使用 RxJS 的 switchMap 操作符将多个 HTTP 调用组合为一个可观察对象，并在组合完成后将其传递给父可观察方法。

示例代码如下：

import { Component } from '@angular/core';
import { HttpClient } from '@angular/common/http';
import { Observable } from 'rxjs/Observable';
import { switchMap } from 'rxjs/operators';

@Component({
  selector: 'app-example',
  template: `
    Get Data
  `
})
export class ExampleComponent {

  constructor(private http: HttpClient) {}

  getData() {
    const getFirstData$ = this.http.get('https://jsonplaceholder.typicode.com/todos/1');
    const getSecondData$ = this.http.get('https://jsonplaceholder.typicode.com/todos/2');

    Observable.combineLatest(getFirstData$, getSecondData$)
      .pipe(
        switchMap(([firstData, secondData]) => {
          // Do something with the data
          console.log('First Data:', firstData);
          console.log('Second Data:', secondData);
          return this.http.get('https://jsonplaceholder.typicode.com/todos/3'); // Return new observable for parent method
        })
      )
      .subscribe((thirdData) => {
        // Do something with the data from the third HTTP call
        console.log('Third Data:', thirdData);
        // Pass data to the parent method
        this.parentHandleData(thirdData);
      });
  }

  parentHandleData(data) {
    // Do something with the data in the parent component
    console.log('Parent Data:', data);
  }
}