fix(list-key-manager): maintain selected index when amount of items changes (#9164)

Currently the `activeItemIndex` and the `activeItem` in the `ListKeyManager` will stay the same if the amount of items changes. This is an issue, because they'll become out of sync and focus could potentially go to a disabled item. With the following changes the key manager will ensure that the `activeItemIndex` and the `activeItem` are in sync and point to the same item.

Author: crisbeto

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

@@ -7,6 +7,7 @@ import {ActiveDescendantKeyManager} from './activedescendant-key-manager';
 import {FocusKeyManager} from './focus-key-manager';
 import {ListKeyManager} from './list-key-manager';
 import {FocusOrigin} from './focus-monitor';
+import {Subject} from 'rxjs/Subject';
 
 
 class FakeFocusable {
@@ -23,11 +24,13 @@ class FakeHighlightable {
 }
 
 class FakeQueryList<T> extends QueryList<T> {
+  changes = new Subject<FakeQueryList<T>>();
   items: T[];
   get length() { return this.items.length; }
   get first() { return this.items[0]; }
   toArray() { return this.items; }
   some() { return this.items.some.apply(this.items, arguments); }
+  notifyOnChanges() { this.changes.next(this); }
 }
 
 
@@ -71,6 +74,17 @@ describe('Key managers', () => {
       spyOn(keyManager, 'setActiveItem').and.callThrough();
     });
 
+    it('should maintain the active item if the amount of items changes', () => {
+      expect(keyManager.activeItemIndex).toBe(0);
+      expect(keyManager.activeItem!.getLabel()).toBe('one');
+
+      itemList.items.unshift(new FakeFocusable('zero'));
+      itemList.notifyOnChanges();
+
+      expect(keyManager.activeItemIndex).toBe(1);
+      expect(keyManager.activeItem!.getLabel()).toBe('one');
+    });
+
     describe('Key events', () => {
 
       it('should emit tabOut when the tab key is pressed', () => {

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

@@ -50,7 +50,18 @@ export class ListKeyManager<T extends ListKeyManagerOption> {
   // Buffer for the letters that the user has pressed when the typeahead option is turned on.
   private _pressedLetters: string[] = [];
 
-  constructor(private _items: QueryList<T>) { }
+  constructor(private _items: QueryList<T>) {
+    _items.changes.subscribe((newItems: QueryList<T>) => {
+      if (this._activeItem) {
+        const itemArray = newItems.toArray();
+        const newIndex = itemArray.indexOf(this._activeItem);
+
+        if (newIndex > -1 && newIndex !== this._activeItemIndex) {
+          this._activeItemIndex = newIndex;
+        }
+      }
+    });
+  }
 
   /**
    * Stream that emits any time the TAB key is pressed, so components can react