data in sync between your model and your widgets.

  • Reactive Forms does not replace native widgets that you commonly use in Flutter like TextFormField, DropdownButtonFormField or CheckboxListTile. It brings new two-way binding capabilities and much more features to those same widgets.

Reactive Forms is much more. This POST was just a preview of some basic features. Please visit Github Repo or plugin page in puv.dev for full documentation.

Posted on by:

joanpablo profile

Joan Pablo

@joanpablo

Full Stack Software Developer. Passionate to constantly study new technologies. Appreciate the use of good practices and the quality of code.

Discussion

markdown guide
 

Hi may I ask how did you put an initial value coming from firebase?

 

Hi John,

You can just set the value to a control as soon as you get it or you can reset the control to and initial value (release 4.0.0)

// `fb.group` is just a syntax sugar, you can simple use FormGroup if you want.
final form = fb.group({
    'name': 'some init value'
});

_executeFirebaseAsyncMethod().then((newValue) {
    // sets the control init value
    form.value = {'name': newValue};
});

or

final form = fb.group({
    'name': 'some init value'
});

_executeFirebaseAsyncMethod().then((newValue) {
    // resets the control to untouched and set init value
    form.reset(value: {'name': newValue});
});
 

One more thing it is possible to untouch a reactiveTextfield upon editing?

For example, I have an async validation that will check if an email is existing then, when I go to the editing profile screen it automatically validates the value of that field which I don't like. I want to validate it when the value of that field changes.

Lastly, is it possible to add an additional parameter to custom validator?

Hello John,

Async validators executes after every Sync validator executes without validation errors. That means that if you add for example Validators.required or Validators.email to your ReactiveTextField only when those validators marks the input as valid then the Async Validator excecutes.

But I think you are right about the issue, I will release soon a new version that let you ask inside your validator if the control is "dirty" or not, so you can check if the value has changed or not and request the server in the async validator. Mean while you can implement a workaround by asking inside your async validator if the value of the FormControl is different from the value of your model.

About the Second question, yes you can create a custom validator with additional params, there are two ways of doing this:
1-) Create a function/method that receives arguments and returns a ValidatorFunction. An example of this is the Validators.mustMatch. You can check the README.md.
2-) Extend Validator class.

Creating a function/method

// controlName and matchingControlName as additional arguments
Map<String, dynamic> _mustMatch(String controlName, String matchingControlName) {
  // returns a ValidatorFunction
  return (AbstractControl control) {
    final form = control as FormGroup;

    final formControl = form.control(controlName);
    final matchingFormControl = form.control(matchingControlName);

    if (formControl.value != matchingFormControl.value) {
      matchingFormControl.addError({'mustMatch': true});

      // force messages to show up as soon as possible
      matchingFormControl.touch(); 
    } else {
      matchingFormControl.setErrors({});
    }

    return null;
  };
}

Usage

final form = fb.group({
   'password': ['', Validators.required],
   'passwordConfirmation': ['', Validators.required],
}, [_mustMatch('password', 'passwordConfirmation')]);

Extend Validator class (this is the implementation of Validators.pattern)

/// Validator that requires the control's value to match a regex pattern.
class PatternValidator extends Validator {
  final Pattern pattern;

  /// Constructs an instance of [PatternValidator].
  ///
  /// The [pattern] argument must not be null.
  PatternValidator(this.pattern) : assert(pattern != null);

  @override
  Map<String, dynamic> validate(AbstractControl control) {
    RegExp regex = new RegExp(this.pattern);
    return (control.value == null ||
            control.value == '' ||
            regex.hasMatch(control.value))
        ? null
        : {
            ValidationMessage.pattern: {
              'requiredPattern': this.pattern.toString(),
              'actualValue': control.value,
            }
          };
  }
}

Usage

final form = fb.group({
   'email': ['', PatternValidator('some pattern').validate],
});

Validators.mustMatch and Validators.pattern are already included in Reactive Forms so you don't have to implement them, the above code are just examples to get the idea.

Thank you so much for your help! I will look forward to the next version of your package! ❤

 

That's great! Thank you so much! 💞

 

Hi! Can I ask if it is possible to add a dynamic reactive text field to a form?