Make Assets know their ID.

pkg/barback/lib/src/asset_set.dart

Index: pkg/barback/lib/src/asset_set.dart
diff --git a/pkg/barback/lib/src/asset_set.dart b/pkg/barback/lib/src/asset_set.dart
new file mode 100644
index 0000000000000000000000000000000000000000..cbbf6c2788a2d271567ee8ac08d9a3c1b31e0b26
--- /dev/null
+++ b/pkg/barback/lib/src/asset_set.dart
@@ -0,0 +1,69 @@
+// 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_set;
+
+import 'dart:async';
+import 'dart:collection';
+import 'dart:io';
+
+import 'asset.dart';
+import 'asset_id.dart';
+
+/// A [Set] of [Asset]s with distinct IDs.
+///
+/// This uses the [AssetId] of each asset to determine uniqueness, so no two
+/// assets with the same ID can be in the set.
+class AssetSet extends IterableBase<Asset> implements Set<Asset> {
+  final _assets = new Map<AssetId, Asset>();
+
+  Iterator<Asset> get iterator => _assets.values.iterator;
+
+  int get length => _assets.length;
+
+  /// Gets the [Asset] in the set with [id], or returns `null` if no asset with
+  /// that ID is present.
+  Asset operator[](AssetId id) => _assets[id];
+
+  /// Adds [asset] to the set.
+  ///
+  /// If there is already an asset with that ID in the set, it is replaced by
+  /// the new one. Returns [asset].
+  Asset add(Asset asset) {
+    _assets[asset.id] = asset;
+    return asset;
+  }
+
+  /// Adds [assets] to the set.
+  void addAll(Iterable<Asset> assets) {
+    assets.forEach(add);
+  }
+
+  /// Returns `true` if the set contains [asset].
+  bool contains(Asset asset) {
+    var other = _assets[asset.id];
+    return other == asset;
+  }

[nweiz, 2013/07/03 22:58:33]

If [contains] has a different notion of equality than the rest of the class,
this shouldn't implement Set.

[Bob Nystrom, 2013/07/08 17:54:01]

It does have the same notion of equality. It looks up the existing asset by ID
for performance but it should otherwise behave like a vanilla Set here.

Where the behavior does differ is add(). With an AssetSet, adding a different
Asset with the same ID will replace the previous one with that ID. A normal Set
will keep them both.

But, either way, implementing Set<T> here wasn't buying us anything yet, so I
removed it.

+
+  /// Returns `true` if the set contains an [Asset] with [id].
+  bool containsId(AssetId id) {
+    return _assets.containsKey(id);
+  }
+
+  /// Removes all assets from the set.
+  void clear() {
+    _assets.clear();
+  }
+
+  // TODO(rnystrom): Implement and test these as needed.

[nweiz, 2013/07/03 22:58:33]

This may be another reason not to implement Set.

[Bob Nystrom, 2013/07/08 17:54:01]

Done.

+  bool containsAll(Iterable<Asset> other) => throw new UnimplementedError();
+  AssetSet difference(Set<Asset> other) => throw new UnimplementedError();
+  AssetSet intersection(Set<Asset> other) => throw new UnimplementedError();
+  bool remove(Object value) => throw new UnimplementedError();
+  void removeAll(Iterable elements) => throw new UnimplementedError();
+  void removeWhere(bool test(Asset element)) => throw new UnimplementedError();
+  void retainAll(Iterable elements) => throw new UnimplementedError();
+  void retainWhere(bool test(Asset element)) => throw new UnimplementedError();
+  AssetSet union(Set<Asset> other) => throw new UnimplementedError();
+}