Change dart:io to use Future for one-shot operations.

runtime/bin/file_impl.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 class _FileInputStream extends _BaseDataInputStream implements InputStream {
   _FileInputStream(String name) {
     var file = new File(name);
     _data = [];
     _position = 0;
-    file.onError = _reportError;
-    file.open(FileMode.READ, (openedFile) {
-      _readDataFromFile(openedFile);
+    var chained = file.open(FileMode.READ).chain((openedFile) {
+      return _readDataFromFile(openedFile);
+    });
+    chained.handleException((e) {
+      _reportError(e);
+      return true;
     });
   }
 
   _FileInputStream.fromStdio(int fd) {
     assert(fd == 0);
     var file = _File._openStdioSync(fd);
     _data = [];
     _position = 0;
-    _readDataFromFile(file);
+    _readDataFromFile(file).handleException((e) {
+      _reportError(e);
+      return true;
+    });
   }
 
-  void _readDataFromFile(RandomAccessFile openedFile) {
-    openedFile.onError = _reportError;
-    openedFile.length((length) {
+  Future<RandomAccessFile> _closeAfterRead(RandomAccessFile openedFile) {
+    return openedFile.close().transform((ignore) {
+      _streamMarkedClosed = true;
+      _checkScheduleCallbacks();
+      return openedFile;
+    });
+  }
+
+  Future<RandomAccessFile> _readDataFromFile(RandomAccessFile openedFile) {
+    return openedFile.length().chain((length) {
       var contents = new Uint8List(length);
       if (length != 0) {
-        openedFile.readList(contents, 0, length, (read) {
+        return openedFile.readList(contents, 0, length).chain((read) {
           if (read != length) {
-            _reportError(new FileIOException(
-                'Failed reading file contents in FileInputStream'));
+            throw new FileIOException(
+                'Failed reading file contents in FileInputStream');
           } else {
             _data = contents;
           }
-          openedFile.close(() {
-            _streamMarkedClosed = true;
-            _checkScheduleCallbacks();
-          });
+          return _closeAfterRead(openedFile);
         });
       } else {
-        openedFile.close(() {
-          _streamMarkedClosed = true;
-          _checkScheduleCallbacks();
-        });
+        return _closeAfterRead(openedFile);
       }
     });
   }
 
   int available() {
     return _closed ? 0 : _data.length - _position;
   }
 
   void pipe(OutputStream output, [bool close = true]) {
     _pipe(this, output, close: close);
@@ skipping 22 matching lines @@
   List<int> _data;
   int _position;
   bool _closed = false;
 }
 
 
 class _FileOutputStream extends _BaseOutputStream implements OutputStream {
   _FileOutputStream(String name, FileMode mode) {
     _pendingOperations = new List<List<int>>();
     var f = new File(name);
-    f.open(mode, (openedFile) {
+    var openFuture = f.open(mode);
+    openFuture.then((openedFile) {
       _file = openedFile;
-      _setupFileHandlers();
       _processPendingOperations();
     });
-    f.onError = _reportError;
+    openFuture.handleException((e) {
+      _reportError(e);
+      return true;
+    });
   }
 
   _FileOutputStream.fromStdio(int fd) {
     assert(1 <= fd && fd <= 2);
     _file = _File._openStdioSync(fd);
-    _setupFileHandlers();
-  }
-
-
-  void _setupFileHandlers() {
-    _file.onError = _reportError;
-    _file.onNoPendingWrites = () {
-      if (!_streamMarkedClosed && _onNoPendingWrites != null) {
-        _onNoPendingWrites();
-      }
-    };
   }
 
   bool write(List<int> buffer, [bool copyBuffer = false]) {
     var data = buffer;
     if (copyBuffer) {
       var length = buffer.length;
       data = new Uint8List(length);
       data.setRange(0, length, buffer, 0);
     }
     if (_file == null) {
@@ skipping 13 matching lines @@
     }
     var copy = new Uint8List(length);
     copy.setRange(0, length, buffer, offset);
     return write(copy);
   }
 
   void close() {
     if (_file == null) {
       _pendingOperations.add(null);
     } else if (!_streamMarkedClosed) {
-      _file.close(() {
+      _file.close().then((ignore) {
         if (_onClosed != null) _onClosed();
       });
       _streamMarkedClosed = true;
     }
   }
 
   void set onNoPendingWrites(void callback()) {
     _onNoPendingWrites = callback;
+    if (((_pendingOperations == null) || (_pendingOperations.length == 0)) &&
+        (outstandingWrites == 0) &&
+        !_streamMarkedClosed &&
+        (_onNoPendingWrites != null)) {
+      new Timer(0, (t) {
+        if (_onNoPendingWrites != null) {
+          _onNoPendingWrites();
+        }
+      });
+    }
   }
 
   void set onClosed(void callback()) {
     _onClosed = callback;
   }
 
   void _processPendingOperations() {
     _pendingOperations.forEach((buffer) {
       (buffer != null) ? write(buffer) : close();
     });
     _pendingOperations = null;
   }
 
   void _write(List<int> buffer, int offset, int len) {
-    _file.writeList(buffer, offset, len);
+    outstandingWrites++;
+    var writeListFuture = _file.writeList(buffer, offset, len);
+    writeListFuture.then((ignore) {
+        outstandingWrites--;
+        if ((outstandingWrites == 0) &&
+            !_streamMarkedClosed &&
+            (_onNoPendingWrites != null)) {
+          _onNoPendingWrites();
+        }
+    });
+    writeListFuture.handleException((e) {
+      outstandingWrites--;
+      _reportError(e);
+      return true;
+    });
   }
 
   RandomAccessFile _file;
 
   // When this is set to true the stream is marked closed. When a
   // stream is marked closed no more data can be written.
   bool _streamMarkedClosed = false;
 
   // When this is set to true the close callback has been called and
   // the stream is fully closed.
   bool _closeCallbackCalled = false;
 
+  // Number of writes that have not yet completed.
+  int outstandingWrites = 0;
+
   // List of pending writes that were issued before the underlying
   // file was successfully opened.
   List<List<int>> _pendingOperations;
 
   Function _onNoPendingWrites;
   Function _onClosed;
 }
 
 
 // Helper class containing static file helper methods.
@@ skipping 172 matching lines @@
 
   static int checkedWriteString(int id, String string) {
     if (string is !String) return -1;
     return writeString(id, string);
   }
 
 }
 
 // Base class for _File and _RandomAccessFile with shared functions.
 class _FileBase {
-  void _reportError(e) {
-    if (_onError != null) {
-      _onError(e);
-    } else {
-      throw e;
-    }
-  }
-
   bool _isErrorResponse(response) {
     return response is List && response[0] != _FileUtils.kSuccessResponse;
   }
 
-  void _handleErrorResponse(response, String message) {
+  Exception _exceptionFromResponse(response, String message) {
     assert(_isErrorResponse(response));
     switch (response[_FileUtils.kErrorResponseErrorType]) {
       case _FileUtils.kIllegalArgumentResponse:
-        _reportError(new IllegalArgumentException());
-        break;
+        return new IllegalArgumentException();
       case _FileUtils.kOSErrorResponse:
         var err = new OSError(response[_FileUtils.kOSErrorResponseMessage],
                               response[_FileUtils.kOSErrorResponseErrorCode]);
-        _reportError(new FileIOException(message, err));
-        break;
+        return new FileIOException(message, err);
       case _FileUtils.kFileClosedResponse:
-        _reportError(new FileIOException("File closed"));
-        break;
+        return new FileIOException("File closed");
       default:
-        _reportError(new Exception("Unknown error"));
-        break;
+        return new Exception("Unknown error");
     }
   }
-
-  void set onError(void handler(e)) {
-    _onError = handler;
-  }
-
-  Function _onError;
 }
 
 // Class for encapsulating the native implementation of files.
 class _File extends _FileBase implements File {
   // Constructor for file.
   _File(String this._name);
 
-  void exists(void callback(bool exists)) {
+  Future<bool> exists() {
     _ensureFileService();
+    Completer<bool> completer = new Completer<bool>();
     List request = new List(2);
     request[0] = _FileUtils.kExistsRequest;
     request[1] = _name;
     _fileService.call(request).then((response) {
       if (_isErrorResponse(response)) {
-        _handleErrorResponse(response, "Cannot open file '$_name'");
+        var e = _exceptionFromResponse(response, "Cannot open file '$_name'");
+        completer.completeException(e);
       } else {
-        callback(response);
+        completer.complete(response);
       }
     });
+    return completer.future;
   }
 
   bool existsSync() {
     return _FileUtils.checkedExists(_name);
   }
 
-  void create(void callback()) {
+  Future<File> create() {
     _ensureFileService();
+    Completer<File> completer = new Completer<File>();
     List request = new List(2);
     request[0] = _FileUtils.kCreateRequest;
     request[1] = _name;
     _fileService.call(request).then((response) {
       if (_isErrorResponse(response)) {
-        _handleErrorResponse(response, "Cannot create file");
+        var e = _exceptionFromResponse(response, "Cannot create file '$_name'");
+        completer.completeException(e);
       } else {
-        callback();
+        completer.complete(this);
       }
     });
+    return completer.future;
   }
 
   void createSync() {
     bool created = _FileUtils.checkedCreate(_name);
     if (!created) {
       throw new FileIOException("Cannot create file '$_name'");
     }
   }
 
-  void delete(void callback()) {
+  Future<File> delete() {
     _ensureFileService();
+    Completer<File> completer = new Completer<File>();
     List request = new List(2);
     request[0] = _FileUtils.kDeleteRequest;
     request[1] = _name;
     _fileService.call(request).then((response) {
       if (_isErrorResponse(response)) {
-        _handleErrorResponse(response, "Cannot delete file '$_name'");
+        var e = _exceptionFromResponse(response, "Cannot delete file '$_name'");
+        completer.completeException(e);
       } else {
-        callback();
+        completer.complete(this);
       }
     });
+    return completer.future;
   }
 
   void deleteSync() {
     _FileUtils.checkedDelete(_name);
   }
 
-  void directory(void callback(Directory dir)) {
+  Future<Directory> directory() {
     _ensureFileService();
+    Completer<Directory> completer = new Completer<Directory>();
     List request = new List(2);
     request[0] = _FileUtils.kDirectoryRequest;
     request[1] = _name;
     _fileService.call(request).then((response) {
       if (_isErrorResponse(response)) {
-        _handleErrorResponse(response, "Cannot retrieve directory for file");
+        var e = _exceptionFromResponse(response,
+                                       "Cannot retrieve directory for "
+                                       "file '$_name'");
+        completer.completeException(e);
       } else {
-        callback(new Directory(response));
+        completer.complete(new Directory(response));
       }
     });
+    return completer.future;
   }
 
   Directory directorySync() {
     _FileUtils.checkedDirectory(_name);
     return new Directory(_FileUtils.directory(_name));
   }
 
-  void open(FileMode mode, void callback(RandomAccessFile file)) {
+  Future<RandomAccessFile> open([FileMode mode = FileMode.READ]) {
     _ensureFileService();
+    Completer<RandomAccessFile> completer = new Completer<RandomAccessFile>();
     if (mode != FileMode.READ &&
         mode != FileMode.WRITE &&
         mode != FileMode.APPEND) {
-      _reportError(new IllegalArgumentException());
-      return;
+      new Timer(0, (t) {
+        completer.completeException(new IllegalArgumentException());
+      });
+      return completer.future;
     }
     List request = new List(3);
     request[0] = _FileUtils.kOpenRequest;
     request[1] = _name;
     request[2] = mode._mode;  // Direct int value for serialization.
     _fileService.call(request).then((response) {
       if (_isErrorResponse(response)) {
-        _handleErrorResponse(response, "Cannot open file '$_name'");
+        var e = _exceptionFromResponse(response, "Cannot open file '$_name'");
+        completer.completeException(e);
       } else {
-        callback(new _RandomAccessFile(response, _name));
+        completer.complete(new _RandomAccessFile(response, _name));
       }
     });
+    return completer.future;
   }
 
-  void length(void callback(int length)) {
+  Future<int> length() {
     _ensureFileService();
+    Completer<int> completer = new Completer<int>();
     List request = new List(2);
     request[0] = _FileUtils.kLengthFromNameRequest;
     request[1] = _name;
     _fileService.call(request).then((response) {
       if (_isErrorResponse(response)) {
-        _handleErrorResponse(response,
-                             "Cannot retrieve length of file '$_name'");
+        var e = _exceptionFromResponse(response,
+                                       "Cannot retrieve length of "
+                                       "file '$_name'");
+        completer.completeException(e);
       } else {
-        callback(response);
+        completer.complete(response);
       }
     });
+    return completer.future;
   }
 
   int lengthSync() {
     var result = _FileUtils.checkedLengthFromName(_name);
     if (result is OSError) {
       throw new FileIOException("Cannot retrieve length of file '$_name'",
                                 result);
     }
     return result;
   }
@@ skipping 11 matching lines @@
   }
 
   static RandomAccessFile _openStdioSync(int fd) {
     var id = _FileUtils.openStdio(fd);
     if (id == 0) {
       throw new FileIOException("Cannot open stdio file for: $fd");
     }
     return new _RandomAccessFile(id, "");
   }
 
-  void fullPath(void callback(String result)) {
+  Future<String> fullPath() {
     _ensureFileService();
+    Completer<String> completer = new Completer<String>();
     List request = new List(2);
     request[0] = _FileUtils.kFullPathRequest;
     request[1] = _name;
     _fileService.call(request).then((response) {
       if (_isErrorResponse(response)) {
-        _handleErrorResponse(response, "Cannot retrieve full path");
+        var e = _exceptionFromResponse(response,
+                                       "Cannot retrieve full path"
+                                       " for '$_name'");
+        completer.completeException(e);
       } else {
-        callback(response);
+        completer.complete(response);
       }
     });
+    return completer.future;
   }
 
   String fullPathSync() {
     return _FileUtils.checkedFullPath(_name);
   }
 
   InputStream openInputStream() {
     return new _FileInputStream(_name);
   }
 
   OutputStream openOutputStream([FileMode mode = FileMode.WRITE]) {
     if (mode != FileMode.WRITE &&
         mode != FileMode.APPEND) {
       throw new FileIOException(
           "Wrong FileMode. Use FileMode.WRITE or FileMode.APPEND");
     }
     return new _FileOutputStream(_name, mode);
   }
 
-  void readAsBytes(void callback(List<int> bytes)) {
+  Future<List<int>> readAsBytes() {
     _ensureFileService();
+    Completer<List<int>> completer = new Completer<List<int>>();
     var chunks = new _BufferList();
     var stream = openInputStream();
     stream.onClosed = () {
-      callback(chunks.readBytes(chunks.length));
+      completer.complete(chunks.readBytes(chunks.length));
     };
     stream.onData = () {
       var chunk = stream.read();
       chunks.add(chunk);
     };
-    stream.onError = (e) => _reportError(e);
+    stream.onError = completer.completeException;
+    return completer.future;
   }
 
   List<int> readAsBytesSync() {
     var opened = openSync();
     var length = opened.lengthSync();
     var result = new Uint8List(length);
     var read = opened.readListSync(result, 0, length);
     if (read != length) {
       throw new FileIOException("Failed to read file");
     }
     opened.closeSync();
     return result;
   }
 
-  void readAsText(Encoding encoding, void callback(String text)) {
+  Future<String> readAsText([Encoding encoding = Encoding.UTF_8]) {
     _ensureFileService();
     var decoder = _StringDecoders.decoder(encoding);
-    readAsBytes((bytes) {
-      try {
-        decoder.write(bytes);
-      } catch (var e) {
-        _reportError(e);
-        return;
-      }
-      callback(decoder.decoded);
+    return readAsBytes().transform((bytes) {
+      decoder.write(bytes);
+      return decoder.decoded;
     });
   }
 
   String readAsTextSync([Encoding encoding = Encoding.UTF_8]) {
     var decoder = _StringDecoders.decoder(encoding);
     List<int> bytes = readAsBytesSync();
     decoder.write(bytes);
     return decoder.decoded;
   }
 
   List<String> _getDecodedLines(_StringDecoder decoder) {
     List<String> result = [];
     var line = decoder.decodedLine;
     while (line != null) {
       result.add(line);
       line = decoder.decodedLine;
     }
     // If there is more data with no terminating line break we treat
     // it as the last line.
     var data = decoder.decoded;
     if (data != null) {
       result.add(data);
     }
     return result;
   }
 
-  void readAsLines(Encoding encoding, void callback(List<String> lines)) {
+  Future<List<String>> readAsLines([Encoding encoding = Encoding.UTF_8]) {
     _ensureFileService();
+    Completer<List<String>> completer = new Completer<List<String>>();
     var decoder = _StringDecoders.decoder(encoding);
-    readAsBytes((bytes) {
-      try {
-        decoder.write(bytes);
-      } catch (var e) {
-        _reportError(e);
-        return;
-      }
-      callback(_getDecodedLines(decoder));
+    return readAsBytes().transform((bytes) {
+      decoder.write(bytes);
+      return _getDecodedLines(decoder);
     });
   }
 
   List<String> readAsLinesSync([Encoding encoding = Encoding.UTF_8]) {
     var decoder = _StringDecoders.decoder(encoding);
     List<int> bytes = readAsBytesSync();
     decoder.write(bytes);
     return _getDecodedLines(decoder);
   }
 
   String get name() => _name;
 
   void _ensureFileService() {
     if (_fileService == null) {
       _fileService = _FileUtils.newServicePort();
     }
   }
 
-  String _name;
+  final String _name;
 
   SendPort _fileService;
 }
 
 
 class _RandomAccessFile extends _FileBase implements RandomAccessFile {
   _RandomAccessFile(int this._id, String this._name);
 
-  void close(void callback()) {
+  Future<RandomAccessFile> close() {
+    Completer<RandomAccessFile> completer = new Completer<RandomAccessFile>();
     if (_id == 0) {
-      _reportError(new FileIOException("Cannot close file '$_name'"));
-      return;
+      // Complete asynchronously so the user has a chance to setup
+      // handlers without getting exceptions when registering the
+      // then handler.
+      new Timer(0, (t) {
+        completer.completeException(
+            new FileIOException("Cannot close file '$_name'"));
+      });
+      return completer.future;
     }
     _ensureFileService();
     List request = new List(2);
     request[0] = _FileUtils.kCloseRequest;
     request[1] = _id;
     // Set the id_ to 0 (NULL) to ensure the no more async requests
     // can be issues for this file.
     _id = 0;
     _fileService.call(request).then((result) {
       if (result != -1) {
         _id = result;
-        callback();
+        completer.complete(this);
       } else {
-        _reportError(new FileIOException("Cannot close file '$_name'"));
+        completer.completeException(
+            new FileIOException("Cannot close file '$_name'"));
       }
     });
+    return completer.future;
   }
 
   void closeSync() {
     var id = _FileUtils.close(_id);
     if (id == -1) {
       throw new FileIOException("Cannot close file '$_name'");
     }
     _id = id;
   }
 
-  void readByte(void callback(int byte)) {
+  Future<int> readByte() {
     _ensureFileService();
+    Completer<int> completer = new Completer<int>();
     List request = new List(2);
     request[0] = _FileUtils.kReadByteRequest;
     request[1] = _id;
     _fileService.call(request).then((response) {
       if (_isErrorResponse(response)) {
-        _handleErrorResponse(response, "readByte failed for file '$_name'");
+        var e = _exceptionFromResponse(response,
+                                       "readByte failed for file '$_name'");
+        completer.completeException(e);
       } else {
-        callback(response);
+        completer.complete(response);
       }
     });
+    return completer.future;
   }
 
   int readByteSync() {
     _checkNotClosed();
     var result = _FileUtils.readByte(_id);
     if (result is OSError) {
       throw new FileIOException("readByte failed for file '$_name'", result);
     }
     return result;
   }
 
-  void readList(List<int> buffer, int offset, int bytes,
-                void callback(int read)) {
+  Future<int> readList(List<int> buffer, int offset, int bytes) {
     _ensureFileService();
+    Completer<int> completer = new Completer<int>();
     if (buffer is !List || offset is !int || bytes is !int) {
-      _reportError(new FileIOException(
-          "Invalid arguments to readList for file '$_name'"));
-      return;
+      // Complete asynchronously so the user has a chance to setup
+      // handlers without getting exceptions when registering the
+      // then handler.
+      new Timer(0, (t) {
+        completer.completeException(new FileIOException(
+            "Invalid arguments to readList for file '$_name'"));
+      });
+      return completer.future;
     };
     List request = new List(3);
     request[0] = _FileUtils.kReadListRequest;
     request[1] = _id;
     request[2] = bytes;
     _fileService.call(request).then((response) {
       if (_isErrorResponse(response)) {
-        _handleErrorResponse(response, "readList failed for file '$_name'");
+        var e = _exceptionFromResponse(response,
+                                       "readList failed for file '$_name'");
+        completer.completeException(e);
       } else {
         var read = response[1];
         var data = response[2];
         buffer.setRange(offset, read, data);
-        callback(read);
+        completer.complete(read);
       }
     });
+    return completer.future;
   }
 
   int readListSync(List<int> buffer, int offset, int bytes) {
     _checkNotClosed();
     if (buffer is !List || offset is !int || bytes is !int) {
       throw new FileIOException(
           "Invalid arguments to readList for file '$_name'");
     }
     if (bytes == 0) return 0;
     int index =
         _FileUtils.checkReadWriteListArguments(buffer.length, offset, bytes);
     if (index != 0) {
       throw new IndexOutOfRangeException(index);
     }
     var result = _FileUtils.readList(_id, buffer, offset, bytes);
     if (result is OSError) {
       throw new FileIOException("readList failed for file '$_name'",
                                 result);
     }
     return result;
   }
 
-  void writeByte(int value) {
+  Future<RandomAccessFile> writeByte(int value) {
     _ensureFileService();
+    Completer<RandomAccessFile> completer = new Completer<RandomAccessFile>();
     if (value is !int) {
-      _reportError(new FileIOException(
-          "Invalid argument to writeByte for file '$_name'"));
-      return;
+      // Complete asynchronously so the user has a chance to setup
+      // handlers without getting exceptions when registering the
+      // then handler.
+      new Timer(0, (t) {
+          completer.completeException(new FileIOException(
+              "Invalid argument to writeByte for file '$_name'"));
+      });
+      return completer.future;
     }
     List request = new List(3);
     request[0] = _FileUtils.kWriteByteRequest;
     request[1] = _id;
     request[2] = value;
-    _writeEnqueued();
     _fileService.call(request).then((response) {
-      _writeCompleted();
       if (_isErrorResponse(response)) {
-        _handleErrorResponse(response, "writeByte failed for file '$_name'");
+        var e = _exceptionFromResponse(response,
+                                       "writeByte failed for file '$_name'");
+        completer.completeException(e);
+      } else {
+        completer.complete(this);
       }
     });
+    return completer.future;
   }
 
   int writeByteSync(int value) {
     _checkNotClosed();
     if (value is !int) {
       throw new FileIOException(
           "Invalid argument to writeByte for file '$_name'");
     }
     var result = _FileUtils.writeByte(_id, value);
     if (result is OSError) {
       throw new FileIOException("writeByte failed for file '$_name'",
                                 result);
     }
     return result;
   }
 
-  void writeList(List<int> buffer, int offset, int bytes) {
+  Future<RandomAccessFile> writeList(List<int> buffer, int offset, int bytes) {
     _ensureFileService();
+    Completer<RandomAccessFile> completer = new Completer<RandomAccessFile>();
     if (buffer is !List || offset is !int || bytes is !int) {
-      _reportError(new FileIOException(
+      // Complete asynchronously so the user has a chance to setup
+      // handlers without getting exceptions when registering the
+      // then handler.
+      new Timer(0, (t) {
+          completer.completeException(new FileIOException(
           "Invalid arguments to writeList for file '$_name'"));
-      return;
+      });
+      return completer.future;
     }
 
     List result =
         _FileUtils.ensureFastAndSerializableBuffer(buffer, offset, bytes);
     List outBuffer = result[0];
     int outOffset = result[1];
 
     List request = new List(5);
     request[0] = _FileUtils.kWriteListRequest;
     request[1] = _id;
     request[2] = outBuffer;
     request[3] = outOffset;
     request[4] = bytes;
-    _writeEnqueued();
     _fileService.call(request).then((response) {
-      _writeCompleted();
       if (_isErrorResponse(response)) {
-        _handleErrorResponse(response, "writeList failed for file '$_name'");
+        var e = _exceptionFromResponse(response,
+                                       "writeList failed for file '$_name'");
+        completer.completeException(e);
+      } else {
+        completer.complete(this);
       }
     });
+    return completer.future;
   }
 
   int writeListSync(List<int> buffer, int offset, int bytes) {
     _checkNotClosed();
     if (buffer is !List || offset is !int || bytes is !int) {
       throw new FileIOException(
           "Invalid arguments to writeList for file '$_name'");
     }
     if (bytes == 0) return 0;
     int index =
         _FileUtils.checkReadWriteListArguments(buffer.length, offset, bytes);
     if (index != 0) {
       throw new IndexOutOfRangeException(index);
     }
     var result = _FileUtils.writeList(_id, buffer, offset, bytes);
     if (result is OSError) {
       throw new FileIOException("writeList failed for file '$_name'", result);
     }
     return result;
   }
 
-  void writeString(String string, [Encoding encoding = Encoding.UTF_8]) {
+  Future<RandomAccessFile> writeString(String string,
+                                       [Encoding encoding = Encoding.UTF_8]) {
     _ensureFileService();
+    Completer<RandomAccessFile> completer = new Completer<RandomAccessFile>();
     List request = new List(3);
     request[0] = _FileUtils.kWriteStringRequest;
     request[1] = _id;
     request[2] = string;
-    _writeEnqueued();
     _fileService.call(request).then((response) {
-      _writeCompleted();
       if (_isErrorResponse(response)) {
-        _handleErrorResponse(response, "writeString failed for file '$_name'");
+        var e = _exceptionFromResponse(response,
+                                       "writeString failed for file '$_name'");
+        completer.completeException(e);
+      } else {
+        completer.complete(this);
       }
     });
+    return completer.future;
   }
 
   int writeStringSync(String string, [Encoding encoding = Encoding.UTF_8]) {
     _checkNotClosed();
     var result = _FileUtils.checkedWriteString(_id, string);
     if (result is OSError) {
       throw new FileIOException("writeString failed for file '$_name'", result);
     }
     return result;
   }
 
-  void position(void callback(int position)) {
+  Future<int> position() {
     _ensureFileService();
+    Completer<int> completer = new Completer<int>();
     List request = new List(2);
     request[0] = _FileUtils.kPositionRequest;
     request[1] = _id;
     _fileService.call(request).then((response) {
       if (_isErrorResponse(response)) {
-        _handleErrorResponse(response, "position failed for file '$_name'");
+        var e = _exceptionFromResponse(response,
+                                       "position failed for file '$_name'");
+        completer.completeException(e);
       } else {
-        callback(response);
+        completer.complete(response);
       }
     });
+    return completer.future;
   }
 
   int positionSync() {
     _checkNotClosed();
     var result = _FileUtils.position(_id);
     if (result is OSError) {
       throw new FileIOException("position failed for file '$_name'", result);
     }
     return result;
   }
 
-  void setPosition(int position, void callback()) {
+  Future<RandomAccessFile> setPosition(int position) {
     _ensureFileService();
+    Completer<RandomAccessFile> completer = new Completer<RandomAccessFile>();
     List request = new List(3);
     request[0] = _FileUtils.kSetPositionRequest;
     request[1] = _id;
     request[2] = position;
     _fileService.call(request).then((response) {
       if (_isErrorResponse(response)) {
-        _handleErrorResponse(response, "setPosition failed for file '$_name'");
+        var e = _exceptionFromResponse(response,
+                                       "setPosition failed for file '$_name'");
+        completer.completeException(e);
       } else {
-        callback();
+        completer.complete(this);
       }
     });
+    return completer.future;
   }
 
   void setPositionSync(int position) {
     _checkNotClosed();
     var result = _FileUtils.setPosition(_id, position);
     if (result is OSError) {
       throw new FileIOException("setPosition failed for file '$_name'", result);
     }
   }
 
-  void truncate(int length, void callback()) {
+  Future<RandomAccessFile> truncate(int length) {
     _ensureFileService();
+    Completer<RandomAccessFile> completer = new Completer<RandomAccessFile>();
     List request = new List(3);
     request[0] = _FileUtils.kTruncateRequest;
     request[1] = _id;
     request[2] = length;
     _fileService.call(request).then((response) {
       if (_isErrorResponse(response)) {
-        _handleErrorResponse(response, "truncate failed for file '$_name'");
+        var e = _exceptionFromResponse(response,
+                                       "truncate failed for file '$_name'");
+        completer.completeException(e);
       } else {
-        callback();
+        completer.complete(this);
       }
     });
+    return completer.future;
   }
 
   void truncateSync(int length) {
     _checkNotClosed();
     var result = _FileUtils.truncate(_id, length);
     if (result is OSError) {
       throw new FileIOException("truncate failed for file '$_name'", result);
     }
   }
 
-  void length(void callback(int length)) {
+  Future<int> length() {
     _ensureFileService();
+    Completer<int> completer = new Completer<int>();
     List request = new List(2);
     request[0] = _FileUtils.kLengthRequest;
     request[1] = _id;
     _fileService.call(request).then((response) {
       if (_isErrorResponse(response)) {
-        _handleErrorResponse(response, "length failed for file '$_name'");
+        var e = _exceptionFromResponse(response,
+                                       "length failed for file '$_name'");
+        completer.completeException(e);
       } else {
-        callback(response);
+        completer.complete(response);
       }
     });
+    return completer.future;
   }
 
   int lengthSync() {
     _checkNotClosed();
     var result = _FileUtils.length(_id);
     if (result is OSError) {
       throw new FileIOException("length failed for file '$_name'", result);
     }
     return result;
   }
 
-  void flush(void callback()) {
+  Future<RandomAccessFile> flush() {
     _ensureFileService();
+    Completer<RandomAccessFile> completer = new Completer<RandomAccessFile>();
     List request = new List(2);
     request[0] = _FileUtils.kFlushRequest;
     request[1] = _id;
     _fileService.call(request).then((response) {
       if (_isErrorResponse(response)) {
-        _handleErrorResponse(response, "flush failed for file '$_name'");
+        var e = _exceptionFromResponse(response,
+                                       "flush failed for file '$_name'");
+        completer.completeException(e);
       } else {
-        callback();
+        completer.complete(this);
       }
     });
+    return completer.future;
   }
 
   void flushSync() {
     _checkNotClosed();
     var result = _FileUtils.flush(_id);
     if (result is OSError) {
       throw new FileIOException("flush failed for file '$_name'", result);
     }
   }
 
   String get name() => _name;
 
-  void set onNoPendingWrites(void handler()) {
-    _onNoPendingWrites = handler;
-    if (_pendingWrites == 0) {
-      _noPendingWriteTimer = new Timer(0, (t) {
-        if (_onNoPendingWrites != null) _onNoPendingWrites();
-      });
-    }
-  }
-
   void _ensureFileService() {
     if (_fileService == null) {
       _fileService = _FileUtils.newServicePort();
     }
   }
 
-  void _writeEnqueued() {
-    _pendingWrites++;
-    if (_noPendingWriteTimer != null) {
-      _noPendingWriteTimer.cancel();
-      _noPendingWriteTimer = null;
-    }
-  }
-
-  void  _writeCompleted() {
-    _pendingWrites--;
-    if (_pendingWrites == 0 && _onNoPendingWrites != null) {
-      _onNoPendingWrites();
-    }
-  }
-
   void _checkNotClosed() {
     if (_id == 0) {
       throw new FileIOException("File closed '$_name'");
     }
   }
 
-  String _name;
+  final String _name;
   int _id;
-  int _pendingWrites = 0;
 
   SendPort _fileService;
-
-  Timer _noPendingWriteTimer;
-
-  Function _onNoPendingWrites;
 }