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Segmented State Pattern (SSP)

When we talk about a single flow state, we end up solving architecture problems early, as we will have only one data flow for each state.

In addition to the maintainability and ease of use architecture, we also have the possibility of increasing this flow with other standards such as the Observer, which gives reactivity to the component when it is modified, and Memento, which makes it possible to revert or redo this state.

A beautiful example of a pattern with a single flow is the BLoC, giving reactivity to a state allowing all transformations in that flow. This (although complicated for some), consolidates very well in the architecture of a project, even the limits of this practice are beneficial for not allowing the developer to resort to other solutions for the architecture and the standard for its resource.

There are other ways to promote reactivity in a property instead of the entire object, such as MobX's Observable and Flutter's ValueNotifier, and that gives us a lot of choices. However, we lost some important limits for architecture, which can put in check the maintenance of the project in the future. Therefore, it needs a standard to force limits on the individual reactivity of each property and thereby improve the maintainability of the components responsible for managing the states of the application.

Single Flow Pattern (BLoC and Flux/Redux)#

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When we work with a single flow state, that is, when the reactivity is in the object and not in its properties, we can have more control over the data being processed before reaching a listener. For example, if your logic manages an X state and wants to make it Y just assign the value.

MyState state = X();state = Y();

However, the flow can contain asynchronous elements and it is always interesting to inform us that the state is being loaded. This is quite common in Mobile development with API for example.

MyState state = X();state = Loading();state = await getY(); // return Y

The recovery of these data can also fail, and this makes the existence of an error state pertinent.

MyState state = X();state = Loading();try{  state = await getY();} catch(e) {  state = Error();}

As we are talking about a single flow, we use POLYMORPHISM of OOP to share these 3 responsibilities (State Value, Loading, or Error).

abstract class MyState {}
class X extends MyStateclass Y extends MyStateclass Loading extends MyStateclass Error extends MyState

With that, we have a unique Flow of MyState, because the objects X, Y, Loading, and Error inherit from MyState.

X is MyState; // it's true!Y is MyState; // it's true!Loading is MyState; // it's true!Error is MyState; // it's also true!

Thanks OOP :)

IMPORTANT: BLoC is an acronym for Bussines Logic Component.

Targeting the state in State, Error, Loading#

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Now, as we have the possibility to have reactivity by property with MobX or ValueNotifier, we would not need Polymorphism if we divide the responsibility of Loading and Error for separate properties within a STORE. And so we have a triple making Loading and Error actions post or pre-change of state. An example using MobX:

...@observable ProductData state = ProductData.empty();
@observable bool isLoading = false;
@observable Exception? error;
@actionFuture<void> fetchProducts() async {  isLoading = true;  try{    state = await repository.getProducts(); // return ProductData  } catch(e){    error = Exception('Error');  }  isLoading = false;}

In short, we have 3 flows, the state that has the state value, the error that holds the exceptions, and the isLoading bool that informs when the loading action is in effect. Being able to listen to these 3 actions separately helps to transform them and combine them into other actions, supplementing your Store (Class with the logic responsible for managing the state of your component). As the movement of the state is always around the trio State, Error, and Loading, it is worth this for standardization.

IMPORTANT: An object called Store is responsible for storing Logic for the state of a component.

Seeing the Flows#

Having 3 separate flows we can have 3 different listeners, for example, we hear the error to launch it in the form of "SnackBar" and when there are Loadings we launch a Dialog, but if we need to add to this state a pattern like "memento" we will have to put the 3 properties in a generic object. To close the pattern of the 3 Flows we can create a generic object, its properties can be reactive as well as the object itself. Let's look at an example with MobX.

class Triple<Error, State> {  final State state;  final Error? error;  final bool isLoading;
  Triple({required this.state, this.error, this.isLoading = false});
  Triple<Error, State> copyWith({State? state, Error? error, bool? isLoading}){    return Triple<Error, State>(      state: state ?? this.state,      error: error ?? this.error,      isLoading: isLoading ?? this.isLoading,    );  }}

So, we can use:

@observable var triple = Triple<Exception, ProductData>(state: ProductData.empty());
@actionFuture<void> fetchProducts() async {  triple = triple.copyWith(isLoading: true);  try{    final state = await repository.getProducts(); // return ProductData    triple = triple.copyWith(isLoading: false, state: state);  } catch(e){    final error = Exception('Error');    triple = triple.copyWith(isLoading: false, error: error);  }}

We now have an object that joins the 3 segmented state properties that can also be accessed and transformed individually using MobX's @computed which automatically distinguishes and only triggers a reaction if the property is really a new object.

@observable var _triple = Triple<Exception, ProductData>(state: ProductData.empty());
@computedProductData get state => _triple.state;
@computedException get error => _triple.error;
@computedbool get isLoading => _triple.isLoading;

With the object bringing together the state and its actions, we can implement other design patterns or just make transformations on the object or separately on its properties. Let's see a small example of implementation of the Design Pattern Memento that will make it possible for the state to rollback, that is, return to the previous states as a time machine.

@observable var _triple = Triple<Exception, ProductData>(state: ProductData.empty());
@computedProductData get state => _triple.state;@computedException get error => _triple.error;@computedbool get isLoading => _triple.isLoading;
//save all changed statesfinal List<Triple<Exception, ProductData>> _history = [];
@actionvoid update(ProductData state){  _history.add(_triple);  _triple = _triple.copyWith(state: state);}
@actionvoid setError(Exception error){  _triple = _triple.copyWith(error: error);}
@actionvoid setLoading(bool loading){  _triple = _triple.copyWith(loading: loading);}
@actionvoid undo(){  if(_history.length > 0){    _triple = _history.last;    _history.remove(_triple);  } else {    throw Exception('Not have history data');  }}
@actionFuture<void> fetchProducts() async {  _triple = setLoading(true);  try{    final state = await repository.getProducts(); // return ProductData    _triple = update(state);  } catch(e){    final error = Exception('Error');    _triple = setError(error);  }  _triple = setLoading(false);}

We implemented something very complex, but it is easier to understand what is happening just by reading the code. So we come to a standard that can be used to manage states and sub-states using reactivity individually by property.

The Segmented State (Or Triple) pattern can be abstracted to make its reuse stronger. We will use MobX again as an example, but we can use it in any type of reactivity by property.

abstract class MobXStore<Error, State> {
  @observable   late Triple<Error, State> _triple;
  MobXStore(State initialState){     _triple = Triple<Error, State>(state: initialState);  }
  @computed  State get state => _triple.state;  @computed  Error get error => _triple.error;  @computed  bool get isLoading => _triple.isLoading;
  //save all changed states  final List<Triple<Error, State>> _history = [];
  @action  void update(State state){    _history.add(_triple);    _triple = _triple.copyWith(state: state);  }
  @action  void setError(Error error){    _triple = _triple.copyWith(error: error);  }
  @action  void setLoading(bool isLoading){    _triple = _triple.copyWith(isLoading: isLoading);  }
  @action  void undo(){    if(_history.length > 0){      _triple = _history.last;      _history.remove(_triple);    } else {      throw Exception('Not have history data');    }  }}

Now, just implement MobXStore in any Store of MobX.

class Product = ProductBase with _$Product;
abstract class ProductBase extends MobXStore<Exception, ProductData> with Store {
  ProductBase(): super(ProductData.empty());
  @action  Future<void> fetchProducts() async {    setLoading(true);    try{      final state = await repository.getProducts(); // return ProductData      update(state);    } catch(e){      final error = Exception('Error');      setError(error);    }    setLoading(false);  }}