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Published 4 years ago • verified publisherxamantra.devDart 3 incompatible• Latest: 2.2.1
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A flutter state management library that focuses on ease of control.

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A flutter state management library that focuses on ease of control.

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The basic idea is that everything can be easily accessed and manipulated. If you are familiar with BLoC package the functions are written in the Bloc class (ex. LoginBloc). In momentum, you write the functions in your MomentumController class (ex. LoginController). One big difference with BLoC package is that there is no "default and easy" way to call another Bloc class, with momentum it's very easy to call another Controller class, like this:

class LoginController extends MomentumController<LoginModel> {
  ...

  void clearSession() {
    // "dependOn" is the magic here. It is a built in method inside MomentumController base class. I can say this is a game changing feature for this library.
    var sessionController = dependOn<SessionController>();
    sessionController.clearSession();

    // that's it, 1 line of code and you are able to access and manipulate other controller (Bloc) class.

    // you can also grab the model value of SessionController:
    var sessionId = sessionController.model.sessionId;
    print(sessionId);
  }

  ...
}

Now, you might be asking where the heck SessionController is instantiated. The answer is right below this section.

Concept #

Momentum - setup class #

  • The Momentum class must be your root widget in order to properly implement it. It acts like a config for your momentum implementation like instantiate all controllers, enabling or disabling logging per controllers, enable time travel feature, lazy loading, etc...

  • First, this is how your fresh flutter app looks like;

      void main() {
    runApp(MyApp());
  }

  • Then, implementing momentum state management will make it look like this:

    void main() {
  runApp(
    Momentum(
      controllers: [
        CounterController(),
        LoginController(),
        SessionController(),
        UserProfileController..config(enableLogging: true), // enable logging and override the global setting.
        RegisterController()..config(maxTimeTravelSteps: 200),
        DashboardController()..config(lazy: true),
      ],
      enableLogging: false, // global setting, all controllers will have logging disabled.
      child: MyApp(),
    ),
  );
}

    Note: The controller classes are just examples, you have to make them yourself. They are like Bloc class in BLoC package. The config(...) method is built in.

MomentumModel - view-model class #

  • The view model for you controller. It is always linked to a specific type of MomentumController. The update(...) method you below is like a copyWith method with just a slight difference. This method is used to update values (immutable). A pair of controller and model can be use in any widgets, they are NOT tied to a single widget only.

    class CounterModel extends MomentumModel<CounterController> {
  final int value;

  CounterModel(
    CounterController controller, {
    this.value,
  }) : super(controller);

  @override
  void update({
    int value,
  }) {
    CounterModel(
      controller,
      value: value ?? this.value,
    ).updateMomentum();
  }
}

MomentumController - logic class #

  • This is where your logic comes (like a "Bloc" class). It is always linked to a specific type of MomentumModel. The MomentumController class is an abstract base class. It requires an implementation of init() method which must return the model type linked with this controller. The library automatically calls this method.

    class CounterController extends MomentumController<CounterModel> {
  @override
  CounterModel init() {
    return CounterModel(
      this,
      value: 0,
    );
  }

  void increment() {
    // let's say model.value is currently = 1
    var value = model.value; // grab the current value.
    model.update(value: value + 1); // calling `model.update(...)` is like calling `setState` so it rebuilds all listeners.
    print('$this -> {value: ${model.value}'); // updated value. model.value is now = 2
  }
}

MomentumBuilder - widget class #

  • Now this is one of the game changer feature of this state management. With the controllers parameter, instead of injecting an instance, you inject a type. It is multiple too which makes it easy to refactor instead of having separate class for single and multiple controller builder class.

    MomentumBuilder(
  controllers: [CounterController], // a type not an instance :)
  builder: (context, snapshot) {
    var counter = snapshot<CounterModel>(); // simply grab a model using generic.
    return Text(
      '${counter.value}',
      style: Theme.of(context).textTheme.display1,
    );
  },
)

Configuration #

enableLogging property #

  • This setting is nothing really important. It only shows logs of a specific controller (number of listeners, if the model is update etc...). Uncaught exceptions are not affected by this setting.
  • Both the Momentum root widget and MomentumController has this config property. Defaults to true.
  • MomentumController overrides Momentum's setting.
  • If this is false from Momentum class but true in MomentumController class, the accepted value will be true. 
      Momentum(
    controller: [
      LoginController()..config(enableLogging: true),
    ]
    enableLogging: false,
    ...
  );

lazy property #

  • Sets when the bootstrap() method will be called. No matter this setting's value, init() method always get called first though.
  • Both the Momentum root widget and MomentumController has this config property. Defaults to true.
  • If this is true, the bootstrap() method will be called when the very first MomentumBuilder that listens to a specific controller will be loaded. 
      Momentum(
    controller: [
      LoginController()..config(lazy: true),
    ]
    ...
  );
    

  MomentumBuilder(
    ...
    controllers: [LoginController], // "bootstrap()" method will be called.
    ...
  );
  • If this is false, the bootstrap() method will be called right when the app starts.

maxTimeTravelSteps property #

  • If the value is greater than 1, time travel methods will be enabled.
  • Both the Momentum root widget and MomentumController has this config property. Defaults to 1, means time travel methods are disabled by default.
  • The value is clamped between 1 and 250.

Managing State #

init() method #

  • MomentumController requires this method to be implemented, but the library will call it for you. This will only be called once. Also, you should NOT put any logic in here, use bootstrap() method instead. 
      class LoginController extends MomentumController<LoginModel> {

    @override
    LoginModel init() {
      return LoginModel(
        this, // inject this controller itself (required by the library).
        username: '',
        password: '',
      );
    }
  }

bootstrap() method #

  • A MomentumController method. Called after init(). This is an optional virtual method. If you override this, the library will call it for you. This is where you put initialization logic instead of init().
  • This is only called once too. 
      class LoginController extends MomentumController<LoginModel> {

    @override
    void bootstrap() async {
      model.update(loadingData: true); // you can use "loadingData" property to display loading indicator in your widget.
      var myProfile = async apiService.getMyProfile(model.userId);
      var friendsList = async apiService.getFriendsList(model.userId); // assuming you actually have friends.
      model.update(
        loadingData: false, // the loading widget will now be hidden :)
        myProfile: myProfile,
        friendsList: friendsList,
      );
    }
  }

Momentum.of<T>(context) method #

  • Get a specific controller to call needed functions. 
      class Login extends StatefulWidget { 
    ...
  }

  class _LoginState extends State<Login> {

    LoginController loginController;

    @override
    didChangeDependencies() {
      loginController = Momentum.of<LoginController>(context); // you can also call this on build but that's dirty, it's up to you.
      super.didChangeDependencies();
    }

    @override
    Widget build(BuildContext context) {
      return Scaffold(
        body: Container(
          child: FlatButton(
            child: Text('Submit'),
            onPressed: () {
              loginController.login(); // call any function you defined in your controller.
            }
          ),
        ),
      );
    }
  }

model.update(...) method #

  • This is the method which is use to rebuild widgets. When this method is called, all MomentumBuilders that listens to a certain controller will rebuild, for example to LoginController. This method is also similar to copyWith. This method must be inside your model class and all properties must be final.

    // declaration
@override
void update({
  String username,
  String password,
}) {
  LoginModel(
    controller,
    username: username ?? this.username,
    password: password ?? this.password,
  ).updateMomentum();
}

// usage
model.update(username: 'momentum', password: '12341234');
print('${model.username}, ${model.password}');
// => "momentum, 12341234"

model.update(password: 'secret'); // only update the password, the username will be retained just like how copyWith works.
print('${model.username}, ${model.password}');
// => "momentum, secret"

    Also, if you are familiar with copyWith method, this method does not accept null values. If it's string just set it as '', if it's integer set it to 0, and you get the idea...

snapshot<T>() builder method #

  • This method is use inside MomentumBuilder's builder method.

    controllers: [LoginController],
builder: (context, snapshot) {
  var loginModel = snapshot<LoginModel>();
  return Text(loginModel.username);
}

  • Multiple controller support:

    controllers: [LoginController, SessionController],
builder: (context, snapshot) {
  var loginModel = snapshot<LoginModel>();
  var sessionModel = snapshot<SessionModel>();
  var username = loginModel.username;
  var sessionId = sessionModel.sessionId;
  return Text('[$sessionId] - $username');
}

  • Of course you can rename the snapshot<T>() method here to even something shorter like use<T> or consume<T>.

reset() method #

  • Reset the model to its initial state. This re-calls init() method. You can also call this anywhere.
  • Call inside the controller itself: 
      class LoginController ... {
    ...

    /// clears the user input for username and password.
    void clearInput() {
      reset();
    }

    ...
  }
  • Call inside a widget: 
      loginController = Momentum.of<LoginController>(context);
  ...
  loginController.reset();
  • Call inside another controller 
      class HomeController ... {
    ...

    /// clears the user session.
    void clearSession() {
      var sessionController = dependOn<SessionController>();
      sessionController.reset();
    }

    ...
  }

backward() time-travel method. #

  • A MomentumController method. Sets the state one step behind from the current model state. Example: 
    loginController.backward();
  • If you are currently in the vey first model state (init), this method will do nothing.
  • You can also call this anywhere like reset() method.

forward() time-travel method. #

  • A MomentumController method. Sets the state one step ahead from the current model state. Example: 
    loginController.forward();
  • If you are currently in the latest model state, this method will do nothing.
  • You can also call this anywhere like reset() method.

Additional note: The library doesn't have any dependencies, except flutter sdk of course.

THE END

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Metadata

A flutter state management library that focuses on ease of control.

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