Add provider_shopper (#87)

This is the code for the sample at https://flutter.dev/docs/development/data-and-backend/state-mgmt/simple.

provider_shopper/lib/models/cart.dart

@@ -0,0 +1,38 @@
+// Copyright 2019 The Flutter team. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter/foundation.dart';
+import 'package:provider_shopper/models/catalog.dart';
+
+class CartModel extends ChangeNotifier {
+  /// The current catalog. Used to construct items from numeric ids.
+  final CatalogModel _catalog;
+
+  /// Internal, private state of the cart. Stores the ids of each item.
+  final List<int> _itemIds;
+
+  /// Construct a CartModel instance that is backed by a [CatalogModel] and
+  /// an optional previous state of the cart.
+  ///
+  /// If [previous] is not `null`, it's items are copied to the newly
+  /// constructed instance.
+  CartModel(this._catalog, CartModel previous)
+      : assert(_catalog != null),
+        _itemIds = previous?._itemIds ?? [];
+
+  /// List of items in the cart.
+  List<Item> get items => _itemIds.map((id) => _catalog.getById(id)).toList();
+
+  /// The current total price of all items.
+  int get totalPrice =>
+      items.fold(0, (total, current) => total + current.price);
+
+  /// Adds [item] to cart. This is the only way to modify the cart from outside.
+  void add(Item item) {
+    _itemIds.add(item.id);
+    // This line tells [Model] that it should rebuild the widgets that
+    // depend on it.
+    notifyListeners();
+  }
+}

provider_shopper/lib/models/catalog.dart

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+// Copyright 2019 The Flutter team. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter/material.dart';
+
+/// A proxy of the catalog of items the user can buy.
+///
+/// In a real app, this might be backed by a backend and cached on device.
+/// In this sample app, the catalog is procedurally generated and infinite.
+///
+/// For simplicity, the catalog is expected to be immutable (no products are
+/// expected to be added, removed or changed during the execution of the app).
+class CatalogModel {
+  static const _itemNames = [
+    'Code Smell',
+    'Control Flow',
+    'Interpreter',
+    'Recursion',
+    'Sprint',
+    'Heisenbug',
+    'Spaghetti',
+    'Hydra Code',
+    'Off-By-One',
+    'Scope',
+    'Callback',
+    'Closure',
+    'Automata',
+    'Bit Shift',
+    'Currying',
+  ];
+
+  /// Get item by [id].
+  ///
+  /// In this sample, the catalog is infinite, looping over [_itemNames].
+  Item getById(int id) => Item(id, _itemNames[id % _itemNames.length]);
+
+  /// Get item by its position in the catalog.
+  Item getByPosition(int position) {
+    // In this simplified case, an item's position in the catalog
+    // is also its id.
+    return getById(position);
+  }
+}
+
+@immutable
+class Item {
+  final int id;
+  final String name;
+  final Color color;
+  final int price = 42;
+
+  Item(this.id, this.name)
+      // To make the sample app look nicer, each item is given one of the
+      // Material Design primary colors.
+      : color = Colors.primaries[id % Colors.primaries.length];
+
+  @override
+  int get hashCode => id;
+
+  @override
+  bool operator ==(Object other) => other is Item && other.id == id;
+}