Start sketching out a buildAll() method.

pkg/barback/lib/src/package_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.package_graph;
 
 import 'dart:async';
 
 import 'asset_cascade.dart';
 import 'asset_id.dart';
 import 'asset_node.dart';
+import 'asset_set.dart';
 import 'build_result.dart';
 import 'errors.dart';
 import 'package_provider.dart';
+import 'transformer.dart';
 import 'utils.dart';
 
 /// The collection of [AssetCascade]s for an entire application.
 ///
 /// This tracks each package's [AssetCascade] and routes asset requests between
 /// them.
 class PackageGraph {
   /// The provider that exposes asset and package information.
   final PackageProvider provider;
 
@@ skipping 20 matching lines @@
   /// A stream that emits any errors from the graph or the transformers.
   ///
   /// This emits errors as they're detected. If an error occurs in one part of
   /// the graph, unrelated parts will continue building.
   ///
   /// This will not emit programming errors from barback itself. Those will be
   /// emitted through the [results] stream's error channel.
   Stream<BarbackException> get errors => _errors;
   Stream<BarbackException> _errors;
 
+  /// The most recent error emitted from a cascade's result stream.
+  ///
+  /// This is used to pipe an unexpected error from a build to the resulting
+  /// [Future] returned by [getAllAssets].
+  var _lastUnexpectedError;
+
   /// Creates a new [PackageGraph] that will transform assets in all packages
   /// made available by [provider].
   PackageGraph(this.provider) {
     for (var package in provider.packages) {
       var cascade = new AssetCascade(this, package);
       // The initial result for each cascade is "success" since the cascade
       // doesn't start building until some source in that graph is updated.
       _cascadeResults[package] = new BuildResult.success();
       _cascades[package] = cascade;
 
       cascade.results.listen((result) {
         _cascadeResults[cascade.package] = result;
         // If any cascade hasn't yet finished, the overall build isn't finished
         // either.
         if (_cascadeResults.values.any((result) => result == null)) return;
 
         // Include all build errors for all cascades. If no cascades have
         // errors, the result will automatically be considered a success.
         _resultsController.add(new BuildResult(unionAll(
             _cascadeResults.values.map((result) => result.errors))));
-      }, onError: _resultsController.addError);
+      }, onError: (error) {
+        _lastUnexpectedError = error;
+        _resultsController.addError(error);
+      });
     }
 
     _errors = mergeStreams(_cascades.values.map((cascade) => cascade.errors));
   }
 
   /// Gets the asset node identified by [id].
   ///
   /// If [id] is for a generated or transformed asset, this will wait until it
   /// has been created and return it. This means that the returned asset will
   /// always be [AssetState.AVAILABLE].
   ///
   /// If the asset cannot be found, returns null.
   Future<AssetNode> getAssetNode(AssetId id) {
     var cascade = _cascades[id.package];
     if (cascade != null) return cascade.getAssetNode(id);
     return new Future.value(null);
   }
 
+  /// Gets all output assets.
+  ///
+  /// If a build is currently in progress, waits until it completes. The
+  /// returned future will complete with an error if the build is not
+  /// successful.
+  Future<AssetSet> getAllAssets() {
+    if (_cascadeResults.values.contains(null)) {
+      // A build is still ongoing, so wait for it to complete and try again.
+      return results.first.then((_) => getAllAssets());
+    }
+
+    // If an unexpected error occurred, complete with that.
+    if (_lastUnexpectedError != null) {
+      var error = _lastUnexpectedError;
+      _lastUnexpectedError = null;
+      return new Future.error(error);
+    }
+
+    // If the build completed with an error, complete the future with it.
+    var errors = unionAll(
+        _cascadeResults.values.map((result) => result.errors));
+    if (errors.isNotEmpty) {
+      return new Future.error(BarbackException.aggregate(errors));
+    }
+
+    // Otherwise, return all of the final output assets.
+    var assets = unionAll(_cascades.values.map(
+        (cascade) => cascade.availableOutputs.toSet()));
+
+    return new Future.value(new AssetSet.from(assets));
+  }
+
   /// Adds [sources] to the graph's known set of source assets.
   ///
   /// Begins applying any transforms that can consume any of the sources. If a
   /// given source is already known, it is considered modified and all
   /// transforms that use it will be re-applied.
   void updateSources(Iterable<AssetId> sources) {
     groupBy(sources, (id) => id.package).forEach((package, ids) {
       var cascade = _cascades[package];
       if (cascade == null) throw new ArgumentError("Unknown package $package.");
       _cascadeResults[package] = null;
@@ skipping 10 matching lines @@
       cascade.removeSources(ids);
     });
   }
 
   void updateTransformers(String package,
       Iterable<Iterable<Transformer>> transformers) {
     _cascadeResults[package] = null;
     _cascades[package].updateTransformers(transformers);
   }
 }