Make Assets know their ID.

pkg/barback/lib/src/asset_graph.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.asset_graph;
 
 import 'dart:async';
 import 'dart:collection';
 
 import 'asset.dart';
@@ skipping 172 matching lines @@
     // changes are processed in a single batch even when the first one starts
     // the build process.
     return new Future(() {
       if (_sourceChanges == null) return null;
 
       // Take the current batch to ensure it doesn't get added to while we're
       // processing it.
       var changes = _sourceChanges;
       _sourceChanges = null;
 
-      var updated = new Map<AssetId, Asset>();
+      var updated = new Set<Asset>();

[nweiz, 2013/07/03 20:08:25]

Does Set<Asset> work? Asset doesn't define [hash] or [operator==]. It would also
be weird to define those as forwarding to [AssetId], because then different
assets with the same id would register as equal.

It may be worth making a custom AssetSet class that wraps a Map<AssetId, Asset>.
That's a pattern that comes up a lot in the code, and it would be nice to have a
class that explicitly expresses "this is a set that's expected to contain only
one asset of a given id" without making the equality semantics of Asset weird.

[Bob Nystrom, 2013/07/03 22:32:11]

Object implements hashCode and == (based on identity), so all objects can be put
in a set. Asset could generate a more appropriate/faster hashCode based on the
ID, but it should work regardless.
If you implement ==, you must implement hashCode, but the converse isn't
necessarily true. It's OK to have your own optimized hashCode but still fallback
on identity for equality.
Great idea. Done.

       var futures = [];
       for (var id in changes.updated) {
         // TODO(rnystrom): Catch all errors from provider and route to results.
         futures.add(_provider.getAsset(id).then((asset) {
-          updated[id] = asset;
+          updated.add(asset);
         }).catchError((error) {
           if (error is AssetNotFoundException) {
             // Handle missing asset errors like regular missing assets.
             reportError(error);
           } else {
             // It's an unexpected error, so rethrow it.
             throw error;
           }
         }));
       }
 
       return Future.wait(futures).then((_) {
         _phases.first.updateInputs(updated, changes.removed);
       });
     });
   }
 }
 
 /// Used to report build results back from the asynchronous build process
 /// running in the background.
 class BuildResult {
   /// The error that occurred, or `null` if the result is not an error.
   final error;
 
   /// `true` if this result is for a successful build.
   bool get succeeded => error == null;
 
   BuildResult([this.error]);
 }