Write logs to file.

lib/foundation/app.dart

@@ -47,6 +47,7 @@ class _App {
 
   late String dataPath;
   late String cachePath;
+  String? externalStoragePath;
 
   final rootNavigatorKey = GlobalKey<NavigatorState>();
 
@@ -77,6 +78,9 @@ class _App {
   Future<void> init() async {
     cachePath = (await getApplicationCacheDirectory()).path;
     dataPath = (await getApplicationSupportDirectory()).path;
+    if (isAndroid) {
+      externalStoragePath = (await getExternalStorageDirectory())!.path;
+    }
   }
 
   Future<void> initComponents() async {

lib/foundation/log.dart

@@ -1,7 +1,9 @@
 import 'dart:io';
 
 import 'package:flutter/foundation.dart';
+import 'package:venera/foundation/app.dart';
 import 'package:venera/utils/ext.dart';
+import 'package:venera/utils/io.dart';
 
 class LogItem {
   final LogLevel level;
@@ -28,9 +30,6 @@ class Log {
 
   static bool ignoreLimitation = false;
 
-  /// only for debug
-  static const String? logFile = null;
-
   static void printWarning(String text) {
     debugPrint('\x1B[33m$text\x1B[0m');
   }
@@ -39,7 +38,32 @@ class Log {
     debugPrint('\x1B[31m$text\x1B[0m');
   }
 
+  static IOSink? _file;
+  static bool _isFlushing = false;
+
+  static void _tryFlush() async {
+    if (_file != null) {
+      while (_isFlushing) {
+        await Future.delayed(const Duration(milliseconds: 20));
+      }
+      _isFlushing = true;
+      await _file!.flush();
+      _isFlushing = false;
+    }
+  }
+
   static void addLog(LogLevel level, String title, String content) {
+    if (_file == null) {
+      Directory dir;
+      if (App.isAndroid) {
+        dir = Directory(App.externalStoragePath!);
+      } else {
+        dir = Directory(App.dataPath);
+      }
+      var file = dir.joinFile("logs.txt");
+      _file = file.openWrite();
+    }
+
     if (!ignoreLimitation && content.length > maxLogLength) {
       content = "${content.substring(0, maxLogLength)}...";
     }
@@ -62,8 +86,9 @@ class Log {
     }
 
     _logs.add(newLog);
-    if(logFile != null) {
-      File(logFile!).writeAsString(newLog.toString(), mode: FileMode.append);
+    if(_file != null) {
+      _file!.write(newLog.toString());
+      _tryFlush();
     }
     if (_logs.length > maxLogNumber) {
       var res = _logs.remove(