在effect中使用RxJS的flatMap操作符等待HttpResponse返回。示例代码如下：

import { Injectable } from '@angular/core';
import { Actions, Effect, ofType } from '@ngrx/effects';
import { Observable, of } from 'rxjs';
import { catchError, map, switchMap } from 'rxjs/operators';
import { HttpClient } from '@angular/common/http';

@Injectable()
export class UserEffects {
  constructor(private actions$: Actions, private http: HttpClient) { }

  @Effect()
  getUser$: Observable = this.actions$.pipe(
    ofType('GET_USER'),
    map((action: any) => action.payload),
    switchMap((id: number) => {
      return this.http.get(`api/user/${id}`).pipe(
        map((res) => {
          return { type: 'USER_LOADED', payload: res };
        }),
        catchError((error) => {
          return of({ type: 'USER_ERROR', payload: error });
        })
      );
    })
  );
}