Tips for efficiently waiting and receiving responses for every HttpClient service call within a loop in Angular 4

Question

Tips for efficiently waiting and receiving responses for every HttpClient service call within a loop in Angular 4

I am facing a situation where I must iterate through a loop to retrieve the descriptions of respective items. These descriptions need to be included in a datatable along with the item Ids and other details.

addDescription(){
this.arrayOfItems.forEach(element => {

   // CALL a function for making a service call
   this.fetchDescription(element);

   //CODE TO DECLARE AN INTERFACE FOR ASSIGNING VALUES, e.g.

   // ITEM_ID : element.id,
   // ITEM_DEF : this.fetchedDescription.join("\n")
}

Function body:

fetchDescription(elementToFetchDesc){

 //Declare HTTPPARAMS in PassingParams variable

 this.componentService.getDesc(PassingParams)
         .subscribe((response: HttpResponse<any>) => {
                if(response.status ==200){
                    this.fetchedDescription = reponse.body.output.slice(6,-1);
                }
                //Code if response status is NOT 200
}

In componentService Service:

construcutor(private httpConn: HttpClient){}

getDesc(Params){
    // Declare URL
    return this.httpConn.get(URL, {params: Params, observe: 'response'});
}

The challenge:

Due to the asynchronous nature of the subscription within the loop, the description is not being assigned to the variable (ITEM_DEF) in the interface after running the loop using forEach.

To address this issue, I made a slight modification by implementing the use of promise. In the service, I added:

 import 'rxjs/add/operator/toPromise';

And updated the service method as follows:

 return this.httpConn.get(URL, {params: Params, observe: 'response'})
                     .toPromise();    // converted observable to promise

Also, I made a change in the component: Within the fetchDescription function:

replaced .subscribe with .then

However, the issue still persists. I would appreciate any insights on where I may have gone wrong in implementing this logic.

angular typescript angular-httpclient angular4-httpclient

Answer 1

Answer №1

To solve this issue, you need to convert observables into promises without relying on the 'then' method.

For example:

Here is a service function that sends the request:

sendMyRequest(num) {
   return this.http.get('http://anotherUrl.com/' + num).toPromise(); 
}

This function can be used to send multiple requests in a component class:

 async sendAllRequests() {
    let responses = [];
    for(let i = 0; i < 5; i++) {
        responses[i] = await this.myService.sendMyRequest();
    }
  // All results have been retrieved!
 }

Answer 2

To solve this issue, you need to convert observables into promises without relying on the 'then' method.

For example:

Here is a service function that sends the request:

sendMyRequest(num) {
   return this.http.get('http://anotherUrl.com/' + num).toPromise(); 
}

This function can be used to send multiple requests in a component class:

 async sendAllRequests() {
    let responses = [];
    for(let i = 0; i < 5; i++) {
        responses[i] = await this.myService.sendMyRequest();
    }
  // All results have been retrieved!
 }

Answer 3

Answer №2

Solution:

Function Body:

fetchingDescriptionData(dataToRetrieve):  Observable<string> {

  //Setting HTTP parameters in postData variable

  return this.componentService.retrieveDesc(postData)
      .map((response: HttpResponse<any>) => {
          if(response.status ==200){
             return reponse.body.result.extract.slice(6,-1);
          }
       });
     }
  }

Call:

this.fetchingDescriptionData(content).subscribe((details: string) => {
   ITEM_NUMBER : content.number,
   ITEM_DETAILS : details
});

Answer 4

Solution:

Function Body:

fetchingDescriptionData(dataToRetrieve):  Observable<string> {

  //Setting HTTP parameters in postData variable

  return this.componentService.retrieveDesc(postData)
      .map((response: HttpResponse<any>) => {
          if(response.status ==200){
             return reponse.body.result.extract.slice(6,-1);
          }
       });
     }
  }

Call:

this.fetchingDescriptionData(content).subscribe((details: string) => {
   ITEM_NUMBER : content.number,
   ITEM_DETAILS : details
});

Answer 5

Answer №3

To achieve this functionality, it is recommended to utilize the rxjs library.

checkConnectionStatus(url: string, id: string): Observable<string> {
const trigger$ = new Subject<string>();
const time = Date.now();

return trigger$.asObservable().startWith(url).concatMap((u) => {
  return this.sendRequest(u).map(() => {
    trigger$.complete();

    return id;
  }).catch((err) => {
      trigger$.next(u);

    return Observable.empty<any>();
  });
});
}


protected sendRequest(url: string): Observable<any> {
return this.http.get(url)
  .catch(err => Observable.throw(err));
}

The key concept in this code snippet is the usage of the concatMap operator, which ensures that the next observable is fired only after the completion of the first one, meaning that the response from the initial API call is received. By invoking the method complete() on the trigger$ subject, you can effectively end the observable and any ongoing API calls. It may be necessary to adjust how you handle calling complete() based on your specific requirements.

Answer 6

To achieve this functionality, it is recommended to utilize the rxjs library.

checkConnectionStatus(url: string, id: string): Observable<string> {
const trigger$ = new Subject<string>();
const time = Date.now();

return trigger$.asObservable().startWith(url).concatMap((u) => {
  return this.sendRequest(u).map(() => {
    trigger$.complete();

    return id;
  }).catch((err) => {
      trigger$.next(u);

    return Observable.empty<any>();
  });
});
}


protected sendRequest(url: string): Observable<any> {
return this.http.get(url)
  .catch(err => Observable.throw(err));
}

The key concept in this code snippet is the usage of the concatMap operator, which ensures that the next observable is fired only after the completion of the first one, meaning that the response from the initial API call is received. By invoking the method complete() on the trigger$ subject, you can effectively end the observable and any ongoing API calls. It may be necessary to adjust how you handle calling complete() based on your specific requirements.

Tips for efficiently waiting and receiving responses for every HttpClient service call within a loop in Angular 4

Answer №1

Answer №2

Answer №3

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