Transition barback's infrastructure to an aggregate-based model.

pkg/barback/lib/src/errors.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.
 
 library barback.errors;
 
 import 'package:stack_trace/stack_trace.dart';
 
 import 'asset/asset_id.dart';
-import 'transformer/transformer.dart';
+import 'transformer/wrapping_aggregate_transformer.dart';
 import 'utils.dart';
 
 /// Error thrown when an asset with [id] cannot be found.
 class AssetNotFoundException implements Exception {
   final AssetId id;
 
   AssetNotFoundException(this.id);
 
   String toString() => "Could not find asset $id.";
 }
@@ skipping 139 matching lines @@
 }
 
 /// Information about a single transform in the barback graph.
 ///
 /// Identifies a single transformation in the barback graph.
 ///
 /// A transformation is uniquely identified by the ID of its primary input, and
 /// the transformer that is applied to it.
 class TransformInfo {
   /// The transformer that's run for this transform.
-  final Transformer transformer;
+  ///
+  /// This may be a [Transformer] or a [WrappingAggregateTransformer]. It may
+  /// also return additional types in the future.

[Bob Nystrom, 2014/05/08 20:30:48]

I know we talked about this a bit before but I don't remember the details.

Is this class just used to associate logs with a specific transformer? If so, I
think it would be good to move to a dumber object here. I worry including the
actual object here is future-API-evolution hostile.

[nweiz, 2014/05/08 21:12:36]

I think it's useful for users to be able to reliably check whether a log entry
came from their transformer, and the best way to do that is to allow them to
write "myTransformer == logEntry.transform.transformer".

[Bob Nystrom, 2014/05/08 23:56:13]

Maybe, but I think we could support that with myTransformer.id ==
logEntry.transformer.id just as well.

But do what you like here.

[nweiz, 2014/05/19 21:11:09]

I'm going to use the pain of backwards incompatibility as a tiebreaker and keep
it as-is.

+  final transformer;
 
   /// The id of this transform's primary asset.
   final AssetId primaryId;

[Bob Nystrom, 2014/05/08 20:30:48]

Mark this deprecated and add a String "key" getter?

[nweiz, 2014/05/08 21:12:36]

In the vast majority of cases this is going to be an actual primary id. I think
it would net add confusion to the API to have separate "package" and "key"
getters that are almost always going to just be the components of the primary id
anyway.

 
-  TransformInfo(this.transformer, this.primaryId);
+  TransformInfo(transformer, this.primaryId)
+      : transformer = transformer is WrappingAggregateTransformer ?
+            transformer.transformer : transformer;
 
   bool operator==(other) =>
       other is TransformInfo &&
       other.transformer == transformer &&
       other.primaryId == primaryId;
 
   int get hashCode => transformer.hashCode ^ primaryId.hashCode;
 
   String toString() => "$transformer on $primaryId";
 }