Improve updating following and fix potential crash.

lib/foundation/comic_source/parser.dart

@@ -759,6 +759,7 @@ class ComicSourceParser {
   LoadComicFunc? _parseLoadComicFunc() {
     return (id) async {
       try {
+        print("Loading comic $id from source $_key. Stacktrace:\n${StackTrace.current}");
         var res = await JsEngine().runCode("""
           ComicSource.sources.$_key.comic.loadInfo(${jsonEncode(id)})
         """);

lib/foundation/follow_updates.dart

@@ -2,6 +2,7 @@ import 'dart:async';
 import 'dart:convert';
 import 'package:venera/foundation/favorites.dart';
 import 'package:venera/foundation/log.dart';
+import 'package:venera/utils/channel.dart';
 
 class ComicUpdateResult {
   final bool updated;
@@ -62,6 +63,7 @@ Future<ComicUpdateResult> updateComic(
       return ComicUpdateResult(updated, null);
     } catch (e, s) {
       Log.error("Check Updates", e, s);
+      await Future.delayed(const Duration(seconds: 2));
       retries--;
       if (retries == 0) {
         return ComicUpdateResult(false, e.toString());
@@ -114,9 +116,36 @@ void updateFolderBase(
   current = 0;
   stream.add(UpdateProgress(total, current, errors, updated));
 
-  var futures = <Future>[];
+  var channel = Channel<FavoriteItemWithUpdateInfo>(10);
+
+  // Producer
+  () async {
+    var c = 0;
     for (var comic in comicsToUpdate) {
-    var future = updateComic(comic, folder).then((result) {
+      await channel.push(comic);
+      c++;
+      // Throttle
+      if (c % 5 == 0) {
+        var delay = c % 100 + 1;
+        if (delay > 10) {
+          delay = 10;
+        }
+        await Future.delayed(Duration(seconds: delay));
+      }
+    }
+    channel.close();
+  }();
+
+  // Consumers
+  var updateFutures = <Future>[];
+  for (var i = 0; i < 5; i++) {
+    var f = () async {
+      while (true) {
+        var comic = await channel.pop();
+        if (comic == null) {
+          break;
+        }
+        var result = await updateComic(comic, folder);
         current++;
         if (result.updated) {
           updated++;
@@ -124,13 +153,13 @@ void updateFolderBase(
         if (result.errorMessage != null) {
           errors++;
         }
-      stream.add(
-          UpdateProgress(total, current, errors, updated, comic, result.errorMessage));
-    });
-    futures.add(future);
+        stream.add(UpdateProgress(total, current, errors, updated, comic, result.errorMessage));
+      }
+    }();
+    updateFutures.add(f);
   }
 
-  await Future.wait(futures);
+  await Future.wait(updateFutures);
 
   if (updated > 0) {
     LocalFavoritesManager().notifyChanges();

lib/utils/channel.dart

@@ -0,0 +1,58 @@
+import 'dart:async';
+import 'dart:collection';
+
+class Channel<T> {
+  final Queue<T> _queue;
+
+  final int size;
+
+  Channel(this.size) : _queue = Queue<T>();
+
+  Completer? _releaseCompleter;
+
+  Completer? _pushCompleter;
+
+  var currentSize = 0;
+
+  var isClosed = false;
+
+  Future<void> push(T item) async {
+    if (currentSize >= size) {
+      _releaseCompleter ??= Completer();
+      return _releaseCompleter!.future.then((_) {
+        if (isClosed) {
+          return;
+        }
+        _queue.addLast(item);
+        currentSize++;
+      });
+    }
+    _queue.addLast(item);
+    currentSize++;
+    _pushCompleter?.complete();
+    _pushCompleter = null;
+  }
+
+  Future<T?> pop() async {
+    while (_queue.isEmpty) {
+      if (isClosed) {
+        return null;
+      }
+      _pushCompleter ??= Completer();
+      await _pushCompleter!.future;
+    }
+    var item = _queue.removeFirst();
+    currentSize--;
+    if (_releaseCompleter != null && currentSize < size) {
+      _releaseCompleter!.complete();
+      _releaseCompleter = null;
+    }
+    return item;
+  }
+
+  void close() {
+    isClosed = true;
+    _pushCompleter?.complete();
+    _releaseCompleter?.complete();
+  }
+}

test/channel_test.dart

@@ -0,0 +1,115 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:venera/utils/channel.dart';
+
+void main() {
+  test("1-1-1", () async {
+    var channel = Channel<int>(1);
+    await channel.push(1);
+    var item = await channel.pop();
+    expect(item, 1);
+  });
+
+  test("1-3-1", () async {
+    var channel = Channel<int>(1);
+
+    // producer
+    () async {
+      await channel.push(1);
+    }();
+    () async {
+      await channel.push(2);
+    }();
+    () async {
+      await channel.push(3);
+    }();
+
+    // consumer
+    var results = <int>[];
+    for (var i = 0; i < 3; i++) {
+      var item = await channel.pop();
+      if (item != null) {
+        results.add(item);
+      }
+    }
+    expect(results.length, 3);
+  });
+
+  test("2-3-1", () async {
+    var channel = Channel<int>(2);
+
+    // producer
+    () async {
+      await channel.push(1);
+    }();
+    () async {
+      await channel.push(2);
+    }();
+    () async {
+      await channel.push(3);
+    }();
+
+    // consumer
+    var results = <int>[];
+    for (var i = 0; i < 3; i++) {
+      var item = await channel.pop();
+      if (item != null) {
+        results.add(item);
+      }
+    }
+    expect(results.length, 3);
+  });
+
+  test("1-1-3", () async {
+    var channel = Channel<int>(1);
+
+    // producer
+    () async {
+      print("push 1");
+      await channel.push(1);
+      print("push 2");
+      await channel.push(2);
+      print("push 3");
+      await channel.push(3);
+      print("push done");
+      channel.close();
+    }();
+
+    // consumer
+    var consumers = <Future>[];
+    var results = <int>[];
+    for (var i = 0; i < 3; i++) {
+      consumers.add(() async {
+        while (true) {
+          var item = await channel.pop();
+          if (item == null) {
+            break;
+          }
+          print("pop $item");
+          results.add(item);
+        }
+      }());
+    }
+
+    await Future.wait(consumers);
+    expect(results.length, 3);
+  });
+
+  test("close", () async {
+    var channel = Channel<int>(2);
+
+    // producer
+    () async {
+      await channel.push(1);
+      await channel.push(2);
+      await channel.push(3);
+      channel.close();
+    }();
+
+    // consumer
+    await channel.pop();
+    await channel.pop();
+    await channel.pop();
+    var item4 = await channel.pop();
+    expect(item4, null);
+  });
+}