Pausing until all RxJS calls are completed before moving on to the next block of code

Question

Pausing until all RxJS calls are completed before moving on to the next block of code

I have a scenario where I am iterating over a list of streets and making API calls for each street. I am storing the responses in an Object Array and want to execute the next code block only after all the requests are finished. However, I am facing an issue where the Object Array appears to be empty when trying to use it in the next code block. Here is the relevant code snippet:

export class MyComponent{
  addressInfoArray: AddressInfo[] = [];

  // some code ...

  prepareStreetInformations(): void {
    // some code ....

    this.fillArray(streets, url);
    this.doSomethingWithArray(this.addressInfoArray); // <--- length = 0 and doesn't wait for finishing the fillArray() method
  }
}

fillArray(streets: Street[], url: string): void { // streets has a length of 150
  for (const street of streets) {
    this.http.get<AddressInfo>(`${url}/street.name`).subscribe(response => {
      this.addressInfoArray.push(response);
    });
  }
}

My question is: How can I make the doSomethingWithArray() method wait for the fillArray() method to completely finish, and why does it not recognize that the Object Array is already filled?

angular typescript asynchronous rxjs race-condition

Answer 1

Answer №1

Utilizing RxJS does not involve forcing a function call to "wait" for a previous function to complete. Instead, you can create an observable that emits the necessary data based on other observables.

In this scenario, it appears that you require an observable that emits an array of AddressInfo.

We can establish a getStreets() method that yields Observable<Street[]> and a getAddressInfo() method that accepts a street parameter and returns Observable<AddressInfo>.

Subsequently, you can generate an observable that will emit your AddressInfo[], utilizing switchMap and forkJoin:

1  export class MyComponent {
2  
3    getStreets(): Observable<Street[]> {
4      this.http.get<Street[]>(...);
5    }
6  
7    getAddressInfo(street: Street): Observable<AddressInfo> {
8      this.http.get<AddressInfo>(`${this.url}/${street.name}`); // avoid subscribing here
9    }
10
11   addressInfos$: Observable<AddressInfo[]> = this.getStreets().pipe(
12     switchMap(streets => forkJoin(
13       streets.map(s => this.getAddressInfo(s))
14     ))
15   );
16 
17 }

The forkJoin function creates a single observable that emits an array consisting of the results from all its input observables. Hence, the function is provided with an array of observables as inputs.

Given that you possess an array of streets. On line #13, we straightforwardly map it to an array of observables that obtain the address information. When the forkJoined observable is subscribed to, it emits an array of AddressInfo (the outcomes of all individual http calls)

To manage the subscription to this "forkjoined observable", we utilize switchMap. Consequently, we obtain a singular addressInfos$ observable that emits the AddressInfo[].

Note that we have not subscribed yet. To conduct your tasks, you can simply subscribe as follows:

addressInfos$.subscibe(
  infos => doSomethingWithArray(infos)
);

Nevertheless, in Angular, a common approach involves further transforming the emitted data into the format required by your template:

templateData$ = this.addressInfos$.pipe(
  map(infos => {
    // carry out operations on the array here :-)
  })
);

Subsequently, within your template, you can leverage the async pipe:

<div *ngIf="templateData$ | async as data">
  <ul>
    <li *ngFor="item of data">{{ item }}</li>
  </ul>
</div>

Answer 2

Utilizing RxJS does not involve forcing a function call to "wait" for a previous function to complete. Instead, you can create an observable that emits the necessary data based on other observables.

In this scenario, it appears that you require an observable that emits an array of AddressInfo.

We can establish a getStreets() method that yields Observable<Street[]> and a getAddressInfo() method that accepts a street parameter and returns Observable<AddressInfo>.

Subsequently, you can generate an observable that will emit your AddressInfo[], utilizing switchMap and forkJoin:

1  export class MyComponent {
2  
3    getStreets(): Observable<Street[]> {
4      this.http.get<Street[]>(...);
5    }
6  
7    getAddressInfo(street: Street): Observable<AddressInfo> {
8      this.http.get<AddressInfo>(`${this.url}/${street.name}`); // avoid subscribing here
9    }
10
11   addressInfos$: Observable<AddressInfo[]> = this.getStreets().pipe(
12     switchMap(streets => forkJoin(
13       streets.map(s => this.getAddressInfo(s))
14     ))
15   );
16 
17 }

The forkJoin function creates a single observable that emits an array consisting of the results from all its input observables. Hence, the function is provided with an array of observables as inputs.

Given that you possess an array of streets. On line #13, we straightforwardly map it to an array of observables that obtain the address information. When the forkJoined observable is subscribed to, it emits an array of AddressInfo (the outcomes of all individual http calls)

To manage the subscription to this "forkjoined observable", we utilize switchMap. Consequently, we obtain a singular addressInfos$ observable that emits the AddressInfo[].

Note that we have not subscribed yet. To conduct your tasks, you can simply subscribe as follows:

addressInfos$.subscibe(
  infos => doSomethingWithArray(infos)
);

Nevertheless, in Angular, a common approach involves further transforming the emitted data into the format required by your template:

templateData$ = this.addressInfos$.pipe(
  map(infos => {
    // carry out operations on the array here :-)
  })
);

Subsequently, within your template, you can leverage the async pipe:

<div *ngIf="templateData$ | async as data">
  <ul>
    <li *ngFor="item of data">{{ item }}</li>
  </ul>
</div>

Answer 3

Answer №2

Consider utilizing forkJoin method for optimal performance;

let streetObservers = this.streets.map(street => this.http.get<AddressInfo>(`${url}/street.name`));
forkJoin(streetObservers).subscribe((responseData: any) => {
  this.addressInfoArray = responseData; // an array containing address information
  this.processArray(this.addressInfoArray); // <--- the array length is 0 and doesn't wait for the fillArray() method to complete

});

Answer 4

Consider utilizing forkJoin method for optimal performance;

let streetObservers = this.streets.map(street => this.http.get<AddressInfo>(`${url}/street.name`));
forkJoin(streetObservers).subscribe((responseData: any) => {
  this.addressInfoArray = responseData; // an array containing address information
  this.processArray(this.addressInfoArray); // <--- the array length is 0 and doesn't wait for the fillArray() method to complete

});

Answer 5

Answer №3

It seems like there may be a misunderstanding of how Observables function. In the provided code snippet, the fillArray function simply dispatches the http calls and then the function ends. As a result, when the doSomethingWithArray function is called next in the code, the addressInfoArray is not yet populated. It is only after some time, when a http call completes and the callback function in the subscribe function is executed, that the addressInfoArray.push is called. This behavior is consistent when using subscribe, as it executes the provided function once the Observable is complete.

If you need to wait for multiple Observables to complete, you can utilize combineLatest. This method creates an Observable that emits an array containing all the values from the original Observables. Here is an example of how you can implement this:

prepareStreetInformations(): void {
  this.fillArray(streets, url).subscribe(addressInfoArray => {
    this.doSomethingWithArray(this.addressInfoArray)
  });
}
  
fillArray(streets: Street[], url: string): Observable<AddressInfo[]> {
  return combineLatest(
    streets.map(street => this.http.get<AddressInfo>(`url/${street.name}`))
  );
}

It is important to note that the fillArray function now returns an Observable that you can subscribe to, and it no longer modifies any class properties directly. If you require access to a class property, you will need to define it within the callback function passed to the subscribe method.

Answer 6

It seems like there may be a misunderstanding of how Observables function. In the provided code snippet, the fillArray function simply dispatches the http calls and then the function ends. As a result, when the doSomethingWithArray function is called next in the code, the addressInfoArray is not yet populated. It is only after some time, when a http call completes and the callback function in the subscribe function is executed, that the addressInfoArray.push is called. This behavior is consistent when using subscribe, as it executes the provided function once the Observable is complete.

If you need to wait for multiple Observables to complete, you can utilize combineLatest. This method creates an Observable that emits an array containing all the values from the original Observables. Here is an example of how you can implement this:

prepareStreetInformations(): void {
  this.fillArray(streets, url).subscribe(addressInfoArray => {
    this.doSomethingWithArray(this.addressInfoArray)
  });
}
  
fillArray(streets: Street[], url: string): Observable<AddressInfo[]> {
  return combineLatest(
    streets.map(street => this.http.get<AddressInfo>(`url/${street.name}`))
  );
}

It is important to note that the fillArray function now returns an Observable that you can subscribe to, and it no longer modifies any class properties directly. If you require access to a class property, you will need to define it within the callback function passed to the subscribe method.

Pausing until all RxJS calls are completed before moving on to the next block of code

Answer №1

Answer №2

Answer №3

Similar questions

Encountering a "Missing Access" error on the Discord.js API when trying to register my slash commands

The issue of sending a POST request with form-data body parameters is not functioning as expected in Angular 2

Encountering an Angular 9 Ivy issue when using the <mat-form-field> with multiple mat-hints

TS1316 Error: You can only have global module exports at the top level of the file

A generic function in Typescript that can accept an object with the first argument as a specified key

The object literal can only define properties that are already known, and 'data' is not found in the type 'PromiseLike<T>'

Deactivate the selection option in Syncfusion NumericTextbox

Tips for updating the date separator in Angular 2

Functions designed to facilitate communication between systems

Issue with CSS files in Jest"errors"

Error message: Trying to use the data type 'String' as an index in the React dynamic component name map won't work

Vue.js with TypeScript: The property 'xxx' is not found on the type 'never'

Looking for Angular 2 material components for dart with CSS styling? Need help centering a glyph on your page?

An issue with event listeners in Vue 3 and Pixi.js arises when working with DisplayObjects that are either created as properties of classes or inherited from parent classes

Creating a table with merged (colspan or rowspan) cells in HTML

Attempting to utilize Array Methods with an Array Union Type

How to conceal parameters in Angular URL?

What's the best way to assign a dual-value condition within a form group field?

Navigating through different routes in an Angular application can be tricky, especially when dealing

How can I display an array in reverse order using *ngFor in a template?