Driving the refactor tractor

- InvenTreeAPI() object now has a method to return a cache-backed image from the server

lib/api.dart

@@ -2,6 +2,8 @@ import 'dart:async';
 import 'dart:convert';
 import 'dart:io';
 
+import 'package:flutter/material.dart';
+import 'package:flutter_advanced_networkimage/provider.dart';
 import 'package:image/image.dart';
 
 import 'package:path/path.dart' as path;
@@ -287,4 +289,20 @@ class InvenTreeAPI {
       return "Basic " + base64Encode(utf8.encode('$_username:$_password'));
     }
   }
+
+  static String get staticImage => "/static/img/blank_image.png";
+
+  static String get staticThumb => "/static/img/blank_image.thumbnail.png";
+
+  /*
+   * Get an image from the server (or, from cache)
+   */
+  AdvancedNetworkImage getImage(String imageUrl) {
+
+    return new AdvancedNetworkImage(makeUrl(imageUrl),
+      header: defaultHeaders(),
+      useDiskCache: true,
+      cacheRule: CacheRule(maxAge: const Duration(days: 5)),
+    );
+  }
 }

lib/inventree/part.dart

@@ -42,25 +42,20 @@ class InvenTreePart extends InvenTreeModel {
 
   String get _thumbnail => jsondata['thumbnail'] ?? '';
 
-  // Return a fully-qualified path to the image for this Part
+  // Return a path to the image for this Part
   String get image {
+    // Use thumbnail as a backup
     String img = _image.isNotEmpty ? _image : _thumbnail;
 
-    if (img.isEmpty) {
-      return InvenTreeAPI().makeUrl('/static/img/blank_image.png');
-    } else {
-      return InvenTreeAPI().makeUrl(img);
-    }
+    return img.isNotEmpty ? img : InvenTreeAPI.staticImage;
   }
 
+  // Return a path to the thumbnail for this part
   String get thumbnail {
+    // Use image as a backup
     String img = _thumbnail.isNotEmpty ? _thumbnail : _image;
 
-    if (img.isEmpty) {
-      return InvenTreeAPI().makeUrl('/static/img/blank_image.thumbnail.png');
-    } else {
-      return InvenTreeAPI().makeUrl(img);
-    }
+    return img.isNotEmpty ? img : InvenTreeAPI.staticThumb;
   }
 
   InvenTreePart() : super();

lib/widget/category_display.dart

@@ -180,12 +180,7 @@ class PartList extends StatelessWidget {
       title: Text("${part.name}"),
       subtitle: Text("${part.description}"),
       leading: Image(
-        image: AdvancedNetworkImage(
-          part.thumbnail,
-          header: InvenTreeAPI().defaultHeaders(),
-          useDiskCache: true,
-          cacheRule: CacheRule(maxAge: const Duration(days: 1)),
-        ),
+        image: InvenTreeAPI().getImage(part.thumbnail),
         width: 48,
       ),
       onTap: () {