Factor a PhaseOutput class out of Phase.

pkg/barback/lib/src/phase_output.dart

Index: pkg/barback/lib/src/phase_output.dart
diff --git a/pkg/barback/lib/src/phase_output.dart b/pkg/barback/lib/src/phase_output.dart
new file mode 100644
index 0000000000000000000000000000000000000000..ae309d9820eba63990cd6e77601aabc457d5a267
--- /dev/null
+++ b/pkg/barback/lib/src/phase_output.dart
@@ -0,0 +1,117 @@
+// 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.phase_output;
+
+import 'dart:async';
+import 'dart:collection';
+
+import 'asset_cascade.dart';
+import 'asset_id.dart';
+import 'asset_node.dart';
+import 'errors.dart';
+import 'phase_input.dart';
+import 'stream_pool.dart';
+import 'transformer.dart';
+import 'utils.dart';
+
+/// A class that handles a single output of a phase.
+///
+/// Normally there's only a single [AssetNode] for a phase's output, but it's
+/// possible that multiple transformers in the same phase emit assets with the
+/// same id, causing collisions. This handles those collisions by forwarding the
+/// chronologically first asset.
+class PhaseOutput {
+  /// The phase for which this is an output.
+  final Phase _phase;
+
+  /// The asset node for this output.
+  AssetNode get output => _outputController.node;
+  final AssetNodeController _outputController;
+
+  /// The assets for this output.
+  ///
+  /// If there's no collision, this will only have one element. Otherwise, it
+  /// will be ordered by which asset was added first.
+  final _assets = new Queue<AssetNode>();
+
+  /// The [AssetCollisionException] for this output, or null if there is no
+  /// collision currently.
+  AssetCollisionException get collisionException {
+    if (_assets.length == 1) return null;
+    return new AssetCollisionException(
+        _assets.where((asset) => asset.transform != null)
+            .map((asset) => asset.transform.info),
+        output.id);
+  }
+
+  PhaseOutput(this._phase, AssetNode output)
+      : _outputController = new AssetNodeController.from(output) {
+    assert(!output.state.isRemoved);
+    add(output);
+  }
+
+  /// Adds an asset node as an output with this id.
+  void add(AssetNode node) {
+    assert(node.id == output.id);
+    _assets.add(node);
+    _watchAsset(node);
+  }
+
+  /// Watches [node] for state changes and adjusts [_assets] and [output]
+  /// appropriately when they occur.
+  void _watchAsset(AssetNode node) {
+    node.onStateChange.listen((state) {
+      if (state.isRemoved) {
+        _removeAsset(node);
+        return;
+      }
+      if (_assets.first != node) return;
+
+      if (state.isAvailable) {
+        _outputController.setAvailable(node.asset);
+      } else {
+        assert(state.isDirty);
+        _outputController.setDirty();
+      }
+    });
+  }
+
+  /// Removes [node] as an output.
+  void _removeAsset(AssetNode node) {
+    if (_assets.length == 1) {
+      assert(_assets.single == node);
+      _outputController.setRemoved();
+      return;
+    }
+
+    // If there was more than one asset, we're resolving a collision --
+    // possibly partially.
+    var wasFirst = _assets.first == node;
+    _assets.remove(node);
+
+    // If this was the first asset, we replace it with the next asset
+    // (chronologically).
+    if (wasFirst) {
+      var newOutput = _assets.first;
+      _outputController.setTransform(newOutput.transform);
+      if (newOutput.state.isAvailable) {
+        if (output.state.isAvailable) _outputController.setDirty();
+        _outputController.setAvailable(newOutput.asset);
+      } else {
+        assert(newOutput.isDirty);
+        if (!output.isDirty) _outputController.setDirty();
+      }
+    }
+
+    // If there's still a collision, report it. This lets the user know
+    // if they've successfully resolved the collision or not.
+    if (_assets.length > 1) {
+      // Pump the event queue to ensure that the removal of the input triggers
+      // a new build to which we can attach the error.
+      // TODO(nweiz): report this through the output asset.
+      newFuture(() => _phase.cascade.reportError(collisionException));
+    }
+  }
+}