counter.state.ts

import { Injectable } from '@angular/core';
import { State } from '@ngxs/store';

interface CountModel {
  value: number;
}

export class IncrementAction {
  public static type: string = '[Increment]';
  constructor(public value: number) {}
}

@State<CountModel>({
  name: 'counter',
  defaults: { value: 0 }
})
@Injectable()
export class CounterState {
   @Action(IncrementAction) 
   public increment(ctx: StateContext<CountModel>, { value }: IncrementAction): void {
     ctx.setState((state) => ({ value: state.value + value }));
   }
}