ListKeyManager: change emission on QueryList changes

[br-kwon]

components/src/cdk/a11y/key-manager/list-key-manager.ts

Line 74 in 4699f30

this._activeItemIndex = newIndex;

In this if block, should this.changes also emit since it seems to emit indices.

[crisbeto]

I'm not sure whether it should emit, because the active item is technically the same, it's just its index that has changed. There's a somewhat-related fix that will emit the event if the active item is removed #20008.

[br-kwon]

Yeah not sure either, but then I feel the changes property is quite ambiguous since it emits a number. I would expect something named changes emits when the state of the instance has changed (activeItemIndex in this case).

One challenge I faced is when I used this class in an unconventional manner. Instead of passing in QueryChildren grabbed from ViewChildren or ContentChildren, I passed in a new QueryList(). The intention here was to listen for ListKeyManager.changes() and markForCheck(). When I reset the list and notifyOnChanges, I would expect an already active item to be "active", but fails to do so in this case.

[crisbeto]

That's totally valid, but I suspect that it may cause some breakages. E.g. in mat-select we have an event tied to the list manager event which will begin firing if items are added to the select automatically, if we were to make the proposed change. We could try it out once #20008 lands and see how many tests break.

[br-kwon]

sounds great! Would love to take a pass at it