Select

Selects are functions that slice a specific portion of state from the global state container.

In CQRS and Redux patterns, we keep READ and WRITE separated. This pattern also exists in NGXS. When we want to read data out of our store, we use a select operator to retrieve this data.

In NGXS, there are two methods to select state, we can either call the select method on the Store service or use the @Select decorator. First let's look at the @Select decorator.

Select Decorators

You can select slices of data from the store using the @Select decorator. It has a few different ways to get your data out, whether passing the state class, a function, a different state class or a memoized selector.

import { Select } from '@ngxs/store';
import { ZooState, ZooStateModel } from './zoo.state';

@Component({ ... })
export class ZooComponent {
  // Reads the name of the state from the state class
  @Select(ZooState) animals$: Observable<string[]>;

  // Uses the pandas memoized selector to only return pandas
  @Select(ZooState.pandas) pandas$: Observable<string[]>;

  // Also accepts a function like our select method
  @Select(state => state.animals) animals$: Observable<string[]>;

  // Reads the name of the state from the parameter
  @Select() zoo$: Observable<ZooStateModel>;
}

Store Select Function

The Store class also has a select function:

import { Store } from '@ngxs/store';

@Component({ ... })
export class ZooComponent {
  animals$: Observable<string[]>;

  constructor(private store: Store) {
    this.animals$ = this.store.select(state => state.zoo.animals);
  }
}

This is most helpful to programmatic selects where we can't statically declare them with the select decorator.

There is also a selectOnce that will basically do select().pipe(take(1)) for you automatically as a shortcut method.

This can be useful in route guards where you only want to check the current state and not continue watching the stream. It can also be useful for unit testing.

Snapshot Selects

On the store, there is a selectSnapshot function that allows you to pull out the raw value. This is helpful for cases where you need to get a static value but can't use Observables. A good use case for this would be an interceptor that needs to get the token from the auth state.

@Injectable()
export class JWTInterceptor implements HttpInterceptor {

  constructor(private _store: Store) {}

  intercept(req: HttpRequest<any>, next: HttpHandler): Observable<HttpEvent<any>> {
    const token = this._store.selectSnapshot<string>((state: AppState) => state.auth.token);
    req = req.clone({
      setHeaders: {
        Authorization: `Bearer ${token}`
      }
    });

    return next.handle(req);
  }

}

Memoized Selectors

Oftentimes you will use the same selectors in several different places or have complex selectors you want to keep separate from your component. NGXS has a @Selector decorator that will help us with that. This decorator will memoize the function for performance as well as automatically slice the state portion you are dealing with.

Let's create a selector that will return a list of pandas from the animals.

import { State, Selector } from '@ngxs/store';

@State<string[]>({
  name: 'animals',
  defaults: []
})
export class ZooState {

  @Selector() 
  static pandas(state: string[]) {
    return state.filter(s => s.indexOf('panda') > -1);
  }

}

Notice, the state is just the local state for this ZooState class. Now in our component, we simply do:

@Component({...})
export class AppComponent {
  @Select(ZooState.pandas) pandas$: Observable<string[]>;
}

and our pandas$ will only return animals with the name panda in them.

Memoized Selectors with Arguments

Selectors can be configured to accept arguments. There are two patterns that allow for this: Lazy Selectors or Dynamic Selectors

Lazy Selectors

To create a lazy selector all that you need to do is return a function from the selector. The function returned by the selector will be memoized automatically and the logic inside this function will be evaluated at a later stage when the consumer of the selector executes the function. Note that this function can take any number of arguments (or zero arguments) as it is the consumer's responsibility to supply them.

For instance, I can have a Lazy Selector that will filter my pandas to the provided type of panda.

@State<string[]>({
  name: 'animals',
  defaults: []
})
export class ZooState {

  @Selector() 
  static pandas(state: string[]) {
    return (type: string) => {
      return state.filter(s => s.indexOf('panda') > -1)
                  .filter(s => s.indexOf(type) > -1);
    };
  }

}

then you can use store.select and evaluate the lazy function using the rxjs map pipeline function.

import { Store } from '@ngxs/store';
import { map } from 'rxjs/operators';

@Component({ ... })
export class ZooComponent {
  babyPandas$: Observable<string[]>;

  constructor(private store: Store) {
    this.babyPandas$ = this.store.select(ZooState.pandas).pipe(map(filterFn => filterFn('baby')));
  }
}

Dynamic Selectors

A dynamic selector is created by using the createSelector function as opposed to the @Selector decorator. It does not need to be created in any special area at any specific time. The typical use case though would be to create a selector that looks like a normal selector but takes an argument to provide to the dynamic selector.

For instance, I can have a Dynamic Selector that will filter my pandas to the provided type of panda.

@State<string[]>({
  name: 'animals',
  defaults: []
})
export class ZooState {

  static pandas(type: string) {
    return createSelector([ZooState], (state: string[]) => {
      return state.filter(s => s.indexOf('panda') > -1)
                  .filter(s => s.indexOf(type) > -1);
    });
  }

}

then you can use @Select to call this function with the parameter provided.

import { Store } from '@ngxs/store';
import { map } from 'rxjs/operators';

@Component({ ... })
export class ZooComponent {

  @Select(ZooState.pandas('baby'))
  babyPandas$: Observable<string[]>;

  @Select(ZooState.pandas('adult'))
  adultPandas$: Observable<string[]>;  

}

Note that each of these selectors have their own separate memoization. Even if two dynamic selectors created in this way are provided the same argument, they will have separate memoization.

These selectors are extremely powerful and are what is used under the hood to create all other selectors.

Dynamic Selectors (dynamic state slice)

An interesting use case would be to allow for a selector to be reused to select from States that have the same structure. For example:

export class SharedSelectors {

  static getEntities(stateClass) {
    return createSelector([stateClass], (state: { entities: any[] }) => {
      return state.entities;
    });
  }

}

then this could be used as follows:

@Component({ ... })
export class ZooComponent {

  @Select(SharedSelectors.getEntities(ZooState))
  zoos$: Observable<Zoo[]>;

  @Select(SharedSelectors.getEntities(ParkState))
  parks$: Observable<Park[]>;

}

Joining Selectors

When defining a selector, you can also pass other selectors into the signature of the Selector decorator to join other selectors with this state selector.

@State<PreferencesStateModel>({ ... })
export class PreferencesState { ... }

@State<string[]>({ ... })
export class ZooState {

  @Selector([PreferencesState])
  static pandas(state: string[], preferencesState: PreferencesStateModel) {
    return state.filter(s =>
      (s.indexOf('panda') > -1 && s.location === preferencesState.location));
  }

}

When using the Selector decorator along with a state class, it will still inject the state class's state first followed by the other selectors in the order they were passed in the signature.

Meta Selectors

By default selectors in NGXS are bound to a state. Sometimes you need the ability to join to un-related states in a high-performance re-usable fashion. A meta selector is a selector allows you to bind N number of selectors together to return a state stream.

Let's say we have 2 states; 'zoos' and 'theme parks'. We have a component that needs to show all the zoos and theme parks for a given city. These are two very distinct state classes that are likely not related in any manner. We can use a meta selector to join these two states together like:

export class CityService {

  @Selector([Zoo, ThemePark]) 
  static zooThemeParks(zoos, themeParks) {
    return [
      ...zoos,
      ...themeParks
    ];
  }

}

now we can use this zooThemeParks selector anywhere in our application.