Guide on Retrieving an Array from an Observable

Question

Guide on Retrieving an Array from an Observable

Hey there! I've come across a function that is supposed to return an Array. In the function below,

this.cordovaFile.readAsArrayBuffer(this.cordovaFile.dataDirectory, storageId)

actually returns a Promise Array. I'm converting it into an Observable and storing it in the timesheetArray variable. Now, timesheetArray will return as an Observable array, but I simply want it to return as just an Array. Here's the code:

I could really use some assistance here. If it could return just an array, then I wouldn't have to make any changes anywhere else because this function is used throughout the application.

public getAllTimesheets(): TimesheetModel[] {
  const storageId = TIMESHEET_KEYS.ALL_TIMESHEET;

  const timesheetArray = from(
    this.cordovaFile
    .readAsArrayBuffer(this.cordovaFile.dataDirectory, storageId)
    .then((compressedTimesheet) => {
      const start = moment();
      const uint8array = new Uint8Array(compressedTimesheet);
      const jsonTimeSheet = this.LZString.decompressFromUint8Array(uint8array);
      this.log.debug(`LocalStorageMaterialService: getMaterials() from files: Decompression took ${moment().subtract(start.valueOf()).valueOf()} ms`);
      return <TimesheetModel[]> JSON.parse(jsonTimeSheet) || [];
    })
    .catch((error) => {
      this.log.debug('LocalStorageMaterialService: Retrieving materials from file storage was not possible: ', JSON.stringify(error));
      return [];
    })
  )
  timesheetArray.subscribe((timesheet) => {
    // How can I return an Array here?
  });
}

Here's just one example of why I want to return an array instead of observable:

let matchedTimesheet = _.find<TimesheetModel>(this.getAllTimesheets(),
(timesheet) => travelToDate 
&& timesheet.startOfWork.isSame(travelToDate.startDate.value, 'days')
            );

In the above code snippet, it expects an array rather than an Observable. I could achieve this by subscribing here as well, but if the function were to return an array instead of an observable, then I would need to make changes everywhere.

angular typescript promise observable

Answer 1

Answer №1

It seems like your approach may need a shift in perspective. The readAsArrayBuffer function operates asynchronously, returning a promise.

In your getAllTimesheets() method, you should not simply return an array of TimesheetModel. Instead, consider returning an

Observable<TimesheetModel[]>

. This change will require adjustments wherever the method is called.

With an

Observable<TimesheetModel[]>

return type, you must either subscribe to each call of getAllTimesheets() or utilize the async pipe in your template to read the Observable data.

The latter option is recommended for simplicity.

To implement this modification in your getAllTimesheets() method, make the following alterations:

public getAllTimesheets(): Observable<TimesheetModel[]> {
  const storageId = TIMESHEET_KEYS.ALL_TIMESHEET;
  return from(
    this.cordovaFile
    .readAsArrayBuffer(this.cordovaFile.dataDirectory, storageId)
    .then((compressedTimesheet) => {
      const start = moment();
      const uint8array = new Uint8Array(compressedTimesheet);
      const jsonTimeSheet = this.LZString.decompressFromUint8Array(uint8array);
      this.log.debug(`LocalStorageMaterialService: getMaterials() from files: Decompression took ${moment().subtract(start.valueOf()).valueOf()} ms`);
      return <TimesheetModel[] > JSON.parse(jsonTimeSheet) || [];
    })
    .catch((error) => {
      this.log.debug('LocalStorageMaterialService: Retrieving materials from file storage was not possible: ', JSON.stringify(error));
      return [];
    })
  );
}

When using this method and subscribing to it, remember to unsubscribe the subscription in the component's ngOnDestroy method:

// unsubscribe this subscription on ngOnDestroy()
const subscription = getAllTimesheets()
  .subscribe(timesheetData => {
    // Handle the data here.
  })

Answer 2

It seems like your approach may need a shift in perspective. The readAsArrayBuffer function operates asynchronously, returning a promise.

In your getAllTimesheets() method, you should not simply return an array of TimesheetModel. Instead, consider returning an

Observable<TimesheetModel[]>

. This change will require adjustments wherever the method is called.

With an

Observable<TimesheetModel[]>

return type, you must either subscribe to each call of getAllTimesheets() or utilize the async pipe in your template to read the Observable data.

The latter option is recommended for simplicity.

To implement this modification in your getAllTimesheets() method, make the following alterations:

public getAllTimesheets(): Observable<TimesheetModel[]> {
  const storageId = TIMESHEET_KEYS.ALL_TIMESHEET;
  return from(
    this.cordovaFile
    .readAsArrayBuffer(this.cordovaFile.dataDirectory, storageId)
    .then((compressedTimesheet) => {
      const start = moment();
      const uint8array = new Uint8Array(compressedTimesheet);
      const jsonTimeSheet = this.LZString.decompressFromUint8Array(uint8array);
      this.log.debug(`LocalStorageMaterialService: getMaterials() from files: Decompression took ${moment().subtract(start.valueOf()).valueOf()} ms`);
      return <TimesheetModel[] > JSON.parse(jsonTimeSheet) || [];
    })
    .catch((error) => {
      this.log.debug('LocalStorageMaterialService: Retrieving materials from file storage was not possible: ', JSON.stringify(error));
      return [];
    })
  );
}

When using this method and subscribing to it, remember to unsubscribe the subscription in the component's ngOnDestroy method:

// unsubscribe this subscription on ngOnDestroy()
const subscription = getAllTimesheets()
  .subscribe(timesheetData => {
    // Handle the data here.
  })

Answer 3

Answer №2

To avoid registering your entire promise as an observable, it's recommended to set up a BehaviorSubject and update the subject with the promise result using .next(). Subscribing to this updated result is then possible.

If you're working within a service, you can establish this setup by following these steps:

@Injectable({ providedIn: 'root' })
export class TimesheetsService {
  /**
   * Initialize a new behavior subject with an empty array which will be
   * accessed from outside of the service through subscription.
   */
  public timesheets$: BehaviorSubject<TimesheetModel[]> = new BehaviorSubject([]);

  constructor() {
    // Invoke your service method to retrieve data via GET request.
    this.getAllTimesheets();
  }

  /**
   * This method updates the behavior subject and can be triggered externally
   * each time there is a need to refresh the result.
   */
  public getAllTimesheets() {
    return this.cordovaFile
      .readAsArrayBuffer(this.cordovaFile.dataDirectory, storageId)
      .then((compressedTimesheet) => {
        const uint8array = new Uint8Array(compressedTimesheet);
        const jsonTimeSheet = this.LZString.decompressFromUint8Array(uint8array);
        // Upon success, update the behavior subject with the fetched result.
        return timesheets$.next(JSON.parse(jsonTimeSheet));
      })
      // In case of error, update the behavior subject accordingly.
      .catch(() => timesheets$.next([]));
  }
}

In your component, subscribe to your result$ observable like so:

export class YourComponent implements OnInit {
  timesheets: TimesheetModel[];

  constructor(private timesheetsService: TimesheetsService) {
    // Optionally trigger your service method again for fresh data retrieval.
    this.timesheetsService.getAllTimesheets();
  }

  ngOnInit() {
    /**
     * Subscribe to the outcome of your service call and assign the result to a
     * variable to be utilized within your component.
     */
    this.timesheetsService.timesheets$
      .subscribe((timesheets) => this.timesheets = timesheets);
  }
}

Answer 4

To avoid registering your entire promise as an observable, it's recommended to set up a BehaviorSubject and update the subject with the promise result using .next(). Subscribing to this updated result is then possible.

If you're working within a service, you can establish this setup by following these steps:

@Injectable({ providedIn: 'root' })
export class TimesheetsService {
  /**
   * Initialize a new behavior subject with an empty array which will be
   * accessed from outside of the service through subscription.
   */
  public timesheets$: BehaviorSubject<TimesheetModel[]> = new BehaviorSubject([]);

  constructor() {
    // Invoke your service method to retrieve data via GET request.
    this.getAllTimesheets();
  }

  /**
   * This method updates the behavior subject and can be triggered externally
   * each time there is a need to refresh the result.
   */
  public getAllTimesheets() {
    return this.cordovaFile
      .readAsArrayBuffer(this.cordovaFile.dataDirectory, storageId)
      .then((compressedTimesheet) => {
        const uint8array = new Uint8Array(compressedTimesheet);
        const jsonTimeSheet = this.LZString.decompressFromUint8Array(uint8array);
        // Upon success, update the behavior subject with the fetched result.
        return timesheets$.next(JSON.parse(jsonTimeSheet));
      })
      // In case of error, update the behavior subject accordingly.
      .catch(() => timesheets$.next([]));
  }
}

In your component, subscribe to your result$ observable like so:

export class YourComponent implements OnInit {
  timesheets: TimesheetModel[];

  constructor(private timesheetsService: TimesheetsService) {
    // Optionally trigger your service method again for fresh data retrieval.
    this.timesheetsService.getAllTimesheets();
  }

  ngOnInit() {
    /**
     * Subscribe to the outcome of your service call and assign the result to a
     * variable to be utilized within your component.
     */
    this.timesheetsService.timesheets$
      .subscribe((timesheets) => this.timesheets = timesheets);
  }
}

Answer 5

Answer №3

If you want to convert an array into a specific type, you can use the as [] syntax in TypeScript.

timesheetArray.subscribe((timesheet) => {
    const time_sheet = timesheet as [];
});

Answer 6

If you want to convert an array into a specific type, you can use the as [] syntax in TypeScript.

timesheetArray.subscribe((timesheet) => {
    const time_sheet = timesheet as [];
});

Guide on Retrieving an Array from an Observable

Answer №1

Answer №2

Answer №3

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