excercises are now cached

src/main/webapp/app/exercise/exercise-details/exercise-details.component.ts

@@ -304,13 +304,13 @@ export class ExerciseBodyComponent implements OnInit, OnDestroy, AfterViewInit {
   }
 
   updateParent(exercise: ExtendedSearchResultDTO): void {
-    if (this.exercise!.file.parentId) {
-      this.exerciseService.loadExercise(this.exercise!.file.parentId).subscribe({
+    if (exercise.file.parentId) {
+      this.exerciseService.loadExercise(exercise.file.parentId).subscribe({
         next: searchResult => {
           this.parent = searchResult;
         },
         error: () => {
-          alert(`exercise ${this.exercise!.file.parentId} cannot be loaded`);
+          alert(`exercise ${exercise.file.parentId} cannot be loaded`);
         },
       });
     }

src/main/webapp/app/exercise/service/exercise.service.ts

@@ -6,7 +6,7 @@ import { StatisticsService } from 'app/entities/statistics/statistics.service';
 import { SearchService } from 'app/search/service/search-service';
 import { ArtemisExerciseInfo } from 'app/shared/model/artemis-exercise-info.model';
 import { ChildInfo, ExtendedSearchResultDTO, hasChildren, SearchResultDTO } from 'app/shared/model/search/search-result-dto.model';
-import { Observable } from 'rxjs';
+import { BehaviorSubject, Observable } from 'rxjs';
 
 @Injectable({ providedIn: 'root' })
 export class ExerciseService {
@@ -34,10 +34,42 @@ export class ExerciseService {
     });
   }
 
+  private exerciseCache: { [id: string]: BehaviorSubject<SearchResultDTO> } = {};
+
   public loadExercise(exerciseId: string): Observable<SearchResultDTO> {
+    if (this.exerciseCache[exerciseId]) {
+      return this.exerciseCache[exerciseId];
+    } else {
+      const call = this.loadExerciseFromServer(exerciseId);
+      this.exerciseCache[exerciseId] = new BehaviorSubject<SearchResultDTO>(undefined!);
+      call.subscribe({
+        next: (data: SearchResultDTO) => {
+          if (this.exerciseCache[exerciseId]) {
+            this.exerciseCache[exerciseId].next(data);
+          } else {
+            // this should never happen, but just in case
+            // we loose all previous listeners to the observable
+            this.exerciseCache[exerciseId] = new BehaviorSubject<SearchResultDTO>(undefined!);
+            this.exerciseCache[exerciseId].next(data);
+          }
+        },
+        error: () => console.warn('Could not load exercise'),
+      });
+      return call;
+    }
+  }
+
+  private loadExerciseFromServer(exerciseId: string): Observable<SearchResultDTO> {
     return this.http.get<SearchResultDTO>(this.exerciseUrl + encodeURIforExerciseId(exerciseId));
   }
 
+  /**
+   * resets all cached exercises
+   */
+  resetExcerciseCache() {
+    this.exerciseCache = {};
+  }
+
   public loadChildInfos(exerciseId: string): Observable<ChildInfo[]> {
     return this.http.get<ChildInfo[]>(this.exerciseChildrenUrl + encodeURIforExerciseId(exerciseId));
   }