Properly handle permissions

The backend has a new state called "needsPermissions" and the app widget
reacts accordingly.

lib/app.dart

@@ -30,9 +30,40 @@ class App extends StatelessWidget {
       home: Builder(
         builder: (context) {
           if (backend.status == BackendStatus.loading) {
-            return Container(
+            return Material(
               color: Theme.of(context).scaffoldBackgroundColor,
             );
+          } else if (backend.status == BackendStatus.needsPermissions) {
+            return Material(
+              color: Theme.of(context).scaffoldBackgroundColor,
+              child: Column(
+                mainAxisAlignment: MainAxisAlignment.center,
+                children: <Widget>[
+                  Text(
+                    'Musicus needs permissions\nto access your files.',
+                    textAlign: TextAlign.center,
+                    style: Theme.of(context).textTheme.headline6,
+                  ),
+                  SizedBox(
+                    height: 16.0,
+                  ),
+                  ListTile(
+                    leading: const Icon(Icons.done),
+                    title: Text('Grant permissions'),
+                    onTap: () {
+                      backend.requestPermissions();
+                    },
+                  ),
+                  ListTile(
+                    leading: const Icon(Icons.settings),
+                    title: Text('Open system\'s app settings'),
+                    onTap: () {
+                      backend.openAppSettings();
+                    },
+                  ),
+                ],
+              ),
+            );
           } else {
             return Content();
           }

lib/backend.dart

@@ -1,10 +1,12 @@
 import 'package:flutter/widgets.dart';
+import 'package:permission_handler/permission_handler.dart';
 import 'package:rxdart/rxdart.dart';
 
 import 'database.dart';
 
 enum BackendStatus {
   loading,
+  needsPermissions,
   ready,
 }
 
@@ -23,6 +25,8 @@ class Backend extends StatefulWidget {
 }
 
 class BackendState extends State<Backend> {
+  final _permissionHandler = PermissionHandler();
+
   final playerActive = BehaviorSubject.seeded(false);
   final playing = BehaviorSubject.seeded(false);
   final position = BehaviorSubject.seeded(0.0);
@@ -47,10 +51,32 @@ class BackendState extends State<Backend> {
   Future<void> _load() async {
     db = Database('musicus.sqlite');
 
+    final permissionStatus =
+        await _permissionHandler.checkPermissionStatus(PermissionGroup.storage);
+
+    if (permissionStatus != PermissionStatus.granted) {
+      setState(() {
+        status = BackendStatus.needsPermissions;
+      });
+    } else {
       setState(() {
         status = BackendStatus.ready;
       });
     }
+  }
+
+  Future<void> requestPermissions() async {
+    final result =
+        await _permissionHandler.requestPermissions([PermissionGroup.storage]);
+    
+    if (result[PermissionGroup.storage] == PermissionStatus.granted) {
+      setState(() {
+        status = BackendStatus.ready;
+      });
+    }
+  }
+
+  Future<void> openAppSettings() => _permissionHandler.openAppSettings();
 
   void startPlayer() {
     playerActive.add(true);

pubspec.yaml

@@ -13,6 +13,7 @@ dependencies:
     sdk: flutter
   moor_flutter:
   path:
+  permission_handler:
   rxdart:
 
 dev_dependencies: