Add support for transformer clusters to barback.

pkg/barback/lib/src/group_runner.dart

Index: pkg/barback/lib/src/group_runner.dart
diff --git a/pkg/barback/lib/src/group_runner.dart b/pkg/barback/lib/src/group_runner.dart
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+++ b/pkg/barback/lib/src/group_runner.dart
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+// 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.group_runner;
+
+import 'dart:async';
+
+import 'asset_cascade.dart';
+import 'asset_node.dart';
+import 'phase.dart';
+import 'stream_pool.dart';
+import 'transformer_group.dart';
+
+/// A class that process all of the phases in a single transformer group.
+///
+/// A group takes many inputs, processes them, and emits many outputs.
+class GroupRunner {
+  /// The phases defined by this group.
+  final _phases = new List<Phase>();
+
+  /// A stream that emits an event whenever this group becomes dirty and needs
+  /// to be run.
+  ///
+  /// This may emit events when the group was already dirty or while processing
+  /// transforms. Events are emitted synchronously to ensure that the dirty
+  /// state is thoroughly propagated as soon as any assets are changed.
+  Stream get onDirty => _onDirtyPool.stream;
+  final _onDirtyPool = new StreamPool.broadcast();
+
+  /// Whether this group is dirty and needs to be run.
+  bool get isDirty => _phases.any((phase) => phase.isDirty);
+
+  // TODO(nweiz): move to a more push-based way of propagating outputs and get
+  // rid of this. Once that's done, see if we can unify GroupRunner and
+  // AssetCascade.
+  /// The set of outputs that has been returned by [process].
+  ///
+  /// [process] is expected to only return new outputs, so this is used to
+  /// ensure that it does so.
+  final _alreadyEmittedOutputs = new Set<AssetNode>();
+
+  GroupRunner(AssetCascade cascade, TransformerGroup group) {
+    var lastPhase = new Phase(cascade, group.phases.first);
+    _phases.add(lastPhase);
+    for (var phase in group.phases.skip(1)) {
+      lastPhase = lastPhase.addPhase(phase);
+      _phases.add(lastPhase);
+    }
+
+    for (var phase in _phases) {
+      _onDirtyPool.add(phase.onDirty);
+    }
+  }
+
+  /// Adds a new asset as an input for this group.
+  void addInput(AssetNode node) {
+    _phases.first.addInput(node);
+  }
+
+  /// Removes this group and all sub-phases within it.
+  void remove() {
+    _phases.first.remove();
+  }
+
+  /// Processes this group.
+  ///
+  /// Returns a future that completes with any new outputs produced by the
+  /// group.
+  Future<Set<AssetNode>> process() {
+    // Process the first phase that needs to do work.
+    for (var phase in _phases) {
+      var future = phase.process();
+      if (future != null) return future.then((_) => process());
+    }
+
+    // If we get here, all phases are done processing.
+    var newOutputs = _phases.last.availableOutputs
+        .difference(_alreadyEmittedOutputs);
+    for (var output in newOutputs) {
+      output.whenRemoved.then((_) => _alreadyEmittedOutputs.remove(output));
+    }
+    _alreadyEmittedOutputs.addAll(newOutputs);
+
+    return new Future.value(newOutputs);
+  }
+}