What is the best way to manage data that arrives late from a service?

Question

What is the best way to manage data that arrives late from a service?

Within my Angular application, I have a requirement to store data in an array that is initially empty.

For example:

someFunction() {

 let array = [];

 console.log("step 1");

 this.service.getRest(url).subscribe(result => { 

   result.data.forEach(element => {

   console.log("step 2");

    array.push(element); // Adding all objects from res.data     

   });

   console.log("step 3");

 });

   console.log("step 4");

}

Here are the console.log() steps in order:

The sequence when calling the function was,

Step 1 Step 4 Step 2 Step 3

After step 1, step 4 is executed followed by step 2. If I console.log(array) instead of step 4, it shows an empty array again.

However, instead of step 2 and 3, it does show values. The issue arises after the service call, resulting in an empty value for the array.

Thus, I consistently encounter an empty array and require assistance in persisting data even during the service call's time duration.

I've made numerous code modifications but haven't been able to resolve the issue.

Edit:

Below is a link to a real-time application I am working on in stackblitz: https://stackblitz.com/edit/angular-x4a5b6-ng8m4z

In this demo, please refer to the file: https://stackblitz.com/edit/angular-x4a5b6-ng8m4z?file=src%2Fapp%2Fquestion.service.ts

Within the service.ts

    jsonData: any = [
    {
      "elementType": "textbox",
      "class": "col-12 col-md-4 col-sm-12",
      "key": "project_name",
      "label": "Project Name",
      "type": "text",
      "value": "",
      "required": false,
      "minlength": 3,
      "maxlength": 20,
      "order": 1
    },
    {
      "elementType": "textbox",
      "class": "col-12 col-md-4 col-sm-12",
      "key": "project_desc",
      "label": "Project Description",
      "type": "text",
      "value": "",
      "required": true,
      "order": 2
    },
    {
      "elementType": "dropdown",
      "key": 'project',
      "label": 'Project Rating',
      "options": [],
      "order": 3
    }
  ];

  getQuestions() {

    let questions: any = [];

    // The JSON above contains empty values in "options": [],

    this.jsonData.forEach(element => {
      if (element.elementType === 'textbox') {
        questions.push(new TextboxQuestion(element));
      } else if (element.elementType === 'dropdown') {

        // Need to populate options with data from the service result (res.data)

        questions.push(new DropdownQuestion(element));

        console.log("step 1");

      // Real-time service call..

        // return this.http.get(element.optionsUrl).subscribe(res => {

        // "res.data" has the following array, iterating through and pushing to elements.options.

      //   [
      //   { "key": 'average', "value": 'Average' },
      //   { "key": 'good', "value": 'Good' },
      //   { "key": 'great', "value": 'Great' }
      // ],

        // res.data.forEach(result => {
          console.log("step 2");
        //   element.options.push(result);
        // });
        // console.log(element.options) gives values as shown above [
      //   { "key": 'average'...
        console.log("step 3");
                // console.log(element.options) gives values as shown above [
      //   { "key": 'average'...
        // });
        console.log("step 4");
      // However, console.log(element.options) results in empty output 
      }
    });

    return questions.sort((a, b) => a.order - b.order);
  }

javascript angular typescript angular-services angular4-router

Answer 1

Answer №1

To begin, you must convert your function getQuestion into an Observable.

Why is this necessary? Because you need to call this.http.get(element.optionsUrl) which is asynchronous (all http.get requests return observables). You have to wait for the call to finish in order to retrieve the data. The advantage of observables is that within the "subscribe function" you will have access to the data.

Therefore, we should consider that "services return observables, and components subscribe to the services."

Now, let's address the main issue. We require multiple calls to http.get. Since all http calls are asynchronous, how can we ensure that we have all the data (keeping in mind that the data is only available within the subscribe function)? Instead of having several subscribe functions in our service (ideally none), we can use forkJoin. ForkJoin takes an array of calls and returns an array of results.

First, create an array of observables, then return this array of observables. However, we don't want to return an array with the options; we want an observable of questions. To achieve this, instead of returning the array of observables directly, we return an object that utilizes this array of observables. Below is a simple example at the end of this response.

getQuestions():Observable<any[]> {

    let questions: any = [];

    let observables:Observable<any[]>[]=[];
    this.jsonData.forEach(element => {
      if (element.elementType === 'dropdown') {
        observables.push(this.http.get(element.optionsUrl))
      }
    }
    
    return forkJoin(observables).pipe(map(res=>
    {  
       let index=0;
       this.jsonData.forEach((element) => { 
          if (element.elementType === 'textbox') {
             questions.push(new TextboxQuestion(element));
          } else if (element.elementType === 'dropdown') {
               element.option=res[index];
               questions.push(new DropdownQuestion(element));
               index++;
          }
       })
       return question
    }))
 }

A component can call these functions as follows:

Let's continue developing methods for incorporating Observables into the workflow.

Please refer to this link for a StackBlitz example.

Answer 2

To begin, you must convert your function getQuestion into an Observable.

Why is this necessary? Because you need to call this.http.get(element.optionsUrl) which is asynchronous (all http.get requests return observables). You have to wait for the call to finish in order to retrieve the data. The advantage of observables is that within the "subscribe function" you will have access to the data.

Therefore, we should consider that "services return observables, and components subscribe to the services."

Now, let's address the main issue. We require multiple calls to http.get. Since all http calls are asynchronous, how can we ensure that we have all the data (keeping in mind that the data is only available within the subscribe function)? Instead of having several subscribe functions in our service (ideally none), we can use forkJoin. ForkJoin takes an array of calls and returns an array of results.

First, create an array of observables, then return this array of observables. However, we don't want to return an array with the options; we want an observable of questions. To achieve this, instead of returning the array of observables directly, we return an object that utilizes this array of observables. Below is a simple example at the end of this response.

getQuestions():Observable<any[]> {

    let questions: any = [];

    let observables:Observable<any[]>[]=[];
    this.jsonData.forEach(element => {
      if (element.elementType === 'dropdown') {
        observables.push(this.http.get(element.optionsUrl))
      }
    }
    
    return forkJoin(observables).pipe(map(res=>
    {  
       let index=0;
       this.jsonData.forEach((element) => { 
          if (element.elementType === 'textbox') {
             questions.push(new TextboxQuestion(element));
          } else if (element.elementType === 'dropdown') {
               element.option=res[index];
               questions.push(new DropdownQuestion(element));
               index++;
          }
       })
       return question
    }))
 }

A component can call these functions as follows:

Let's continue developing methods for incorporating Observables into the workflow.

Please refer to this link for a StackBlitz example.

Answer 3

Answer №2

Ensure that Step 4 is incorporated within the subscription logic to execute it last, following Step 3.

Observables transmit next, error, and complete notifications. For handling positive responses, all logic should be enclosed within the next notification. https://angular.io/guide/observables

myObservable.subscribe(
 x => console.log('Observer got a next value: ' + x),
 err => console.error('Observer got an error: ' + err),
 () => console.log('Observer got a complete notification')
);

Consider Flattening Strategies like concatMap if dealing with multiple observables sequentially interests you. https://medium.com/@shairez/a-super-ninja-trick-to-learn-rxjss-switchmap-mergemap-concatmap-and-exhaustmap-forever-88e178a75f1b

Answer 4

Ensure that Step 4 is incorporated within the subscription logic to execute it last, following Step 3.

Observables transmit next, error, and complete notifications. For handling positive responses, all logic should be enclosed within the next notification. https://angular.io/guide/observables

myObservable.subscribe(
 x => console.log('Observer got a next value: ' + x),
 err => console.error('Observer got an error: ' + err),
 () => console.log('Observer got a complete notification')
);

Consider Flattening Strategies like concatMap if dealing with multiple observables sequentially interests you. https://medium.com/@shairez/a-super-ninja-trick-to-learn-rxjss-switchmap-mergemap-concatmap-and-exhaustmap-forever-88e178a75f1b

Answer 5

Answer №3

It's important to note that your function is making an asynchronous API call, which means you won't be able to access the array value before or after the .subscribe() function. To address this issue, make sure to declare your array outside of the function.

Once you have done that, you can simply call another function once you receive the data.

let array = [];

someFunction() {

 this.service.getRest(url).subscribe(result => { 

   result.data.forEach(element => {

    array.push(element); // Storing all objects from result.data in the array     

   });

   this.anotherFunction();

 });

}

anotherFunction()
{
   console.log(this.array) // You can now access the array here 
}

Answer 6

It's important to note that your function is making an asynchronous API call, which means you won't be able to access the array value before or after the .subscribe() function. To address this issue, make sure to declare your array outside of the function.

Once you have done that, you can simply call another function once you receive the data.

let array = [];

someFunction() {

 this.service.getRest(url).subscribe(result => { 

   result.data.forEach(element => {

    array.push(element); // Storing all objects from result.data in the array     

   });

   this.anotherFunction();

 });

}

anotherFunction()
{
   console.log(this.array) // You can now access the array here 
}

Answer 7

Answer №4

Examine the timeline presented below: https://i.sstatic.net/WxOOF.png

It cannot be guaranteed that the service return will occur before step 4, thus there is no assurance that the array will be populated in step 4. The suggested approach to ensure effective handling of a filled array is to relocate the array processing logic within the service callback, aligning with the second downward arrow illustrated in the image.

Answer 8

Examine the timeline presented below: https://i.sstatic.net/WxOOF.png

It cannot be guaranteed that the service return will occur before step 4, thus there is no assurance that the array will be populated in step 4. The suggested approach to ensure effective handling of a filled array is to relocate the array processing logic within the service callback, aligning with the second downward arrow illustrated in the image.

Answer 9

Answer №5

1-

To achieve the same outcome, there are multiple approaches available depending on the specific use case at hand. One option is to utilize async await:

async someFunction() {
    this.asyncResult = await this.httpClient.get(yourUrl).toPromise();
    console.log("step 4");
  }

With this method, there is no need to subscribe anymore. Once the data is fetched from "yourUrl", the Observable will be converted to a promise and resolved, storing the returned data in the "asyncResult" variable. This ensures that the final console log will be executed as expected. For a demonstration, check out this example.

PS: this.httpClient.get(yourUrl) corresponds to what is implemented in your this.service.getRest(url)

2-

Alternatively, you can simply relocate your console.log("step 4"); within the scope of the subscribe method to ensure proper sequencing. Keep in mind that JavaScript exhibits well-known asynchronous behavior; further information can be found through research.

Answer 10

1-

To achieve the same outcome, there are multiple approaches available depending on the specific use case at hand. One option is to utilize async await:

async someFunction() {
    this.asyncResult = await this.httpClient.get(yourUrl).toPromise();
    console.log("step 4");
  }

With this method, there is no need to subscribe anymore. Once the data is fetched from "yourUrl", the Observable will be converted to a promise and resolved, storing the returned data in the "asyncResult" variable. This ensures that the final console log will be executed as expected. For a demonstration, check out this example.

PS: this.httpClient.get(yourUrl) corresponds to what is implemented in your this.service.getRest(url)

2-

Alternatively, you can simply relocate your console.log("step 4"); within the scope of the subscribe method to ensure proper sequencing. Keep in mind that JavaScript exhibits well-known asynchronous behavior; further information can be found through research.

What is the best way to manage data that arrives late from a service?

Answer №1

Answer №2

Answer №3

Answer №4

Answer №5

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