feat: use http interceptor for backendless development

Note: intercept method of the FakeBackendInterceptor should
be developed further whenever it is needed to fake a response
from the backend.

Closes #37

src/app/_helpers/fake-backend.interceptor.ts

@@ -0,0 +1,54 @@
+/*
+ * Copyright (C) 2018, 2019 StApps
+ * This program is free software: you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License as published by the Free
+ * Software Foundation, version 3.
+ *
+ * This program is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for
+ * more details.
+ *
+ * You should have received a copy of the GNU General Public License along with
+ * this program. If not, see <https://www.gnu.org/licenses/>.
+ */
+import {HTTP_INTERCEPTORS, HttpEvent,
+  HttpHandler, HttpInterceptor, HttpRequest, HttpResponse} from '@angular/common/http';
+import {Injectable} from '@angular/core';
+import {SCThing} from '@openstapps/core';
+import {Observable, of} from 'rxjs';
+import {getSampleThings} from './data/sampleThings';
+
+@Injectable()
+export class FakeBackendInterceptor implements HttpInterceptor {
+  // tslint:disable-next-line:no-empty
+  constructor() {
+  }
+
+  intercept(request: HttpRequest<any>, next: HttpHandler): Observable<HttpEvent<any>> {
+    let data: SCThing[] = [];
+    // fake responses for search requests for easier (backendless) development process
+    if (request.url.endsWith('/search') && request.method === 'POST') {
+      if (typeof request.body.filter !== 'undefined' && typeof request.body.filter.arguments !== 'undefined') {
+        if (request.body.filter.arguments.field === 'uid') {
+          // provide items with given uid for search requests requesting single items (detail view)
+          data = getSampleThings(request.body.filter.arguments.value);
+        }
+      } else {
+        // if filter and arguments are not set, then provide all sample items (things)
+        data = getSampleThings();
+      }
+      // fake a response of the backend with previously defined data
+      return of(new HttpResponse({status: 200, body: {data: data}}));
+    } else {
+      // for all other requests, forward the requests to actually requested URL (backend)
+      return next.handle(request);
+    }
+  }
+}
+
+export const fakeBackendProvider = {
+  multi: true,
+  provide: HTTP_INTERCEPTORS,
+  useClass: FakeBackendInterceptor,
+};