Child component's view is not re-rendered when using OnPush strategy

Question

@Component({
    selector: "parent",
    template: `<child [userId]="(userId$ | async)"></child>`,
    changeDetection: ChangeDetectionStrategy.OnPush
})
export class ParentCmp implements OnInit {
    userId$: BehaviorSubject<string> = new BehaviorSubject<string>(null);

    constructor(private activatedRoute: ActivatedRoute) { }
    ngOnInit() {
        this.activatedRoute.queryParams.subscribe(query => {
            //notify child with BehaviorSubject
            this.userId$.next(query["userid"])
        }
    }
}

@Component({
    selector: "child",
    template: `<div *ngIf="(userState$ | async) && userId">
                    {{(userState$ | async).user.id)}}
               </div>`,
    changeDetection: ChangeDetectionStrategy.OnPush
})
export class ChildCmp implements OnChanges {
    @Input() userId: string;
    private userState$: Observable<User>;

    constructor(private store: Store<store.AppState>) { }
    ngOnChanges(changes: SimpleChanges) { 
        //when it gets userId it starts to track fit user in ngrx store
        this.userState$ = this.store
                .select(state => state.user-list)                 
                .map(userList => userList[this.userId])
                .filter(user => !!user);
    }
}

The Child cmp gets userId from Parent one and needed user is contained in ngrx store (userList), but child's view is not re-rendered. It works perfectly when ChangeDetectionStrategy for the Child is default. What can be wrong here? Angular v2.4

Answer 1

If you change the model in ngOnChanges() you need to invoke change detection explicitely

export class ChildCmp implements OnChanges {
    @Input() userId: string;
    private userState$: Observable<User>;

    constructor(
      private store: Store<store.AppState>,
      private cdRef:ChangeDetectorRef
    ) { }
    ngOnChanges(changes: SimpleChanges) { 
        //when it gets userId it starts to track fit user in ngrx store
        this.userState$ = this.store
                .select(state => state.user-list)                 
                .map(userList => userList[this.userId])
                .filter(user => !!user);
        this.cdRef.detectChanges();
    }
}

or probably better make userStates$ an Observable and keep the same instance instead of creating a new one each time ngOnChanges is called:

userId$: Subject<User> = new Subject<User>();

ngOnChanges(changes: SimpleChanges) { 
    //when it gets userId it starts to track fit user in ngrx store
    this.store
            .select(state => state.user-list)                 
            .map(userList => userList[this.userId])
            .filter(user => !!user)
            .subscribe((user) => this.userId.next(user));
}