Angular 如何使用来自不同可观测对象的值更新可观测对象

发布于11月08日

我很难弄清楚如何用一个观察点的结果来更新另一个观察点的结果:

this._http.get<JSON>(url, { params })
.pipe(map((res: JSON) => this.getObjectFromJson(res)))
.subscribe(myObject => {
    this.getValueToUpdate(myObject).subscribe(newValue => {
        myObject.some.value = newValue;
    });
});
// return Observable<myObect>;

Rxjs运算符的正确顺序是什么，才能在没有这subscribe的情况下实现这一点，并能够返回生成的可观测对象以供以后使用？

推荐答案

您要找的是switchMap:

this._http.get<JSON>(url, { params }).pipe(
  map((res: JSON) => this.getObjectFromJson(res)),
  switchMap(myObject => {
    return this.getValueToUpdate(myObject).pipe(
      // probably here you need also a catchError(err => ...),
      map(newValue => {
        myObject.some.value = newValue;
        return myObject;
      })
    );
  })
).subscribe(myObject => {
  // here myObject is updated with newValue
});

It allows to invoke another async operation and switch (i.e. observe) on that.
The inner pipe, using the map operator, allows to combine the result of the nested async operation with the previous result.