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/screens/cart.dart

@@ -0,0 +1,83 @@
+// 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';
+import 'package:provider/provider.dart';
+import 'package:provider_shopper/models/cart.dart';
+
+class MyCart extends StatelessWidget {
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      appBar: AppBar(
+        title: Text('Cart', style: Theme.of(context).textTheme.display4),
+        backgroundColor: Colors.white,
+      ),
+      body: Container(
+        color: Colors.yellow,
+        child: Column(
+          children: [
+            Expanded(
+              child: Padding(
+                padding: const EdgeInsets.all(32),
+                child: _CartList(),
+              ),
+            ),
+            Divider(height: 4, color: Colors.black),
+            _CartTotal()
+          ],
+        ),
+      ),
+    );
+  }
+}
+
+class _CartList extends StatelessWidget {
+  @override
+  Widget build(BuildContext context) {
+    var itemNameStyle = Theme.of(context).textTheme.title;
+    var cart = Provider.of<CartModel>(context);
+
+    return ListView.builder(
+      itemCount: cart.items.length,
+      itemBuilder: (context, index) => ListTile(
+        leading: Icon(Icons.done),
+        title: Text(
+          cart.items[index].name,
+          style: itemNameStyle,
+        ),
+      ),
+    );
+  }
+}
+
+class _CartTotal extends StatelessWidget {
+  @override
+  Widget build(BuildContext context) {
+    var hugeStyle = Theme.of(context).textTheme.display4.copyWith(fontSize: 48);
+
+    return SizedBox(
+      height: 200,
+      child: Center(
+        child: Row(
+          mainAxisAlignment: MainAxisAlignment.center,
+          children: [
+            Consumer<CartModel>(
+                builder: (context, cart, child) =>
+                    Text('\$${cart.totalPrice}', style: hugeStyle)),
+            SizedBox(width: 24),
+            FlatButton(
+              onPressed: () {
+                Scaffold.of(context).showSnackBar(
+                    SnackBar(content: Text('Buying not supported yet.')));
+              },
+              color: Colors.white,
+              child: Text('BUY'),
+            ),
+          ],
+        ),
+      ),
+    );
+  }
+}

provider_shopper/lib/screens/catalog.dart

@@ -0,0 +1,97 @@
+// 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';
+import 'package:provider/provider.dart';
+import 'package:provider_shopper/models/cart.dart';
+import 'package:provider_shopper/models/catalog.dart';
+
+class MyCatalog extends StatelessWidget {
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      body: CustomScrollView(
+        slivers: [
+          _MyAppBar(),
+          SliverToBoxAdapter(child: SizedBox(height: 12)),
+          SliverList(
+            delegate: SliverChildBuilderDelegate(
+                (context, index) => _MyListItem(index)),
+          ),
+        ],
+      ),
+    );
+  }
+}
+
+class _AddButton extends StatelessWidget {
+  final Item item;
+
+  const _AddButton({Key key, @required this.item}) : super(key: key);
+
+  @override
+  Widget build(BuildContext context) {
+    var cart = Provider.of<CartModel>(context);
+
+    return FlatButton(
+      onPressed: cart.items.contains(item) ? null : () => cart.add(item),
+      splashColor: Theme.of(context).primaryColor,
+      child: cart.items.contains(item)
+          ? Icon(Icons.check, semanticLabel: 'ADDED')
+          : Text('ADD'),
+    );
+  }
+}
+
+class _MyAppBar extends StatelessWidget {
+  @override
+  Widget build(BuildContext context) {
+    return SliverAppBar(
+      title: Text('Catalog', style: Theme.of(context).textTheme.display4),
+      floating: true,
+      actions: [
+        IconButton(
+          icon: Icon(Icons.shopping_cart),
+          onPressed: () => Navigator.pushNamed(context, '/cart'),
+        ),
+      ],
+    );
+  }
+}
+
+class _MyListItem extends StatelessWidget {
+  final int index;
+
+  _MyListItem(this.index, {Key key}) : super(key: key);
+
+  @override
+  Widget build(BuildContext context) {
+    var catalog = Provider.of<CatalogModel>(context);
+    var item = catalog.getByPosition(index);
+    var textTheme = Theme.of(context).textTheme.title;
+
+    return Padding(
+      padding: const EdgeInsets.symmetric(horizontal: 16, vertical: 8),
+      child: LimitedBox(
+        maxHeight: 48,
+        child: Row(
+          children: [
+            AspectRatio(
+              aspectRatio: 1,
+              child: Container(
+                color: item.color,
+              ),
+            ),
+            SizedBox(width: 24),
+            Expanded(
+              child: Text(item.name, style: textTheme),
+            ),
+            SizedBox(width: 24),
+            _AddButton(item: item),
+          ],
+        ),
+      ),
+    );
+  }
+}