Serialize barback assets more efficiently.

pkg/barback/lib/src/internal_asset.dart

Index: pkg/barback/lib/src/internal_asset.dart
diff --git a/pkg/barback/lib/src/asset.dart b/pkg/barback/lib/src/internal_asset.dart
similarity index 56%
copy from pkg/barback/lib/src/asset.dart
copy to pkg/barback/lib/src/internal_asset.dart
index fda813380dc1965b6b901db7e043f531256eed41..85ce4dce6eb994ef217f8defc04f9a7f6ebe9c10 100644
--- a/pkg/barback/lib/src/asset.dart
+++ b/pkg/barback/lib/src/internal_asset.dart
@@ -2,64 +2,79 @@
 // 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.
 
-library barback.asset;
+library barback.internal_asset;
 
 import 'dart:async';
 import 'dart:convert';
 import 'dart:io';
 
+import 'asset.dart';
 import 'asset_id.dart';
 import 'file_pool.dart';
+import 'serialize.dart';
 import 'stream_replayer.dart';
 import 'utils.dart';
 
-/// A blob of content.
-///
-/// Assets may come from the file system, or as the output of a [Transformer].
-/// They are identified by [AssetId].
-abstract class Asset {
-  /// The ID for this asset.
-  final AssetId id;
-
-  Asset(this.id);
-
-  factory Asset.fromBytes(AssetId id, List<int> bytes) =>
-      new _BinaryAsset(id, bytes);
-
-  factory Asset.fromFile(AssetId id, File file) =>
-      new _FileAsset(id, file);
-
-  factory Asset.fromString(AssetId id, String content) =>
-      new _StringAsset(id, content);
-
-  factory Asset.fromPath(AssetId id, String path) =>
-      new _FileAsset(id, new File(path));
-
-  /// Creates an asset from a stream.
-  ///
-  /// This immediately starts draining [stream].
-  factory Asset.fromStream(AssetId id, Stream<List<int>> stream) =>
-      new _StreamAsset(id, stream);
-
-  /// Returns the contents of the asset as a string.
-  ///
-  /// If the asset was created from a [String] the original string is always
-  /// returned and [encoding] is ignored. Otherwise, the binary data of the
-  /// asset is decoded using [encoding], which defaults to [UTF8].
-  Future<String> readAsString({Encoding encoding});
+/// Serialize an asset to a form that's safe to send across isolates.
+Map serializeAsset(Asset asset) {
+  var id = serializeId(asset.id);
+  if (asset is BinaryAsset) {
+    // If [_contents] is a custom subclass of List, it won't be serializable, so
+    // we have to convert it to a built-in [List] first.
+    // TODO(nweiz): Send a typed array if that works after issue 14703 is fixed.
+    var contents = asset._contents.runtimeType == List ?
+        asset._contents : asset._contents.toList();
+
+    return {
+      'type': 'binary',
+      'id': id,
+      'contents': contents
+    };
+  } else if (asset is FileAsset) {
+    return {
+      'type': 'file',
+      'id': id,
+      'path': asset._path
+    };
+  } else if (asset is StringAsset) {
+    return {
+      'type': 'string',
+      'id': id,
+      'contents': asset._contents
+    };
+  } else {
+    // [asset] is probably a [StreamAsset], but it's possible that the user has
+    // created a custom subclass, in which case we just serialize the stream
+    // anyway.
+    return {
+      'type': 'stream',
+      'id': id,
+      'stream': serializeStream(asset.read())
+    };
+  }
+}
 
-  /// Streams the binary contents of the asset.
-  ///
-  /// If the asset was created from a [String], this returns its UTF-8 encoding.
-  Stream<List<int>> read();
+/// Deserialize an asset from the form returned by [serialize].
+Asset deserializeAsset(Map asset) {
+  var id = deserializeId(asset['id']);
+  switch (asset['type']) {
+    case 'binary': return new BinaryAsset(id, asset['contents']);
+    case 'file': return new FileAsset(id, asset['path']);
+    case 'string': return new StringAsset(id, asset['contents']);
+    case 'stream':
+      return new StreamAsset(id, deserializeStream(asset['stream']));
+    default:
+      throw new FormatException('Unknown asset type "${asset['type']}".');
+  }
 }
 
 /// An asset whose data is stored in a list of bytes.
-class _BinaryAsset extends Asset {
+class BinaryAsset implements Asset {
+  final AssetId id;
+
   final List<int> _contents;
 
-  _BinaryAsset(AssetId id, this._contents)
-      : super(id);
+  BinaryAsset(this.id, this._contents);
 
   Future<String> readAsString({Encoding encoding}) {
     if (encoding == null) encoding = UTF8;
@@ -99,31 +114,33 @@ class _BinaryAsset extends Asset {
 }
 
 /// An asset backed by a file on the local file system.
-class _FileAsset extends Asset {
+class FileAsset implements Asset {
+  final AssetId id;
+
   /// Use a [FilePool] to handle reads so we can try to cope with running out
   /// of file descriptors more gracefully.
   static final _pool = new FilePool();
 
-  final File _file;
-  _FileAsset(AssetId id, this._file)
-      : super(id);
+  final String _path;
+  FileAsset(this.id, this._path);
 
   Future<String> readAsString({Encoding encoding}) {
     if (encoding == null) encoding = UTF8;
-    return _pool.readAsString(_file, encoding);
+    return _pool.readAsString(_path, encoding);
   }
 
-  Stream<List<int>> read() => _pool.openRead(_file);
+  Stream<List<int>> read() => _pool.openRead(_path);
 
-  String toString() => 'File "${_file.path}"';
+  String toString() => 'File "${_path}"';
 }
 
 /// An asset whose data is stored in a string.
-class _StringAsset extends Asset {
+class StringAsset implements Asset {
+  final AssetId id;
+
   final String _contents;
 
-  _StringAsset(AssetId id, this._contents)
-      : super(id);
+  StringAsset(this.id, this._contents);
 
   Future<String> readAsString({Encoding encoding}) =>
       new Future.value(_contents);
@@ -153,14 +170,15 @@ class _StringAsset extends Asset {
 }
 
 /// An asset whose data is available from a stream.
-class _StreamAsset extends Asset {
+class StreamAsset implements Asset {
+  final AssetId id;
+
   /// A stream replayer that records and replays the contents of the input
   /// stream.
   final StreamReplayer<List<int>> _replayer;
 
-  _StreamAsset(AssetId id, Stream<List<int>> stream)
-      : _replayer = new StreamReplayer(stream),
-        super(id);
+  StreamAsset(this.id, Stream<List<int>> stream)
+      : _replayer = new StreamReplayer(stream);
 
   Future<String> readAsString({Encoding encoding}) {
     if (encoding == null) encoding = UTF8;