add server ping check

src/app/service/api.service.ts

@@ -11,8 +11,6 @@ import {Database} from './db/Database';
 import {DatabaseColumn} from './db/DatabaseColumn';
 import {DatabaseEntry} from './db/DatabaseEntry';
 import {FileGroup} from './db/FileGroup';
-import {Page} from './db/Page'
-import {PageNode} from "./db/PageNode";
 
 export class ResourceNotAvailableOfflineError extends Error {
   constructor(msg = 'This resource is not yet available offline on this device.') {
@@ -43,10 +41,30 @@ export class ApiService {
       if ( !isConnected ) {
         this.makeOffline();
       } else {
-        this.makeOnline(); // TODO add checks for server.
+        this.checkOnline().then(isOnline => {
+          if ( isOnline ) {
+            this.makeOnline();
+          } else {
+            this.makeOffline();
           }
         });
       }
+    });
+
+    this.checkOnline().then(isConnected => {
+      if ( !isConnected ) {
+        this.makeOffline();
+      } else {
+        this.makeOnline();
+      }
+    });
+  }
+
+  public checkOnline(): Promise<boolean> {
+    return fetch(this.statUrl).then(resp => {
+      return resp && (resp.ok || resp.type === 'opaque');
+    });
+  }
 
   public makeOffline() {
     this.offline = true;