Function _subscribeToMenuOpen

src/cdk/menu/menu-base.ts:216–234

* Subscribe to the menu trigger's open events in order to track the trigger which opened the menu * and stop tracking it when the menu is closed.

()

   * and stop tracking it when the menu is closed.
   */
  private _subscribeToMenuOpen() {
    const exitCondition = merge(this.items.changes, this.destroyed);
    this.items.changes
      .pipe(
        startWith(this.items),
        mergeMap((list: QueryList<CdkMenuItem>) =>
          list
            .filter(item => item.hasMenu)
            .map(item => item.getMenuTrigger()!.opened.pipe(mapTo(item), takeUntil(exitCondition))),
        ),
        mergeAll(),
        switchMap((item: CdkMenuItem) => {
          this.triggerItem = item;
          return item.getMenuTrigger()!.closed;
        }),
        takeUntil(this.destroyed),
      )
      .subscribe(() => (this.triggerItem = undefined));
  }

  /** Subscribe to the MenuStack close events. */
  private _subscribeToMenuStackClosed() {