How to implement in angular multiple sequential Http requests in a loop (each new request depends of the previous results)

Question

I have a grid that contains heavy data to be loaded and so the loading time became not manageable. The solution I found was do a sequential number of http requests, each one retrieving batches of 100 rows, till fulfill all data on the grid. I know how to implement 2 sequential http requests using concatMap and it works perfectly but I would like to have a while loop that would validate each response and if the current number of rows < total number of rows then subscribe a new Http request. It's strange I don't find any solution for this, maybe I'm thinking wrong from the very beginning this solution :D Any help would be very welcome! Thanks in advance!

Forwarding code used for execute 2 http requests using concatMap:

private LoadGridStringResourcesInProject(projectId: number) {
let allData: StringResource[] = [];
const batchSize = 100;

this.stringResourcesService.getStringResourcesInProject(projectId, 0, batchSize)
    .pipe(
      concatMap(firstData => {
        const stringResourcesInProject = firstData as StringResource[];

        // Loads first 100 rows on the Grid
        this.gridApi.updateRowData({ add: stringResourcesInProject });
        this.agGridService.refreshSizeColumns(this.agGrid);

        // Fetch data returned by partial Http requests
        allData = stringResourcesInProject;

        if (allData && allData.length == batchSize) {

          // Do new Http request to fetch the remaining data
          return this.stringResourcesService
            .getStringResourcesInProject(projectId, batchSize, 0);
        }

        return [];
      })
    ).subscribe(data => {
        const stringResourcesInProject = data as StringResource[];

        // Loads the remaining rows in the Grid
        this.gridApi.updateRowData({ add: stringResourcesInProject });

        // Fetch data returned by partial Http requests
        allData = allData.concat(stringResourcesInProject);
      },
      error => of(null),
      () => {
        this.agGridService.refreshSizeColumns(this.agGrid);
      });

}

The operator you're looking for is expand :)

maxime1992
– maxime1992

2020-01-16 11:47:00 +00:00
Commented Jan 16, 2020 at 11:47 — maxime1992
– maxime1992, Commented Jan 16, 2020 at 11:47
Thank you so much @Maxim1992. Working like a charme :)

Ricardo Magalhães
– Ricardo Magalhães

2020-01-16 14:49:07 +00:00
Commented Jan 16, 2020 at 14:49 — Ricardo Magalhães
– Ricardo Magalhães, Commented Jan 16, 2020 at 14:49

Ricardo Magalhães · Accepted Answer · 2020-01-16 14:54:26Z

3

Like @Maxim1992 said, the solution is using the Expand RxJS operator in order to call recursively! Thank you so much @Maxim1992!

More information here: Example

Here is the code you can use also (hope it can help someone in the future):

private LoadGridStringResourcesInProject(projectId: number) {
const batchSize = 1000;
let iteraction = 0;


this.stringResourcesService.getStringResourcesInProject(projectId, false, false, false, 0, batchSize)
  .pipe(
    expand(partialData => {
      if (partialData) {
        let partialStringResourcesInProject = partialData as StringResource[];

        if (partialStringResourcesInProject.length > batchSize) {

          // Loads the remaining rows in the Grid
          this.gridApi.updateRowData({ add: partialStringResourcesInProject });

          iteraction += 1;

          return this.stringResourcesService.getStringResourcesInProject(projectId, false, false, false,
                       batchSize * (iteraction - 1), batchSize);
        }

        return EMPTY;
      }
    })
  ).subscribe(data => {

        //... 
  },
  error => of(null),
  () => {
    this.agGridService.refreshSizeColumns(this.agGrid);
  });

answered Jan 16, 2020 at 14:54

Ricardo Magalhães

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1 Comment

maxime1992 Over a year ago

Glad you figured out the solution once you heard of expand :) well done

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