Refactor the Transform classes.

pkg/barback/lib/src/base_transform.dart

 // Copyright (c) 2014, 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.base_transform;
 
 import 'dart:async';
 import 'dart:convert';
 
 import 'asset.dart';
 import 'asset_id.dart';
 import 'asset_node.dart';
 import 'errors.dart';
+import 'log.dart';
 import 'transform_logger.dart';
 import 'transform_node.dart';
 import 'utils.dart';
 
 /// The base class for the ephemeral transform objects that are passed to
 /// transformers.
 ///
 /// This class provides the transformers with inputs, but its up to the
 /// subclasses to provide a means of emitting outputs.
 abstract class BaseTransform {
   final TransformNode _node;
-  final TransformLogger _logger;
+
+  /// The controller for the stream of log entries emitted by the transformer.
+  ///
+  /// This is exposed via [BaseTransformController].
+  ///
+  /// This is synchronous because error logs can cause the transform to fail, so
+  /// we need to ensure that their processing isn't delayed until after the
+  /// transform or build has finished.
+  final _onLogController = new StreamController<LogEntry>.broadcast(sync: true);
 
   /// A logger so that the [Transformer] can report build details.
   TransformLogger get logger => _logger;
+  TransformLogger _logger;
 
   /// Gets the primary input asset.
   ///
   /// While a transformation can use multiple input assets, one must be a
   /// special "primary" asset. This will be the "entrypoint" or "main" input
   /// file for a transformation.
   ///
   /// For example, with a dart2js transform, the primary input would be the
   /// entrypoint Dart file. All of the other Dart files that that imports
   /// would be secondary inputs.
   ///
   /// This method may fail at runtime if called asynchronously after the
   /// transform begins running. The primary input may become unavailable while
   /// this transformer is running due to asset changes earlier in the graph.
   /// You can ignore the error if this happens: the transformer will be re-run
   /// automatically for you.
   Asset get primaryInput {
     if (_node.primary.state != AssetState.AVAILABLE) {
       throw new AssetNotFoundException(_node.primary.id);
     }
 
     return _node.primary.asset;
   }
 
-  BaseTransform(this._node, LogFunction logFunction)
-    : _logger = new TransformLogger(logFunction);
+  BaseTransform(this._node) {
+    _logger = new TransformLogger((asset, level, message, span) {
+      // If the log isn't already associated with an asset, use the primary.
+      if (asset == null) asset = _node.primary.id;
+      var entry = new LogEntry(_node.info, asset, level, message, span);
+      _onLogController.add(entry);
+    });
+  }
 
   /// Gets the asset for an input [id].
   ///
   /// If an input with that ID cannot be found, throws an
   /// [AssetNotFoundException].
   Future<Asset> getInput(AssetId id) => _node.getInput(id);
 
   /// A convenience method to the contents of the input with [id] as a string.
   ///
   /// This is equivalent to calling `getInput()` followed by `readAsString()`.
   ///
   /// If the asset was created from a [String] the original string is always
   /// returned and [encoding] is ignored. Otherwise, the binary data of the
   /// asset is decoded using [encoding], which defaults to [UTF8].
   Future<String> readInputAsString(AssetId id, {Encoding encoding}) {
     if (encoding == null) encoding = UTF8;
     return getInput(id).then((input) => input.readAsString(encoding: encoding));
   }
 
   /// A convenience method to the contents of the input with [id].
   ///
   /// This is equivalent to calling `getInput()` followed by `read()`.
   ///
   /// If the asset was created from a [String], this returns its UTF-8 encoding.
   Stream<List<int>> readInput(AssetId id) =>
       futureStream(getInput(id).then((input) => input.read()));
 }
+
+/// The base class for controllers of subclasses of [BaseTransform].
+///
+/// Controllers are used so that [TransformNode]s can get values from a
+/// [BaseTransform] without exposing getters in the public API.
+abstract class BaseTransformController {
+  /// The [BaseTransform] controlled by this controller.
+  final BaseTransform transform;
+
+  /// The stream of log entries emitted by the transformer during a run.
+  Stream<LogEntry> get onLog => transform._onLogController.stream;
+
+  BaseTransformController(this.transform);
+
+  /// Notifies the [BaseTransform] that the transformation has finished being
+  /// applied.
+  ///
+  /// This will close any streams and release any resources that were allocated
+  /// for the duration of the transformation.
+  void close() {
+    transform._onLogController.close();
+  }
+}