Add "Category display" view

- Select "parts" from the app drawer
- Allow traversal of sub-categories

lib/widget/category_display.dart

@@ -0,0 +1,187 @@
+
+import 'package:InvenTree/inventree/part.dart';
+import 'package:flutter/cupertino.dart';
+import 'package:flutter/foundation.dart';
+import 'package:flutter/material.dart';
+
+class CategoryDisplayWidget extends StatefulWidget {
+
+  CategoryDisplayWidget(int catId, {this.category, Key key}) : categoryId = catId, super(key: key);
+
+  InvenTreePartCategory category = null;
+
+  final String title = "Category";
+
+  final int categoryId;
+
+  @override
+  _CategoryDisplayState createState() => _CategoryDisplayState(categoryId, category: category);
+}
+
+
+class _CategoryDisplayState extends State<CategoryDisplayWidget> {
+
+  _CategoryDisplayState(int id, {this.category}) : categoryId = id {
+    _requestData();
+  }
+
+  final int categoryId;
+
+  // The local InvenTreePartCategory object
+  InvenTreePartCategory category = null;
+
+  List<InvenTreePartCategory> subcategories = List<InvenTreePartCategory>();
+
+  List<InvenTreePart> parts = List<InvenTreePart>();
+
+  String get _titleString {
+
+    if (category == null) {
+      return "Part Categories";
+    } else {
+      return "Part Category '${category.name}'";
+    }
+  }
+
+  /*
+   * Request data from the server
+   */
+  void _requestData() {
+
+    // Request a list of sub-categories under this one
+    InvenTreePartCategory().list(filters: {"parent": "$categoryId"}).then((var cats) {
+      subcategories.clear();
+
+      print("Returned categories: ${cats.length}");
+
+      for (var cat in cats) {
+        if (cat is InvenTreePartCategory) {
+          subcategories.add(cat);
+        }
+      }
+
+      // Update state
+      setState(() {});
+    });
+
+    // Request a list of parts under this category
+    InvenTreePart().list(filters: {"category": "$categoryId"}).then((var parts) {
+      parts.clear();
+
+      print("Returned parts: ${parts.length}");
+
+      for (var part in parts) {
+        if (part is InvenTreePart) {
+          parts.add(part);
+          print("Adding part: ${part.name}");
+        }
+      }
+
+      // Update state
+      setState(() {});
+    });
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      appBar: AppBar(
+        title: Text(_titleString),
+      ),
+      body: Center(
+        child: Column(
+          mainAxisAlignment: MainAxisAlignment.center,
+          children: <Widget>[
+            Text(
+              "Subcategories",
+            ),
+            Expanded(child: SubcategoryList(subcategories)),
+            Divider(),
+            Text("Parts"),
+            Expanded(child: PartList(parts)),
+            Spacer(),
+          ]
+        )
+      )
+    );
+  }
+}
+
+
+/*
+ * Builder for displaying a list of PartCategory objects
+ */
+class SubcategoryList extends StatelessWidget {
+  final List<InvenTreePartCategory> _categories;
+
+  SubcategoryList(this._categories);
+
+  void _openCategory(BuildContext context, int pk) {
+
+    // Attempt to load the sub-category.
+    InvenTreePartCategory().get(pk).then((var cat) {
+      if (cat is InvenTreePartCategory) {
+        print("Found cat: <${cat.pk}> : ${cat.name} - ${cat.description}");
+
+        Navigator.push(context, MaterialPageRoute(builder: (context) => CategoryDisplayWidget(pk, category: cat)));
+      }
+    });
+  }
+
+  Widget _build(BuildContext context, int index) {
+    InvenTreePartCategory cat;
+
+    if (index < _categories.length) {
+      cat = _categories[index];
+    }
+
+    return Card(
+      child: InkWell(
+        child: Column(
+          children: <Widget>[
+            Text('${cat.name} - ${cat.description}'),
+          ],
+        ),
+        onTap: () {
+          _openCategory(context, cat.pk);
+        }
+      ),
+    );
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return ListView.builder(itemBuilder: _build, itemCount: _categories.length);
+  }
+}
+
+
+/*
+ * Builder for displaying a list of Part objects
+ */
+class PartList extends StatelessWidget {
+  final List<InvenTreePart> _parts;
+
+  PartList(this._parts);
+
+  Widget _build(BuildContext context, int index) {
+    InvenTreePart part;
+
+    if (index < _parts.length) {
+      part = _parts[index];
+    }
+
+    return Card(
+      child: Column(
+        children: <Widget> [
+          Text('${part.name} - ${part.description}'),
+        ]
+      )
+    );
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return ListView.builder(itemBuilder: _build, itemCount: _parts.length);
+  }
+}