Add 'start' and 'end' optional arguments to File.openRead(). This makes it possible to stream a sub…

sdk/lib/io/file_impl.dart

 // Copyright (c) 2013, 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.
 
 part of dart.io;
 
 // Read the file in blocks of size 64k.
 const int _BLOCK_SIZE = 64 * 1024;
 
 
 class _FileStream extends Stream<List<int>> {
   // Stream controller.
   StreamController<List<int>> _controller;
 
   // Information about the underlying file.
   String _path;
   RandomAccessFile _openedFile;
   int _position;
+  int _end;
 
   // Has the stream been paused or unsubscribed?
   bool _paused = false;
   bool _unsubscribed = false;
 
   // Is there a read currently in progress?
   bool _readInProgress = false;
 
   // Block read but not yet send because stream is paused.
   List<int> _currentBlock;
 
-  _FileStream(String this._path) : _position = 0 {
+  _FileStream(String this._path, this._position, this._end) {
     _setupController();
   }
 
   _FileStream.forStdin() : _position = 0 {
     _setupController();
   }
 
   StreamSubscription<List<int>> listen(void onData(List<int> event),
                                        {void onError(error),
                                         void onDone(),
@@ skipping 23 matching lines @@
       return closeFuture;
     } else {
       return new Future.value();
     }
   }
 
   void _readBlock() {
     // Don't start a new read if one is already in progress.
     if (_readInProgress) return;
     _readInProgress = true;
-    _openedFile.read(_BLOCK_SIZE)
+    int readBytes = _BLOCK_SIZE;
+    if (_end != null) {
+      readBytes = min(readBytes, _end - _position);
+      if (readBytes < 0) {
+        throw new RangeError("Bad end position: $_end");
+      }
+    }
+    _openedFile.read(readBytes)
       .then((block) {
         _readInProgress = false;
         if (block.length == 0) {
           if (!_unsubscribed) {
             _closeFile().then((_) { _controller.close(); });
             _unsubscribed = true;
           }
           return;
         }
         _position += block.length;
@@ skipping 16 matching lines @@
   void _start() {
     Future<RandomAccessFile> openFuture;
     if (_path != null) {
       openFuture = new File(_path).open(mode: FileMode.READ);
     } else {
       openFuture = new Future.value(_File._openStdioSync(0));
     }
     openFuture
       .then((RandomAccessFile opened) {
         _openedFile = opened;
-        _readBlock();
+        if (_position == null) {
+          _position = 0;
+        }
+        if (_position > 0) {
+          return opened.setPosition(_position);
+        } else if (_position < 0) {
+          throw new RangeError("Bad start position: $_position");
+        }
       })
+      .then((_) => _readBlock())
       .catchError((e) {
         _controller.addError(e);
         _controller.close();
       });
   }
 
   void _resume() {
     _paused = false;
     if (_currentBlock != null) {
       _controller.add(_currentBlock);
@@ skipping 300 matching lines @@
   }
 
   external static _fullPath(String path);
 
   String fullPathSync() {
     var result = _fullPath(_path);
     throwIfError(result, "Cannot retrieve full path for file '$_path'");
     return result;
   }
 
-  Stream<List<int>> openRead() {
-    return new _FileStream(_path);
+  Stream<List<int>> openRead([int start, int end]) {
+    return new _FileStream(_path, start, end);
   }
 
   IOSink openWrite({FileMode mode: FileMode.WRITE,
                     Encoding encoding: Encoding.UTF_8}) {
     if (mode != FileMode.WRITE &&
         mode != FileMode.APPEND) {
       throw new FileIOException(
           "Wrong FileMode. Use FileMode.WRITE or FileMode.APPEND");
     }
     var consumer = new _FileStreamConsumer(this, mode);
@@ skipping 566 matching lines @@
           new FileIOException("File closed '$_path'"));
     });
     return completer.future;
   }
 
   final String _path;
   int _id;
 
   SendPort _fileService;
 }