export class SampleComponent {
  @Output somethingSelected = new EventEmitter<boolean>()
  ...
  private _selected = false;
  toggleSelected() {
      this._selected = !this._selected;
      this.somethingSelected.emit(this._selected);
  }
}

export class SampleComponent {
  @Output somethingSelected = new EventEmitter<boolean>()
  ...
  private _selected$ = new BehaviorSubject<boolean>(false);
  toggleSelected() {
      this._selected$.next(!this._selected$.value);
      this.somethingSelected.emit(this._selected$.value);
  }
}

export class ToggleSubject extends BehaviorSubject<boolean> {
    toogle() {
        this.next(!this.value);
    }
}

export class SampleComponent {
    @Output somethingSelected = new EventEmitter<boolean>()
  ...
  private _selected$ = new ToggleSubject(false);
  toggleSelected() {
      this._selected$.toggle();
      this.somethingSelected.emit(this._selected$.value);
  }
}

export class ToggleSwitcher extends EventEmitter<boolean> {
    get value(): boolean {
        return this._value
    }
    constructor(private _value = false) {
        super();
    }
    toggle() {
        this.emit(!this.value);
    }
    emit(v: boolean) {
        this._value = v;
        super.emit(v);
    }
}

export class SampleComponent {
    @Output somethingSelected = new ToggleSwitcher()
   ...
}

@Output somethingSelected = new ToggleSwitcher(true)

export class ToggleSwitcher extends BehaviorSubject<boolean> {
    eventEmitter = new EventEmitter<boolean>();
    next(v: boolean) {
        this.eventEmitter.emit(v);
        super.next(v);
    }
    toggle() {
        this.next(!this.value)
    }
}

export class SampleComponent {
    somethingSwitcher = new ToggleSwitcher(false);
    @Output somethingSelected = this.somethingSwitcher.eventEmitter;
}

export class EntityCheckedState<T> {
    entity: T;
    checked: boolean
}
export class EntityMultiSelector<T> extends BehaviorSubject<T[]> {
    private _list: EntityCheckedState<T>[];
    eventEmitter = new EventEmitter<T[]>();
    get list(): EntityCheckedState<T>[] {
        return this._list;
    }
    set list(v: EntityCheckedState<T>[]) {
         this._list = v;
        this.next(this.list.filter(({checked}) => checked).map(({entity}) => entity));
    }
    constructor(v: T[], defaultChecked = false) {
        super(defaultChecked? v : []);
        this.eventEmitter.emit(defaultChecked? v : []);
        this._list = v.map(entity => ({entity, checked: defaultChecked}));
    }
    setCheckedForEntity(entity: T, checked: boolean) {
         this.list = this.list.map(v => (v.entity === entity ? { ...v, checked } : v));
    }
    setCheckedForAll(checked: boolean) {
          this.list = this.list.map(v => ({...v, checked}));
    }
    next(v: T[]) {
        this.eventEmitter.emit(v);
        super.next(v);
    }
}

export class SampleComponent {
    @Input() set data(v: SampleDto[]) {
        this.multiSelector = new EntityMultiSelector<SampleDto>(v);
        this.selectedSamples = this.multiSelector.eventEmitter;
  }
  multiSelector: EntityMultiSelector<SampleDto>;
  @Output() selectedSamples: EventEmitter<SampleDto[]>
}

<app-sample-entity *ngFor = "let state of multiSelector.list"
                    [data] = "state.entity"
                    [checked] = "state.checked"
                    (checked) = "multiSelector.setCheckedForEntity(state.entity, $event)"
></app-sample-entity>
Всего: {{multiSelector.list.length}} Выбрано: {{multiSelector.value.lenght}}
<button (click) = "multiSelector.setSelectedForAll(false)">Очистить</button>