Angular mat-sidenav property isHandset$ | async explain
问题 <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$ | async)"> <mat-toolbar color="primary">Menu</mat-toolbar> <mat-nav-list> <a mat-list-item href="#">Link 1</a> <a mat-list-item href="#">Link 2</a> <a mat-list-item href="#">Link 3</a> </mat-nav-list> </mat-sidenav> I do not understand what is written in