[DURACOM-234] Migrate to functional guards WIP

src/app/core/server-check/server-check.guard.ts

@@ -1,14 +1,13 @@
-import { Injectable } from '@angular/core';
+import { inject } from '@angular/core';
 import {
   ActivatedRouteSnapshot,
-  NavigationStart,
+  CanActivateChildFn,
   Router,
   RouterStateSnapshot,
   UrlTree,
 } from '@angular/router';
 import { Observable } from 'rxjs';
 import {
-  filter,
   map,
   take,
 } from 'rxjs/operators';
@@ -16,52 +15,18 @@ import {
 import { getPageInternalServerErrorRoute } from '../../app-routing-paths';
 import { RootDataService } from '../data/root-data.service';
 
-@Injectable({
-  providedIn: 'root',
-})
 /**
  * A guard that checks if root api endpoint is reachable.
  * If not redirect to 500 error page
  */
-export class ServerCheckGuard  {
-  constructor(private router: Router, private rootDataService: RootDataService) {
-  }
-
-  /**
-   * True when root api endpoint is reachable.
-   */
-  canActivateChild(
-    route: ActivatedRouteSnapshot,
-    state: RouterStateSnapshot,
-  ): Observable<boolean | UrlTree> {
-
-    return this.rootDataService.checkServerAvailability().pipe(
-      take(1),
-      map((isAvailable: boolean) => {
-        if (!isAvailable) {
-          return this.router.parseUrl(getPageInternalServerErrorRoute());
-        } else {
-          return true;
-        }
-      }),
-    );
-  }
-
-  /**
-   * Listen to all router events. Every time a new navigation starts, invalidate the cache
-   * for the root endpoint. That way we retrieve it once per routing operation to ensure the
-   * backend is not down. But if the guard is called multiple times during the same routing
-   * operation, the cached version is used.
-   */
-  listenForRouteChanges(): void {
-    // we'll always be too late for the first NavigationStart event with the router subscribe below,
-    // so this statement is for the very first route operation.
-    this.rootDataService.invalidateRootCache();
-
-    this.router.events.pipe(
-      filter(event => event instanceof NavigationStart),
-    ).subscribe(() => {
-      this.rootDataService.invalidateRootCache();
-    });
-  }
-}
+export const ServerCheckGuard: CanActivateChildFn = (
+  route: ActivatedRouteSnapshot,
+  state: RouterStateSnapshot,
+  rootDataService: RootDataService = inject(RootDataService),
+  router: Router = inject(Router),
+): Observable<boolean | UrlTree> => {
+  return rootDataService.checkServerAvailability().pipe(
+    take(1),
+    map((isAvailable: boolean) => isAvailable ? true : router.parseUrl(getPageInternalServerErrorRoute())),
+  );
+};