Improve init. Close #236

lib/utils/data.dart

@@ -103,7 +103,7 @@ Future<void> importAppData(File file, [bool checkVersion = false]) async {
           await file.copy(targetFile);
         }
       }
-      await ComicSource.reload();
+      await ComicSourceManager().reload();
     }
   } finally {
     cacheDir.deleteIgnoreError(recursive: true);

lib/utils/data_sync.dart

@@ -19,7 +19,7 @@ class DataSync with ChangeNotifier {
       downloadData();
     }
     LocalFavoritesManager().addListener(onDataChanged);
-    ComicSource.addListener(onDataChanged);
+    ComicSourceManager().addListener(onDataChanged);
   }
 
   void onDataChanged() {

lib/utils/init.dart

@@ -0,0 +1,40 @@
+import 'dart:async';
+
+import 'package:flutter/foundation.dart';
+
+/// A mixin class that provides a way to ensure the class is initialized.
+abstract mixin class Init {
+  bool _isInit = false;
+
+  final _initCompleter = <Completer<void>>[];
+
+  /// Ensure the class is initialized.
+  Future<void> ensureInit() async {
+    if (_isInit) {
+      return;
+    }
+    var completer = Completer<void>();
+    _initCompleter.add(completer);
+    return completer.future;
+  }
+
+  Future<void> _markInit() async {
+    _isInit = true;
+    for (var completer in _initCompleter) {
+      completer.complete();
+    }
+    _initCompleter.clear();
+  }
+
+  @protected
+  Future<void> doInit();
+
+  /// Initialize the class.
+  Future<void> init() async {
+    if (_isInit) {
+      return;
+    }
+    await doInit();
+    await _markInit();
+  }
+}