如何将选定的角度下拉值绑定到标量值的Observable？

我有一个角度材质选择组件,我想将下拉列表的选定值绑定到firebase的一个Observable of scalar.我想在不打开组件中的observable的情况下这样做.它看起来我无法使用异步管道绑定值.下面的代码抛出一个异常,因为mat-select的值不能绑定到“uiStateSvc.currentClient $| async”.

<mat-form-field *ngIf="(store.authorizedClients$| async)?.length > 0">
  <mat-select [(value)]="uiStateSvc.currentClient$| async" [compareWith]="compareFn">
    <mat-option *ngFor="let client of store.authorizedClients$| async" [value]="client"
    (onSelectionChange)="changeCurrentClient($event,client)">
      {{ client.name}}
    </mat-option>
  </mat-select>
</mat-form-field>

我从firebase中提取下拉列表的当前选定值,如下所示：

this.selectedClient$= this.authSvc.currentUser$.pipe(
      switchMap((x: firebase.User) => {
        if (x != null) {
          return this.afs.doc(`uistate/${x.uid}`).valueChanges().map((y: any) => y.selectedclient);
        } else {
          return Observable.of(null);
        }
      })
    );

<mat-form-field *ngIf="(store.authorizedClients$| async)?.length > 0">
  <mat-select [value]="uiStateSvc.currentClient$| async" [compareWith]="compareFn">
    <mat-option *ngFor="let client of store.authorizedClients$| async" [value]="client"
    (onSelectionChange)="changeCurrentClient($event,client)">
      {{client.name}}
    </mat-option>
  </mat-select>
</mat-form-field>