Factor out an input-handling class from Phase in barback.

pkg/barback/lib/src/asset_forwarder.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_forwarder;
+
+import 'dart:async';
+
+import 'asset_node.dart';
+
+/// A wrapper for an [AssetNode] that forwards events to a new node.
+///
+/// This allows the output node to be marked as removed without the input node
+/// having been removed.
+class AssetForwarder {
+  /// The subscription on the input node.
+  StreamSubscription _subscription;
+
+  /// The controller for the output node.
+  final AssetNodeController _controller;
+
+  /// The node to which events are forwarded.
+  AssetNode get node => _controller.node;
+
+  AssetForwarder(AssetNode node)
+      : _controller = new AssetNodeController(node.id, node.transform) {
+    _subscription = node.onStateChange.listen((state) {
+      if (state.isAvailable) {
+        _controller.setAvailable(node.asset);
+      } else if (state.isDirty) {
+        _controller.setDirty();
+      } else {
+        assert(state.isRemoved);
+        close();
+      }
+    });
+
+    if (node.state.isAvailable) {
+      _controller.setAvailable(node.asset);
+    } else if (node.state.isRemoved) {
+      close();
+    }
+  }
+
+  /// Closes the forwarder and marks [node] as removed.
+  void close() {
+    if (_controller.node.state.isRemoved) return;
+    _subscription.cancel();
+    _controller.setRemoved();
+  }
+}