add download threads setting

assets/translation.json

@@ -155,7 +155,8 @@
     "Start": "开始",
     "Export App Data": "导出应用数据",
     "Import App Data": "导入应用数据",
-    "Export": "导出"
+    "Export": "导出",
+    "Download Threads": "下载线程数"
   },
   "zh_TW": {
     "Home": "首頁",
@@ -313,6 +314,7 @@
     "Start": "開始",
     "Export App Data": "匯出應用數據",
     "Import App Data": "匯入應用數據",
-    "Export": "匯出"
+    "Export": "匯出",
+    "Download Threads": "下載線程數"
   }
 }

lib/foundation/appdata.dart

@@ -110,6 +110,7 @@ class _Settings with ChangeNotifier {
     'enablePageAnimation': true,
     'language': 'system', // system, zh-CN, zh-TW, en-US
     'cacheSize': 2048, // in MB
+    'downloadThreads': 5,
   };
 
   operator [](String key) {

lib/network/download.dart

@@ -1,6 +1,7 @@
 import 'dart:async';
 
 import 'package:flutter/widgets.dart' show ChangeNotifier;
+import 'package:venera/foundation/appdata.dart';
 import 'package:venera/foundation/comic_source/comic_source.dart';
 import 'package:venera/foundation/comic_type.dart';
 import 'package:venera/foundation/local.dart';
@@ -155,7 +156,7 @@ class ImagesDownloadTask extends DownloadTask with _TransferSpeedMixin {
 
   var tasks = <int, _ImageDownloadWrapper>{};
 
-  int get _maxConcurrentTasks => 5;
+  int get _maxConcurrentTasks => (appdata.settings["downloadThreads"] as num).toInt();
 
   void _scheduleTasks() {
     var images = _images![_images!.keys.elementAt(_chapter)]!;

lib/pages/settings/network.dart

@@ -17,6 +17,13 @@ class _NetworkSettingsState extends State<NetworkSettings> {
           title: "Proxy".tl,
           builder: () => const _ProxySettingView(),
         ).toSliver(),
+        _SliderSetting(
+          title: "Download Threads".tl,
+          settingsIndex: 'downloadThreads',
+          interval: 1,
+          min: 1,
+          max: 16,
+        ).toSliver(),
       ],
     );
   }