feat: add server-side pagination sort and search for centers component (#506)

- Add server-side pagination, sort and search for centers component
- Refactor code to meet current guidelines

See: #234

src/app/centers/centers.datasource.ts

@@ -0,0 +1,61 @@
+/** Angular Imports */
+import { CollectionViewer, DataSource } from '@angular/cdk/collections';
+
+/** rxjs Imports */
+import { Observable, BehaviorSubject } from 'rxjs';
+
+/** Custom Services */
+import { CentersService } from './centers.service';
+
+/**
+ * Centers custom data source to implement server side filtering, pagination and sorting.
+ */
+export class CentersDataSource implements DataSource<any> {
+
+  /** centers behavior subject to represent loaded centers page. */
+  private centersSubject = new BehaviorSubject<any[]>([]);
+  /** Records subject to represent total number of filtered centers records. */
+  private recordsSubject = new BehaviorSubject<number>(0);
+  /** Records observable which can be subscribed to get the value of total number of filtered centers records. */
+  public records$ = this.recordsSubject.asObservable();
+
+  /**
+   * @param {CentersService} centersService Centers Service
+   */
+  constructor(private centersService: CentersService) { }
+
+  /**
+   * Gets centers on the basis of provided parameters and emits the value.
+   * @param {any} filterBy Properties by which entries should be filtered.
+   * @param {string} orderBy Property by which entries should be sorted.
+   * @param {string} sortOrder Sort order: ascending or descending.
+   * @param {number} pageIndex Page number.
+   * @param {number} limit Number of entries within the page.
+   * @param {boolean} centerActive Specify whether to only filter active centers.
+   */
+  getCenters(filterBy: any, orderBy: string = '', sortOrder: string = '', pageIndex: number = 0, limit: number = 10, centerActive: boolean = true) {
+    this.centersSubject.next([]);
+    this.centersService.getCenters(filterBy, orderBy, sortOrder, pageIndex * limit, limit)
+      .subscribe((centers: any) => {
+        centers.pageItems = (centerActive) ? (centers.pageItems.filter((center: any) => center.active)) : centers.pageItems;
+        this.recordsSubject.next(centers.totalFilteredRecords);
+        this.centersSubject.next(centers.pageItems);
+      });
+  }
+
+  /**
+   * @param {CollectionViewer} collectionViewer
+   */
+  connect(collectionViewer: CollectionViewer): Observable<any[]> {
+    return this.centersSubject.asObservable();
+  }
+
+  /**
+   * @param {CollectionViewer} collectionViewer
+   */
+  disconnect(collectionViewer: CollectionViewer): void {
+    this.centersSubject.complete();
+    this.recordsSubject.complete();
+  }
+
+}