angular - 类型 Observable> 不可分配给类型 Observable

Question

我正在使用来自数据库服务的数据实现自动完成:

@Injectable()
export class SchoolService {
  constructor(private db: AngularFirestore) {
  }

  getSchools(): Observable<School[]> {
    return this.db.collection<School>('schools').valueChanges();
  }
}

在我的组件中:

export class SchoolComponent implements OnInit {
  formControl: FormControl = new FormControl();
  schools: Observable<School[]>;
  filteredSchools: Observable<School[]>;

  constructor(private schoolService: SchoolService) {
  }

  ngOnInit() {
    this.schools = this.schoolService.getSchools();

    //Below line gives error "Type Observable<Observable<School[]>> is not assignable to type Observable<School[]>".
    this.filteredSchools = this.formControl.valueChanges.pipe(
      startWith(''),
      map(name => this.filterSchools(name))
    );
  }

  filterSchools(name: string): Observable<School[]> {
    return this.schools.map(school => school.filter(s => s.name.toLowerCase().includes(name)));
  }
}

还有我的 html:

<form>
  <mat-form-field>
    <input type="text" matInput placeholder="Type Your School to Continue" [matAutocomplete]="auto"
           [formControl]="formControl">
    <mat-autocomplete #auto="matAutocomplete">
      <mat-option *ngFor="let school of filteredSchools | async" [value]="school">
        <span>{{school.name}}</span> |
        <small>{{school.city}}</small>
      </mat-option>
    </mat-autocomplete>
  </mat-form-field>
</form>

ngOnInit中的函数给出 Type Observable<Observable<School[]>> is not assignable to type Observable<School[]>错误。我该如何解决这个问题？

Answer 1

错误准确地说明了问题所在。您定义了:

filteredSchools: Observable<School[]>;

但稍后您创建了一个 Observable，它发出(映射每个值)filterSchools(name: string): Observable<School[]> 的结果它也返回一个 Observable，所以最后你有一个 Observable 链 Observable<Observable<School[]>> .

实际上看起来您只是想使用 switchMap (或 mergeMap 或 concatMap )而不是 map . switchMap将订阅嵌套的 Observable 并将链变为 Observable<School[]> :

this.filteredSchools = this.formControl.valueChanges.pipe(
  startWith(''),
  switchMap(name => this.filterSchools(name))
);