Fetch new content without refetching everything

Closes #14

lib/main.dart

@@ -92,71 +92,98 @@ class MyHomePage extends StatefulWidget {
   State<MyHomePage> createState() => _MyHomePageState();
 }
 
-class _MyHomePageState extends State<MyHomePage> {
+class _MyHomePageState extends State<MyHomePage> with WidgetsBindingObserver {
   var _index = 0;
   var _fetched = false;
   var _fetchedCount = 0;
   var _knownPostsSize = -1;
   var _isDemoUser = false;
+  AppLifecycleState? _appState;
   List<int> _feedFilter = [];
 
-  Future<void> refresh() async {
-    widget.api.cache.clear();
+  @override
+  void didChangeAppLifecycleState(AppLifecycleState state) {
+    print("Lifecycle state changed from $_appState to $state");
+
+    if (_appState != state) {
+      if (state == AppLifecycleState.resumed) {
+        refresh(fullRefresh: false);
+      }
+    }
+
     setState(() {
-      _fetchedCount = 0;
-      _knownPostsSize = -1;
-      _feedFilter = [];
+      _appState = state;
+    });
+  }
+
+  @override
+  void initState() {
+    super.initState();
+
+    WidgetsBinding.instance.addObserver(this);
+
+    setState(() {
+      print("Demo: ${widget.prefs.getBool("isDemo")}");
+      _isDemoUser = widget.prefs.getBool("isDemo") ?? false;
     });
 
+    refresh(fullRefresh: true).then((_) => setState(() {
+      _fetched = true;
+    }));
+  }
+
+  @override
+  void dispose() {
+    super.dispose();
+    WidgetsBinding.instance.removeObserver(this);
+  }
+
+  Future<void> refresh({ required bool fullRefresh }) async {
     print("Starting refresh");
 
+    listIds() => widget.api.cache.items
+      .getAll()
+      .values
+      .map((value) => value.id)
+      .toList()
+      ..sort((a, b) => b - a);
+
+    if (fullRefresh) {
+      widget.api.cache.clear();
+      setState(() {
+        _fetchedCount = 0;
+        _knownPostsSize = -1;
+        _feedFilter = [];
+      });
+    }
+
     var data = await widget.api
         .request()
         .withFeeds()
         .withGroups()
         .withFavicons()
         .withItems()
-        .sinceId(0)
+        .sinceId(fullRefresh ? 0 : listIds().first)
         .execute();
 
     while ((data['items'] as List<dynamic>).isNotEmpty &&
         _fetchedCount < fetchMaxPosts) {
       print("Fetching more items");
 
-      var ids = widget.api.cache.items
-          .getAll()
-          .values
-          .map((value) => value.id)
-          .toList()
-        ..sort((a, b) => b - a);
+      var ids = listIds();
 
       setState(() {
         _fetchedCount = ids.length;
         _knownPostsSize = data['total_items'] ?? -1;
       });
 
-      data =
-          await widget.api.request().withItems().sinceId(ids.first).execute();
+      data = await widget.api.request().withItems().sinceId(ids.first).execute();
 
       print(
           "Item cache size is now ${widget.api.cache.items.size} ${ids.first} ${ids.last}");
     }
   }
 
-  @override
-  void initState() {
-    super.initState();
-
-    setState(() {
-      print("Demo: ${widget.prefs.getBool("isDemo")}");
-      _isDemoUser = widget.prefs.getBool("isDemo") ?? false;
-    });
-
-    refresh().then((_) => setState(() {
-          _fetched = true;
-        }));
-  }
-
   @override
   Widget build(BuildContext context) {
     var isDemo = widget.prefs.getBool("isDemo");
@@ -218,7 +245,7 @@ class _MyHomePageState extends State<MyHomePage> {
 
         if (loggedIn) {
           timer.cancel();
-          refresh().then((_) => setState(() => _fetched = true));
+          refresh(fullRefresh: true).then((_) => setState(() => _fetched = true));
         }
       });
     }
@@ -285,7 +312,7 @@ class _MyHomePageState extends State<MyHomePage> {
                         backgroundColor: theme.colorScheme.secondaryContainer,
                       ),
                     );
-                    refresh().then((_) {
+                    refresh(fullRefresh: false).then((_) {
                       setState(() {});
                     });
                   },