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Simple state management in Angular with only Services and RxJS

avatsaev profile image Aslan Vatsaev Updated on ・5 min read

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 what if you don't want to learn, setup, and use an entire state management library 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.


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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

Posted on by:

avatsaev profile

Aslan Vatsaev

@avatsaev

I do web engineering with Angular, Node, Rails, Docker... Currently R&D engineer at IRCAD/IHU France

Discussion

markdown guide
 

Definitely my preference. Generally the applications I find myself building don't need anything more than this, and pure observable contexts make things so much more composable, no magic strings anywhere.

 

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.

 

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.

 

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.

 

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

 

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

 

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.

 

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.

 

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.
 

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.

 

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.

 

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.

 

I notice that the StackBlitz project refers to an undefined trackBy function:

todosTrackFn

I can't see any problem that it is causing. The project compiles ok. TSLint flags the problem in my vscode edit buffer.

 

yep it's a typo, fixed it on stackblitz

 

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

 

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.

 

Thanks for the feedback, updated.

 

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?
 

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

 

If you are reaching this level of complexity with more than 1-2 services then i would suggest start looking into using a library with some more restraints.

At least if you are more than 1-2 devs touching the codebase.

Otherwise long-term feature additions and bugfixes are likely to turn this into state soup eventually, once developers start to slack a bit or implement newer idea's.

Which is bound to happen sooner or later in a system without opinionated restraints.

I liked the demo service example, but would have liked to see some with less observable bootstrapping, and maybe just some general guidelines.

Since that's what i find smaller projects benefit more from in my experience. :)

Found the article a good read however, thanks!

 

This was my thought as well.

From looking around it seems like (as of today) Akita basically is this approach, plus some organizational patterns/practices that should help encourage consistency.

I do wish there was a library out there with a longer track record that was lighter than NGRX.

 

Thanks for this post, it's help us very well.

When you define the "readonly completedTodos$" in the service, you use the getter (this.todos) in the map operator to filter the stream. What doesn't use directly the "todos" var entry from the map operator like this:
readonly completedTodos$ = this.todos$.pipe(
map(todos => todos.filter(todo => todo.isCompleted))
) ?

 

Hey, both do the same thing, but yes you are correct, using todos from the map makes more sense, i updated the article.

 

Whats your opinion about using this method in a complex application that doesn't use any state management at all yet (due to lack of initial planning), but our team would like to change this in the future regarding modules one by one starting with the most complex one. Is it a good idea to go with this one, or would it be better idea to go with ngrx because of the complexity?

 

Depends on the scale, if you have a big app with a lot of components than need to share state, and a lot of moving parts, i strongly suggest ngrx, i used this method (the one described in the article) to build several libraries that have internal state, and it works very well, so no need to use ngrx in component libs.

 

Thanks for this. Have been exploring NgRx and have been feeling "this isn't worth it". Did a google search for "simple Angular state management" and here I am.

 

"this isn't worth it"

depends on the case, when I work on large scale apps with a lot of moving parts, it's very worth it

 

Absolutely brilliant! Always know your actual toolset before buying into new ones!

 

First of all, thanks for the article.
Its really great, and certainly expanded my horizons on the use of RxJS.

Just one question, on stack blitz you use toPromise() on the calls to the rest api. This could not also be achieved using observables?

this.todosService.setCompleted(id, isCompleted)
.pipe(take(1))
.subscribe({
next: value => {},
error: err => {
console.error(e);
this.todos[index] = {
...todo,
isCompleted: !isCompleted
}
}
});

I did see that you use it because of async/wait but not understand why.
I'm just trying to understand the difference between the two approaches.

Thanks!

 

Hey Aslan, I do the same thing, except I don't understand the need for sharedReplay. You're already sharing the same observable because it's assigned to a public readonly property. And you're also using a BehaviourSubject which should always return the current value to new subscribers. Also, why not just use a ReplaySubject if you need the previous value? You lose getValue, but it's not such a big deal because you probably want to subscribe anyway. What am I missing?

 

BehaviourSubject is not a simple observable, it's an observer AND an observable at the same time that can give you its latest state synchronously.

You're already sharing the same observable because it's assigned to a public readonly property

That doesn't mean the observer is shared, and that doesn't change the fact that when someone subscribes to todos$, a new dedicated observer will be created for it. A shared observable means that all subscribers are getting their data stream from a single observable source. (lookup rxjs multicasting)

More info here: learnrxjs.io/operators/multicastin...

Also, why not just use a ReplaySubject if you need the previous value? You lose getValue...

I need to be able to get the previous state synchronously in the reducers, as I already explained in my other comment, reducers are synchronous functions that take previous state and reduce it to a new state in an immutable way.

The reason we need the replay part is because some components might be rendered asynchronously (after fetching some data from api for ex), and might miss the emits in todos$ observable, so when they subscribe to it they won't know what happened in the source before component came to life, the replay will emit the latest value to all observers upon subscription.

On the other hand, the second share replay is not necessary, I updated the code.

Hope I answered your questions.

 

Hi, in the user's code I've noticed, that you pass 'event' to the remove method:
(remove)="todosStore.removeTodo($event)"

Shouldn't it be just an id?:
(remove)="todosStore.removeTodo(todo.id)"

 

both are correct, if the app-todo-component outputs the todo id in the event, it'll still work

depends on the implementation of the @Ouptut

 

Yea, I completely agree, they are the same, I guess there is a real small advantage to use todo.id instead of $event - if we use todo.id we explicitly say that we wanna to use id property from list of todos, otherwise if we use $event we delegate the responsibility to the child component to use 'remove' event properly (pass the correct value).

 

Great article. Thank you.
I'm trying to adapt this kind of state managment to my app.
In my scenario I'm subscribing to some events (adding, removing) and do some other logic (for example showing message "you have removed something").
How would you modify your todo example to add this kind of logic in a separate file/ or in other components?

 

Shouldn't the getter and setter for todos be private as well?

 

I really liked this approach and I'm definitively going to use it! But I wonder how could it be used when we actually have to fetch data from an API?

 

It's easy, i'll make an example soon

 

Aslan, I like this approach quite a bit, but I have the same questions. What would it look like when fetching data from an API?

Hey @klouddy @gabrielaraujof , I've updated the stackblitz example with a real REST API and some interesting techniques on how to do optimistic updates and rollbacks: stackblitz.com/edit/angular-rxjs-s...

 

I've updated the stackblitz example with a rest api

 

How do you organize your stores files on the project structure? Do you keep in a separate folder or within the component itself? Thanks!

 

separate modules for each store, inside the module: reducers, effects, facades.

 

What difference is there in using BehaviorSubject in a state manager class instead of sharing a service class with properties passed by reference?

 

onPush change detection, combining/manipulating data streams with RxJS, and it's really fast.

 
 

If this works out for me, and I implement it in 4 hours on my very crazy codebase... Im buying u coffee or a beer...

 

good luck haha, 4 hours is quite a challenge, let me know how it went

 

A great example of clean and organized code.

 

Thank you for sharing Aslan!
Would you recommend this solution for large Enterprise Angular applications with +200 views?

 

Useful and clean implementation of a simple state management.

 

For me Redux has no interest at all and must add at least 50% more work (for nothing).