Move BuildStep to an interface and add BuildStepImpl with the gory details

Author: jakemac53

lib/src/builder/build_step.dart

@@ -2,77 +2,30 @@
 // 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.
 import 'dart:async';
-import 'dart:collection';
 import 'dart:convert';
 
 import '../asset/asset.dart';
 import '../asset/id.dart';
-import '../asset/reader.dart';
-import '../asset/writer.dart';
 
 /// A single step in the build processes. This represents a single input and
-/// its expected and real outputs. It also handles tracking of dependencies.
+/// it also handles tracking of dependencies.
 class BuildStep {
   /// The primary input for this build step.
   final Asset input;
 
-  /// The list of all outputs which are expected/allowed to be output from this
-  /// step.
-  final List<AssetId> expectedOutputs;
-
-  /// The actual outputs of this build step.
-  UnmodifiableListView<Asset> get outputs => new UnmodifiableListView(_outputs);
-  final List<Asset> _outputs = [];
-
-  /// A future that completes once all outputs current are done writing.
-  ///
-  /// TODO(jakemac): This is not typically needed inside of a Builder, we could
-  /// consider moving it out to a separate class which wraps a [BuildStep] and
-  /// this future? Another option would be an `Output` class which wraps an
-  /// [Asset] and a [Future] that completes once its done writing.
-  Future get outputsCompleted => _outputsCompleted;
-  Future _outputsCompleted = new Future(() {});
-
-  /// The dependencies read in during this build step.
-  UnmodifiableListView<AssetId> get dependencies =>
-      new UnmodifiableListView(_dependencies);
-  final Set<AssetId> _dependencies = new Set<AssetId>();
-
-  /// Used internally for reading files.
-  final AssetReader _reader;
-
-  /// Used internally for writing files.
-  final AssetWriter _writer;
-
-  BuildStep(
-      this.input, List<AssetId> expectedOutputs, this._reader, this._writer)
-      : expectedOutputs = new List.unmodifiable(expectedOutputs) {
-    /// The [input] is always a dependency.
-    _dependencies.add(input.id);
-  }
-
   /// Reads an [Asset] by [id] as a [String] using [encoding].
   ///
   /// If [trackAsDependency] is true, then [id] will be marked as a dependency
   /// of all [expectedOutputs].
   Future<String> readAsString(AssetId id,
-      {Encoding encoding: UTF8, bool trackAsDependency: true}) {
-    if (trackAsDependency) _dependencies.add(id);
-    return _reader.readAsString(id, encoding: encoding);
-  }
+      {Encoding encoding: UTF8, bool trackAsDependency: true});
 
   /// Outputs an [Asset] using the current [AssetWriter], and adds [asset] to
   /// [outputs].
-  void writeAsString(Asset asset, {Encoding encoding: UTF8}) {
-    _outputs.add(asset);
-    var done = _writer.writeAsString(asset, encoding: encoding);
-    _outputsCompleted = _outputsCompleted.then((_) => done);
-  }
+  void writeAsString(Asset asset, {Encoding encoding: UTF8});
 
   /// Explicitly adds [id] as a dependency of all [expectedOutputs]. This is
   /// not generally necessary unless forcing `trackAsDependency: false` when
   /// calling [readAsString].
-  void addDependency(AssetId id) {
-    _dependencies.add(id);
-  }
+  void addDependency(AssetId id);
 }

lib/src/builder/build_step_impl.dart

@@ -0,0 +1,83 @@
+// Copyright (c) 2016, 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.
+import 'dart:async';
+import 'dart:collection';
+import 'dart:convert';
+
+import '../asset/asset.dart';
+import '../asset/id.dart';
+import '../asset/reader.dart';
+import '../asset/writer.dart';
+import 'build_step.dart';
+
+/// A single step in the build processes. This represents a single input and
+/// its expected and real outputs. It also handles tracking of dependencies.
+class BuildStepImpl implements BuildStep {
+  /// The primary input for this build step.
+  @override
+  final Asset input;
+
+  /// The list of all outputs which are expected/allowed to be output from this
+  /// step.
+  final List<AssetId> expectedOutputs;
+
+  /// The actual outputs of this build step.
+  UnmodifiableListView<Asset> get outputs => new UnmodifiableListView(_outputs);
+  final List<Asset> _outputs = [];
+
+  /// A future that completes once all outputs current are done writing.
+  ///
+  /// TODO(jakemac): This is not typically needed inside of a Builder, we could
+  /// consider moving it out to a separate class which wraps a [BuildStep] and
+  /// this future? Another option would be an `Output` class which wraps an
+  /// [Asset] and a [Future] that completes once its done writing.
+  Future get outputsCompleted => _outputsCompleted;
+  Future _outputsCompleted = new Future(() {});
+
+  /// The dependencies read in during this build step.
+  UnmodifiableListView<AssetId> get dependencies =>
+      new UnmodifiableListView(_dependencies);
+  final Set<AssetId> _dependencies = new Set<AssetId>();
+
+  /// Used internally for reading files.
+  final AssetReader _reader;
+
+  /// Used internally for writing files.
+  final AssetWriter _writer;
+
+  BuildStepImpl(
+      this.input, List<AssetId> expectedOutputs, this._reader, this._writer)
+      : expectedOutputs = new List.unmodifiable(expectedOutputs) {
+    /// The [input] is always a dependency.
+    _dependencies.add(input.id);
+  }
+
+  /// Reads an [Asset] by [id] as a [String] using [encoding].
+  ///
+  /// If [trackAsDependency] is true, then [id] will be marked as a dependency
+  /// of all [expectedOutputs].
+  @override
+  Future<String> readAsString(AssetId id,
+      {Encoding encoding: UTF8, bool trackAsDependency: true}) {
+    if (trackAsDependency) _dependencies.add(id);
+    return _reader.readAsString(id, encoding: encoding);
+  }
+
+  /// Outputs an [Asset] using the current [AssetWriter], and adds [asset] to
+  /// [outputs].
+  @override
+  void writeAsString(Asset asset, {Encoding encoding: UTF8}) {
+    _outputs.add(asset);
+    var done = _writer.writeAsString(asset, encoding: encoding);
+    _outputsCompleted = _outputsCompleted.then((_) => done);
+  }
+
+  /// Explicitly adds [id] as a dependency of all [expectedOutputs]. This is
+  /// not generally necessary unless forcing `trackAsDependency: false` when
+  /// calling [readAsString].
+  @override
+  void addDependency(AssetId id) {
+    _dependencies.add(id);
+  }
+}

lib/src/generate/build.dart

@@ -14,7 +14,7 @@ import '../asset/id.dart';
 import '../asset/reader.dart';
 import '../asset/writer.dart';
 import '../builder/builder.dart';
-import '../builder/build_step.dart';
+import '../builder/build_step_impl.dart';
 import 'build_result.dart';
 import 'input_set.dart';
 import 'phase.dart';
@@ -109,7 +109,7 @@ Stream<Asset> _runBuilder(Builder builder, List<AssetId> inputs) async* {
     var expectedOutputs = builder.declareOutputs(input);
     var inputAsset = new Asset(input, await _reader.readAsString(input));
     var buildStep =
-        new BuildStep(inputAsset, expectedOutputs, _reader, _writer);
+        new BuildStepImpl(inputAsset, expectedOutputs, _reader, _writer);
     await builder.build(buildStep);
     await buildStep.outputsCompleted;
     for (var output in buildStep.outputs) {

lib/src/transformer/transformer.dart

@@ -12,7 +12,7 @@ import '../asset/id.dart' as build;
 import '../asset/reader.dart';
 import '../asset/writer.dart';
 import '../builder/builder.dart';
-import '../builder/build_step.dart';
+import '../builder/build_step_impl.dart';
 
 abstract class BuilderTransformer implements Transformer, DeclaringTransformer {
   List<Builder> get builders;
@@ -35,7 +35,7 @@ abstract class BuilderTransformer implements Transformer, DeclaringTransformer {
       var expected = _expectedOutputs(transform.primaryInput.id, [builder]);
       if (expected.isEmpty) continue;
 
-      var buildStep = new BuildStep(input, expected, reader, writer);
+      var buildStep = new BuildStepImpl(input, expected, reader, writer);
       futures.add(builder.build(buildStep));
     }