Angular2: *ngFor does not update when array is updated

Question

I have an array of objects (let's call it arr). In one of my component's inputs in the (change) method I modify one of these object's attribute, but in the view (*ngFor) nothing changes. I read that Angular2 change detection doesn't check the contents of arrays or object, so I tried these:

this.arr = this.arr.slice();

and

this.arr = [...this.arr];

But the view doesn't change, it still shows the old attribute. In the (change) method with console.log() I got the correct array. Weird, but this one works: this.arr = []; I tried NgZone and markForCheck() too.

Answer 1

You could also use the trackBy option in your *ngFor expression, providing a unique ID for every item inside the array. This does make you 100% responsible for change detection, so update this (unique) property every time the item in the array changes. Angular will then only re-render the list if any item inside your array has been given a different trackBy property:

*ngFor="let item of (itemList$ | async); trackBy: trackItem"

or:

*ngFor="let item of itemList; trackBy: trackItem"

where:

trackItem is a public method in your component:

public trackItem (index: number, item: Item) {
  return item.trackId;
}