Angular 5 and transition only in 'new' items

Question

My component's template renders a list:

<div class="row">
  <div *ngFor="let m of activeModules" [@enterAnimation]>
    {{ m.name }}
  </div>
</div>

The component animates it

@Component({
  selector: 'app-client-detail',
  templateUrl: './client-detail.component.html',
  styleUrls: ['./client-detail.component.scss'],
  animations: [

    trigger(
      'enterAnimation', [
        transition(':enter', [
          style({ opacity: 0 }),
          animate('1000ms', style({ opacity: 1}))
        ]),
        transition(':leave', [
          style({ opacity: 1 }),
          animate('1000ms', style({ opacity: 0}))
        ])
      ]
    )
  ]
})
export class ClientDetailComponent implements OnInit {
   activeModules = [{ name: 'modulo 1'}, { name: 'modulo 2'}];
}

This works fine, the 'divs' have the transition from the opacity. If I do something like this:

ngOnInit() {
  Observable.timer(2000).subscribe(() => {
    this.activeModules =  [{ name: 'modulo 1'}, { name: 'modulo 3'}, { name: 'modulo 2'}];
}

the activeModules' array is updated and the templates renders again the list with the transition. My problem is that I need only the 'new' items from the array (in this case, the one in the middle) to have the transition. Is it possible?

Answer 1

Sure, you just need to push your item instead of creating your array again.

Let's make a function for that :

pushAt(arr, index, item) {
  arr.splice(index, 0, item); 
}

Now you can just

ngOnInit() {
  Observable.timer(2000).subscribe(() => this.pushAt(this.activeModules, 1, { name: 'Modulo 2' }));
}

EDIT maybe you can try with a custom trackby function :

customTrackBy(index, item) { return index; }

In your HTML

<div *ngFor="let m of activeModules; trackBy: customTrackBy;" [@enterAnimation]>