Angular mat-sidenav propiedad isHandset $ | asincrónico explicar

<mat-sidenav-container class="sidenav-container"> <mat-sidenav #drawer class="sidenav" fixedInViewport="true" [attr.role]="(isHandset$ | async) ? 'dialog' : 'navigation'" [mode]="(isHandset$ | async) ? 'push' : 'push'" [opened]="!(isHandset$ | ...