Add a connection checker

lib/api.dart

@@ -72,6 +72,41 @@ class InvenTreeAPI {
   // Authentication token (initially empty, must be requested)
   String _token = "";
 
+  bool isConnected() {
+    return _token.isNotEmpty;
+  }
+
+  /*
+   * Check server connection and display messages if not connected.
+   * Useful as a precursor check before performing operations.
+   */
+  bool checkConnection(BuildContext context) {
+
+    // Firstly, is the server connected?
+    if (!isConnected()) {
+      showDialog(
+          context: context,
+          child: new SimpleDialog(
+              title: new Text("Not Connected"),
+              children: <Widget> [
+                ListTile(
+                  title: Text("Server not connected"),
+                )
+              ]
+          )
+      );
+
+      return false;
+    }
+
+    // Is the server version too old?
+    // TODO
+
+    // Finally
+    return true;
+
+  }
+
   // Server instance information
   String instance = '';
 

lib/main.dart

@@ -179,18 +179,27 @@ class _MyHomePageState extends State<MyHomePage> {
   }
 
   void _search() {
+    if (!InvenTreeAPI().checkConnection(context)) return;
+
     // TODO
   }
 
   void _scan() {
+    if (!InvenTreeAPI().checkConnection(context)) return;
+
     scanQrCode(context);
   }
 
   void _parts() {
+    if (!InvenTreeAPI().checkConnection(context)) return;
+
     Navigator.push(context, MaterialPageRoute(builder: (context) => CategoryDisplayWidget(null)));
   }
 
   void _stock() {
+
+    if (!InvenTreeAPI().checkConnection(context)) return;
+
     Navigator.push(context, MaterialPageRoute(builder: (context) => LocationDisplayWidget(null)));
   }
 

lib/widget/drawer.dart

@@ -1,6 +1,9 @@
+import 'package:InvenTree/api.dart';
 import 'package:InvenTree/barcode.dart';
 import 'package:flutter/material.dart';
 
+import 'package:InvenTree/api.dart';
+
 import 'package:InvenTree/widget/category_display.dart';
 import 'package:InvenTree/widget/location_display.dart';
 
@@ -33,6 +36,7 @@ class InvenTreeDrawer extends StatelessWidget {
    * Upon successful scan, data are passed off to be decoded.
    */
   void _scan() async {
+    if (!InvenTreeAPI().checkConnection(context)) return;
 
     _closeDrawer();
     scanQrCode(context);
@@ -42,6 +46,7 @@ class InvenTreeDrawer extends StatelessWidget {
    * Display the top-level PartCategory list
    */
   void _showParts() {
+    if (!InvenTreeAPI().checkConnection(context)) return;
 
     _closeDrawer();
     Navigator.push(context, MaterialPageRoute(builder: (context) => CategoryDisplayWidget(null)));
@@ -51,6 +56,7 @@ class InvenTreeDrawer extends StatelessWidget {
    * Display the top-level StockLocation list
    */
   void _showStock() {
+    if (!InvenTreeAPI().checkConnection(context)) return;
     _closeDrawer();
     Navigator.push(context, MaterialPageRoute(builder: (context) => LocationDisplayWidget(null)));
   }
@@ -72,6 +78,7 @@ class InvenTreeDrawer extends StatelessWidget {
                 leading: new Image.asset(
                   "assets/image/icon.png",
                   fit: BoxFit.scaleDown,
+                  width: 40,
                 ),
                 title: new Text("InvenTree"),
                 onTap: _home,