How to trigger change detection in Angular2?

Question

I'm creating a Facebook service that calls the Facebook javascript api and am wondering how to best implement change detection when my values are updated.

I have a UserService which has a currentUser property that is a BehaviorSubject:

currentUser: Subject<User> = new BehaviorSubject<User>(new User(null));

And when I want to update the user in response to the facebook javascript sdk telling me the user has logged in or logged out, I update that and need to call tick() on an ApplicationRef:

updateUser(user: User) {
  console.log('UserService.updateUser:', user);
  this.currentUser.next(user);
  this.appRef.tick(); // UI does not update without this
}

constructor(facebook: Facebook) {
  this.facebook.facebookEvents.filter(x => x != null 
         && x.eventName == 'auth.authResponseChange')
      .subscribe((event) => {
        this.updateUser(new User(event.data));
      }
}

In my component I store the 'currentUser' from the user service in the constructor and bind to the value property:

<h2>Logged into Facebook as {{currentUser.value.name}}</h2>
<p>Is this you? &nbsp; <img src="{{currentUser.value.profilePicUrl}}"></p>

Am I doing something wrong? Is there a better way than having to call ApplicationRef.tick() after a change triggered from an external library?

Edit

I tried using NgZone and that doesn't work, using a different event that returns posts in a feed as the service pages through them:

constructor(userService: UserService, private ref: ApplicationRef, private zone: NgZone)

...

this.postsSubject.subscribe((post) => {
  this.zone.runOutsideAngular(() => { // doesn't do anything
    this.posts.push(post);
    console.log('postsSubject POST, count is ', this.posts.length);
    ref.tick(); // required to update bindings
  });
}

The console shows the count incrementing, but the html binding {{posts.length}} is only updated if I add the ref.tick() call...

I think I saw somewhere that you can make available 'inputs' to any component from the top-level app component which might be the way to go for the logged in user, but not other calls like getting posts in a feed...

Answer 1

Am I doing something wrong? Is there a better way than having to call ApplicationRef.tick() after a change triggered from an external library?

It depends. If you're calling the Facebook APIs outside of Angular (i.e., registering asynchronous event handlers outside Angular), then you'll need to manually run change detection as part of handling any events. You can either

• inject NgZone and then call run() on that object, which will execute the passed (callback) function inside the Angular zone and then automatically call ApplicationRef().tick(), which will run change detection for the entire app, or
• inject ApplicationRef and call tick() yourself, or
• inject ChangeDetectorRef and call detectChanges() yourself -- this will only run change detection on the one component and its children

Note, if you inject NgZone, you don't want to use runOutsideAngular(). That will execute the passed (callback) function outside the Angular zone, and it will not call ApplicationRef().tick(), so change detection will not execute.

If you call the Facebook APIs inside Angular, you might be able to avoid all of the above, because then Angular (via its use of Zone.js) should monkey-patch the asynchronous event calls. Then, when your events fire, ApplicationRef.tick() will automatically be called. See https://stackoverflow.com/a/34593821/215945 for more discussion on this approach.