rewrite file api

lib/platform/path.dart

@@ -31,38 +31,57 @@ class Path {
     return _currentExe;
   }
 
-  /// 保存文件
   Future<void> saveFile(
       String filenameWithoutExtension, String mimeType, Uint8List bytes,
-      {String dir = ""}) async {
+      {String dir = "", bool saveAs = true}) async {
     if (kIsWeb) {
       return saveFileWeb(bytes, mimeType, filenameWithoutExtension);
     }
-    return _safChannel.invokeMethod(
-        "saveFile", [filenameWithoutExtension, dir, mimeType, bytes]);
+    final f = await openFile(filenameWithoutExtension, mimeType,
+        dir: dir, saveAs: saveAs);
+    try {
+      await f.write(bytes);
+    } finally {
+      await f.dispose();
+    }
   }
 
   Future<SAFFile> openFile(String filenameWithoutExtension, String mimeType,
-      {String dir = ""}) async {
-    final fd = await _safChannel.invokeMethod<int>(
-        "openFile", [filenameWithoutExtension, dir, mimeType]);
-    return SAFFile(fd!);
+      {String dir = "",
+      bool read = false,
+      bool write = true,
+      bool append = false,
+      bool saveAs = true}) async {
+    if (!write) {
+      append = false;
+    }
+    final fd = await _safChannel.invokeMethod<int>("openFile",
+        [filenameWithoutExtension, dir, mimeType, read, write, append, saveAs]);
+    return SAFFile(fd!, read, write);
   }
 }
 
 class SAFFile {
-  SAFFile(this._fd);
+  SAFFile(this._fd, this._read, this._write);
 
   final int _fd;
+  final bool _read;
+  final bool _write;
   bool _disposed = false;
 
+  Future<Uint8List> read(int maxLen) async {
+    if (_disposed) throw Exception("File already closed");
+    if (!_read) throw Exception("File not opened for read");
+    return await Path._safChannel.invokeMethod("readFile", [_fd, maxLen]);
+  }
+
   /// 写入文件，返回此次写入的字节数
   ///
   /// [data] 要写入文件的数据
   Future<int> write(Uint8List data) async {
     if (_disposed) throw Exception("File already closed");
-    return await Path._safChannel
-        .invokeMethod("writeFile", [_fd, data]);
+    if (!_write) throw Exception("File not opened for write");
+    return await Path._safChannel.invokeMethod("writeFile", [_fd, data]);
   }
 
   Future<void> dispose() async {