Angular State Management Simple state management in Angular with only Services and RxJS

One of the most challenging things in software development is state management. Currently there are several state management libraries for Angular apps: NGRX, NGXS, Akita... All of them have different styles of managing state, the most popular being NGRX, which pretty much follows the FLUX/Redux principles from React world (basically using one way data flow and immutable data structures but with RxJS observable streams).

But what if you don't want to learn, setup, use an entire state management library, and deal with all the boilerplate for a simple project, what if you want to manage state by only using tools you already know well as an Angular developer, and still get the performance optimisations and coherency that state management libraries provide (On Push Change Detection, one way immutable data flow).

DISCLAIMER: This is not a post against state management libraries. We do use NGRX at work, and it really helps us to manage very complex states in very big and complex applications, but as I always say, NGRX complicates things for simple applications, and simplifies things for complex applications, keep that in mind.

In this write up, I'll show you a simple way of managing state by only using RxJS and Dependency Injection, all of our component tree will use OnPush change detection strategy.

Imagine we have simple Todo app, and we want to manage its state, we already have our components setup and now we need a service to manage the state, let's create a simple Angular Service:

// todos-store.service.ts

@Injectable({provideIn: 'root'})
export class TodosStoreService {


}

So what we need is, a way to provide a list of todos, a way to add todos, remove, filter, and complete them, we'll use getters/setters and RxJS's Behaviour Subject to do so:

First we create ways to read and write in todos:

// todos-store.service.ts

@Injectable({provideIn: 'root'})
export class TodosStoreService {

  // - We set the initial state in BehaviorSubject's constructor
  // - Nobody outside the Store should have access to the BehaviorSubject 
  //   because it has the write rights
  // - Writing to state should be handled by specialized Store methods (ex: addTodo, removeTodo, etc)
  // - Create one BehaviorSubject per store entity, for example if you have TodoGroups
  //   create a new BehaviorSubject for it, as well as the observable$, and getters/setters
  private readonly _todos = new BehaviorSubject<Todo[]>([]);

  // Expose the observable$ part of the _todos subject (read only stream)
  readonly todos$ = this._todos.asObservable();


  // the getter will return the last value emitted in _todos subject
  get todos(): Todo[] {
    return this._todos.getValue();
  }


  // assigning a value to this.todos will push it onto the observable 
  // and down to all of its subsribers (ex: this.todos = [])
  private set todos(val: Todo[]) {
    this._todos.next(val);
  }

  addTodo(title: string) {
    // we assaign a new copy of todos by adding a new todo to it 
    // with automatically assigned ID ( don't do this at home, use uuid() )
    this.todos = [
      ...this.todos, 
      {id: this.todos.length + 1, title, isCompleted: false}
    ];
  }

  removeTodo(id: number) {
    this.todos = this.todos.filter(todo => todo.id !== id);
  }


}

Now let's create a method that will allow us to set todo's completion status:

// todos-store.service.ts


setCompleted(id: number, isCompleted: boolean) {
  let todo = this.todos.find(todo => todo.id === id);

  if(todo) {
    // we need to make a new copy of todos array, and the todo as well
    // remember, our state must always remain immutable
    // otherwise, on push change detection won't work, and won't update its view    

    const index = this.todos.indexOf(todo);
    this.todos[index] = {
      ...todo,
      isCompleted
    }
    this.todos = [...this.todos];
  }
}

And finally an observable source that will provide us with only completed todos:

// todos-store.service.ts

// we'll compose the todos$ observable with map operator to create a stream of only completed todos
readonly completedTodos$ = this.todos$.pipe(
  map(todos => todos.filter(todo => todo.isCompleted))
)

Now, our todos store looks something like this:

// todos-store.service.ts


@Injectable({providedIn: 'root'})
export class TodosStoreService {

  // - We set the initial state in BehaviorSubject's constructor
  // - Nobody outside the Store should have access to the BehaviorSubject 
  //   because it has the write rights
  // - Writing to state should be handled by specialized Store methods (ex: addTodo, removeTodo, etc)
  // - Create one BehaviorSubject per store entity, for example if you have TodoGroups
  //   create a new BehaviorSubject for it, as well as the observable$, and getters/setters
  private readonly _todos = new BehaviorSubject<Todo[]>([]);

  // Expose the observable$ part of the _todos subject (read only stream)
  readonly todos$ = this._todos.asObservable();


  // we'll compose the todos$ observable with map operator to create a stream of only completed todos
  readonly completedTodos$ = this.todos$.pipe(
    map(todos => todos.filter(todo => todo.isCompleted))
  )

  // the getter will return the last value emitted in _todos subject
  get todos(): Todo[] {
    return this._todos.getValue();
  }


  // assigning a value to this.todos will push it onto the observable 
  // and down to all of its subsribers (ex: this.todos = [])
  private set todos(val: Todo[]) {
    this._todos.next(val);
  }

  addTodo(title: string) {
    // we assaign a new copy of todos by adding a new todo to it 
    // with automatically assigned ID ( don't do this at home, use uuid() )
    this.todos = [
      ...this.todos, 
      {id: this.todos.length + 1, title, isCompleted: false}
    ];
  }

  removeTodo(id: number) {
    this.todos = this.todos.filter(todo => todo.id !== id);
  }

  setCompleted(id: number, isCompleted: boolean) {
    let todo = this.todos.find(todo => todo.id === id);

    if(todo) {
      // we need to make a new copy of todos array, and the todo as well
      // remember, our state must always remain immutable
      // otherwise, on push change detection won't work, and won't update its view
      const index = this.todos.indexOf(todo);
      this.todos[index] = {
        ...todo,
        isCompleted
      }
      this.todos = [...this.todos];
    }
  }

}

Now our smart components can access the store and manipulate it easily:

(PS: Instead of managing immutability by hand, I'd recommend using something ImmutableJS)

// app.component.ts

export class AppComponent  {
  constructor(public todosStore: TodosStoreService) {}
}

<!-- app.component.html -->

<div class="all-todos">

  <p>All todos</p>

  <app-todo 
    *ngFor="let todo of todosStore.todos$ | async"
    [todo]="todo"
    (complete)="todosStore.setCompleted(todo.id, $event)"
    (remove)="todosStore.removeTodo($event)"
  ></app-todo>
</div>

And here is the complete and final result:

Full example on StackBlitz with a real REST API

This is a scalable way of managing state too, you can easily inject other store services into each other by using Angular's powerful DI system, combine their observables with pipe operator to create more complex observables, and inject services like HttpClient to pull data from your server for example. No need for all the NGRX boilerplate or installing other State Management libraries. Keep it simple and light when you can.

---

Follow me on Twitter for more interesting Angular related stuff: https://twitter.com/avatsaev

Comments

[Oleg Varaksin]

Exactly what I'm trying to do in my app. An advice: don't put HTTP, async services, etc. into the store. Keep it separate. State management has nothing to do with such services and business logic. Ngrx Effects is a terrible mix of two concepts.

[Aslan Vatsaev]

You're right, side effects must always be separated from state management, this was a quick example, i'll try to clean it up when I have some free time.

[Esteban Hernández]

I've almost made a career out of reducing the complexity and size of Angular 2+ apps by appropriately using services and RxJs instead of convoluted, home-grown solutions to state management.

My resume:

• Read the Angular docs past the 2nd page.

[Ronan Connolly 🛠]

Very well put together, succinct article!
We don't use NgRX at my workplace, but we do have a couple of data source services.
They kind of just grew organically.

I see the benefits of following your rules of immutability, having a private behaviourSubject, and the getter and setter. Also the second readonly observable which pipes the behaviourSubject is very nice.

Is the term storeService a widely used suffix?
I currently use dataSourceService.

[Aslan Vatsaev]

Thanks!

Is the term storeService a widely used suffix?

nope it's up to you to name it, I usually name it something like TodosStore

[Ronan Connolly 🛠]

I think I prefer the suffix store over dataSource.
Also the ngrx stuff talks about stores a lot. Seems to be a popular term.

[Iain Adams]

This is exactly what I do in my project. I trialled ngrx/store but found it so overly bloated with a ridiculous amount of boilerplate for very little gain. This pattern is simple, elegant, easy to reason about.

[Esteban Hernández]

Give ngrx/data a look for a more streamlined version of ngrx/store. It's excellent for managing collections of objects. It provides almost all of the functionality you might need out of the box and is based on configuration so you end up writing very little logic.

[Cameron Spear]

This looks really nice. After 4ish projects across React and Angular, I have yet to find the Redux pattern remotely worth it... But I'll say that has a lot to do with particular executions...

Regardless, this seems like this would make life a lot easier. :)

One question, tho: is the shareReplay in completedTodos$ not redundant? It would call shareReplay twice in a row in the pipe.

[Aslan Vatsaev]

To be honest I'm not sure, I've put it for good measure, but theoretically yes, i didn't have to use shareReplay on filtering considering that the original source is already multicast, I'll do some tests to confirm, and remove it later.

[chrismarx]

As a few others have pointed out, this is basically Akita. Akita gives you all the basic crud methods you need to start managing either an array of entities (todos) or a single entity, but with none of the boilerplate required by ngrx or ngxs. There's almost no setup, and in addition to getting a rxjs-based approach to state management, you also get all the other bells and whistles, like action tracking with the redux dev tools, time travelling, action tracing, etc. I started out with rxjs stores, and then tried ngrx, ngxs, went back to rxjs stores, then finally found akita, and I see no reason to ever not use it-

github.com/datorama/akita

[Evan BOISSONNOT]

Hi, some perf prob from Akita ? medium.com/@vpranskunas/deep-compa...

I figure out if you get these probs too ?

[Drew Taylor]

Honestly, unless your app is extremely complicated, anyone using a state management library for Angular has misunderstood the component life cycle and is pretty much trying to make Angular into a React application.

[pavelNM]

why use behavior subject with sharereplay?
get todos() is also overkill. It's not a good way to get data from observer, except in async pipe. For adding todo we should use scan operator, same as for mark as completed. Subject with shareReplay (or ReplaySubject) solves all the problems and BehaviorSubject is really redundant here.

[Aslan Vatsaev]

Thanks for the feedback, updated.

[Jason Politis]

Accessors for the same member name must specify the same accessibility

todos getter/setters must either both be public or both be private. My preference is they are private, so that publicly, only the observable is accessible.

Another alternative is make private set _todos and public get todos.

You probably already know better than me which options are best, but thought I throw some options out there for others that come across this.

Thanks for the excellent example. Do you by any chance have an example for todosGroup and todos, and how they would work with each other when navigating from group to single todo?

Thank you for sharing. I value your input as a developer and person. :D

[David]

Thanks for the article! Some points/questions for someone who is quite new to Angular:

1. What made you choose to use a view child?
2. Thank you for including todosTrackFn. Didn't know that existed!
3. Can the getter and setter be private? Or are they used outside of the store?
4. Nice work with the optimistic updates!
5. Since you are defining the todo id in the client, does it need to be optional in the model? Or is it not set somewhere?