mobile: Support using an account

mobile/lib/screens/account_settings.dart

@@ -0,0 +1,110 @@
+import 'dart:async';
+
+import 'package:flutter/material.dart';
+import 'package:musicus_common/musicus_common.dart';
+
+import 'register.dart';
+
+/// A screen for logging in using a Musicus account.
+class AccountSettingsScreen extends StatefulWidget {
+  @override
+  _AccountSettingsScreenState createState() => _AccountSettingsScreenState();
+}
+
+class _AccountSettingsScreenState extends State<AccountSettingsScreen> {
+  final nameController = TextEditingController();
+  final passwordController = TextEditingController();
+
+  MusicusBackendState backend;
+  StreamSubscription<MusicusAccountSettings> accountSubscription;
+
+  @override
+  void didChangeDependencies() {
+    super.didChangeDependencies();
+
+    backend = MusicusBackend.of(context);
+
+    if (accountSubscription != null) {
+      accountSubscription.cancel();
+    }
+
+    _settingsChanged(backend.settings.account.value);
+    accountSubscription = backend.settings.account.listen((settings) {
+      _settingsChanged(settings);
+    });
+  }
+
+  void _settingsChanged(MusicusAccountSettings settings) {
+    nameController.text = settings?.username ?? '';
+    passwordController.text = settings?.password ?? '';
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      appBar: AppBar(
+        title: Text('Account settings'),
+        actions: <Widget>[
+          FlatButton(
+            onPressed: () async {
+              await backend.settings.setAccount(MusicusAccountSettings(
+                username: nameController.text,
+                password: passwordController.text,
+              ));
+
+              Navigator.pop(context);
+            },
+            child: Text('LOGIN'),
+          ),
+        ],
+      ),
+      body: ListView(
+        children: <Widget>[
+          Padding(
+            padding: const EdgeInsets.all(16.0),
+            child: TextField(
+              controller: nameController,
+              decoration: InputDecoration(
+                labelText: 'User name',
+              ),
+            ),
+          ),
+          Padding(
+            padding: const EdgeInsets.all(16.0),
+            child: TextField(
+              controller: passwordController,
+              obscureText: true,
+              decoration: InputDecoration(
+                labelText: 'Password',
+              ),
+            ),
+          ),
+          ListTile(
+            title: Text('Create a new account'),
+            onTap: () {
+              Navigator.pushReplacement(
+                context,
+                MaterialPageRoute(
+                  builder: (context) => RegisterScreen(),
+                ),
+              );
+            },
+          ),
+          ListTile(
+            title: Text('Don\'t use an account'),
+            onTap: () {
+              backend.settings.clearAccount();
+              Navigator.pop(context);
+            },
+          ),
+        ],
+      ),
+    );
+  }
+
+  @override
+  void dispose() {
+    super.dispose();
+    accountSubscription.cancel();
+  }
+}