add transformer tests

lib/src/builder/build_step_impl.dart

@@ -42,7 +42,7 @@ class BuildStepImpl implements BuildStep {
   final AssetWriter _writer;
 
   BuildStepImpl(
-      this.input, List<AssetId> expectedOutputs, this._reader, this._writer)
+      this.input, Iterable<AssetId> expectedOutputs, this._reader, this._writer)
       : expectedOutputs = new List.unmodifiable(expectedOutputs) {
     /// The [input] is always a dependency.
     _dependencies.add(input.id);

lib/src/transformer/transformer.dart

@@ -14,7 +14,17 @@ import '../asset/writer.dart';
 import '../builder/builder.dart';
 import '../builder/build_step_impl.dart';
 
-abstract class BuilderTransformer implements Transformer, DeclaringTransformer {
+/// A [Transformer] which runs multiple [Builder]s.
+/// Extend this class and define the [builders] getter to create a [Transformer]
+/// out of your custom [Builder]s.
+abstract class BuilderTransformer implements Transformer {
+  /// The only thing you need to implement when extending this class. This
+  /// declares which builders should be ran.
+  ///
+  /// **Note**: All [builders] are ran in the same phase, and there are no
+  /// ordering guarantees. Thus, none of the [builders] can use the outputs of
+  /// other [builders]. In order to do this you must create a [TransformerGroup]
+  /// with multiple [BuilderTransformer]s.
   List<Builder> get builders;
 
   @override
@@ -30,23 +40,36 @@ abstract class BuilderTransformer implements Transformer, DeclaringTransformer {
     var reader = new _TransformAssetReader(transform);
     var writer = new _TransformAssetWriter(transform);
 
-    var futures = <Future>[];
-    for (var builder in builders) {
-      var expected = _expectedOutputs(transform.primaryInput.id, [builder]);
-      if (expected.isEmpty) continue;
+    // Tracks all the expected outputs for each builder to make sure they don't
+    // overlap.
+    var allExpected = new Set<build.AssetId>();
 
+    // Run all builders at the same time.
+    await Future.wait(builders.map((builder) async {
+      var expected = _expectedOutputs(transform.primaryInput.id, [builder]);
+      if (expected.isEmpty) return;
+
+      // Check that no expected outputs already exist.
+      var preExistingFiles = [];
+      for (var output in expected) {
+        if (!allExpected.add(output) ||
+            await transform.hasInput(_toBarbackAssetId(output))) {
+          preExistingFiles.add(_toBarbackAssetId(output));
+        }
+      }
+      if (preExistingFiles.isNotEmpty) {
+        transform.logger.error(
+            'Builder `$builder` declared outputs `$preExistingFiles` but those '
+            'files already exist. That build step has been skipped.',
+            asset: transform.primaryInput.id);
+        return;
+      }
+
+      // Run the build step.
       var buildStep = new BuildStepImpl(input, expected, reader, writer);
-      futures.add(builder.build(buildStep));
-    }
-
-    await Future.wait(futures);
-  }
-
-  @override
-  void declareOutputs(DeclaringTransform transform) {
-    for (var outputId in _expectedOutputs(transform.primaryId, builders)) {
-      transform.declareOutput(_toBarbackAssetId(outputId));
-    }
+      await builder.build(buildStep);
+      await buildStep.outputsCompleted;
+    }));
   }
 }
 
@@ -68,8 +91,10 @@ class _TransformAssetWriter implements AssetWriter {
   _TransformAssetWriter(this.transform);
 
   @override
-  Future writeAsString(build.Asset asset, {Encoding encoding: UTF8}) async =>
-      transform.addOutput(_toBarbackAsset(asset));
+  Future writeAsString(build.Asset asset, {Encoding encoding: UTF8}) {
+    transform.addOutput(_toBarbackAsset(asset));
+    return new Future.value(null);
+  }
 }
 
 /// All the expected outputs for [id] given [builders].

pubspec.yaml

@@ -16,3 +16,4 @@ dependencies:
 
 dev_dependencies:
   test: ^0.12.0
+  transformer_test: ^0.1.0

test/common/common.dart

@@ -0,0 +1,5 @@
+// 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.
+export 'copy_builder.dart';
+export 'generic_builder_transformer.dart';

test/common/copy_builder.dart

@@ -0,0 +1,30 @@
+// 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 'package:build/build.dart';
+
+class CopyBuilder implements Builder {
+  int numCopies;
+
+  CopyBuilder({this.numCopies: 1});
+
+  Future build(BuildStep buildStep) async {
+    var ids = declareOutputs(buildStep.input.id);
+    for (var id in ids) {
+      buildStep.writeAsString(new Asset(id, buildStep.input.stringContents));
+    }
+  }
+
+  List<AssetId> declareOutputs(AssetId input) {
+    var outputs = [];
+    for (int i = 0; i < numCopies; i++) {
+      outputs.add(_copiedAssetId(input, numCopies == 1 ? null : i));
+    }
+    return outputs;
+  }
+}
+
+AssetId _copiedAssetId(AssetId inputId, int copyNum) =>
+    inputId.addExtension('.copy${copyNum == null ? '' : '.$copyNum'}');

test/all_test.dart → test/common/generic_builder_transformer.dart

@@ -1,14 +1,10 @@
 // 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 'package:build/build.dart';
-import 'package:test/test.dart';
 
-main() {
-  group('A group of tests', () {
-    test('First Test', () {
-      expect(true, isTrue);
-    });
-  });
+class GenericBuilderTransformer extends BuilderTransformer {
+  final List<Builder> builders;
+
+  GenericBuilderTransformer(this.builders);
 }

test/transformer/transformer_test.dart

@@ -0,0 +1,117 @@
+// 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.
+@TestOn('vm')
+import 'package:test/test.dart';
+import 'package:transformer_test/utils.dart';
+
+import '../common/common.dart';
+
+main() {
+  var singleCopyTransformer =
+      new GenericBuilderTransformer([new CopyBuilder()]);
+  var multiCopyTransformer =
+      new GenericBuilderTransformer([new CopyBuilder(numCopies: 2)]);
+  var singleAndMultiCopyTransformer = new GenericBuilderTransformer(
+      [new CopyBuilder(), new CopyBuilder(numCopies: 2)]);
+
+  testPhases('single builder, single output', [
+    [singleCopyTransformer],
+  ], {
+    'a|web/a.txt': 'hello',
+  }, {
+    'a|web/a.txt.copy': 'hello',
+  });
+
+  testPhases('single builder, multiple outputs', [
+    [multiCopyTransformer],
+  ], {
+    'a|web/a.txt': 'hello',
+  }, {
+    'a|web/a.txt.copy.0': 'hello',
+    'a|web/a.txt.copy.1': 'hello',
+  });
+
+  testPhases('multiple builders, different outputs', [
+    [singleAndMultiCopyTransformer],
+  ], {
+    'a|web/a.txt': 'hello',
+  }, {
+    'a|web/a.txt.copy': 'hello',
+    'a|web/a.txt.copy.0': 'hello',
+    'a|web/a.txt.copy.1': 'hello',
+  });
+
+  testPhases('multiple builders, same outputs', [
+    [
+      new GenericBuilderTransformer([new CopyBuilder(), new CopyBuilder()])
+    ],
+  ], {
+    'a|web/a.txt': 'hello',
+  }, {}, [
+    _fileExistsError('CopyBuilder', ['a|web/a.txt.copy']),
+  ]);
+
+  testPhases('multiple phases', [
+    [singleCopyTransformer],
+    [multiCopyTransformer],
+  ], {
+    'a|web/a.txt': 'hello',
+  }, {
+    'a|web/a.txt.copy': 'hello',
+    'a|web/a.txt.copy.0': 'hello',
+    'a|web/a.txt.copy.1': 'hello',
+    'a|web/a.txt.copy.copy.0': 'hello',
+    'a|web/a.txt.copy.copy.1': 'hello',
+  });
+
+  testPhases('multiple transformers in the same phase', [
+    [singleCopyTransformer, multiCopyTransformer],
+  ], {
+    'a|web/a.txt': 'hello',
+  }, {
+    'a|web/a.txt.copy': 'hello',
+    'a|web/a.txt.copy.0': 'hello',
+    'a|web/a.txt.copy.1': 'hello',
+  });
+
+  testPhases('cant overwrite files', [
+    [singleCopyTransformer]
+  ], {
+    'a|web/a.txt': 'hello',
+    'a|web/a.txt.copy': 'hello',
+  }, {}, [
+    _fileExistsError("CopyBuilder", ["a|web/a.txt.copy"]),
+  ]);
+
+  // TODO(jakemac): Skipped because we can't detect this situation today.
+  // Instead you get a barback error, see
+  // https://github.com/dart-lang/transformer_test/issues/2
+  //
+  // testPhases('builders in the same phase can\'t output the same file', [
+  //   [singleCopyTransformer, new GenericBuilderTransformer([new CopyBuilder()])]
+  // ], {
+  //   'a|web/a.txt': 'hello',
+  // }, {
+  //   'a|web/a.txt.copy': 'hello',
+  // }, [
+  //   _fileExistsError("CopyBuilder", ["a|web/a.txt.copy"]),
+  // ]);
+
+  testPhases('builders in separate phases can\'t output the same file', [
+    [singleCopyTransformer],
+    [singleCopyTransformer],
+  ], {
+    'a|web/a.txt': 'hello',
+  }, {
+    'a|web/a.txt.copy': 'hello',
+  }, [
+    _fileExistsError("CopyBuilder", ["a|web/a.txt.copy"]),
+  ]);
+}
+
+String _fileExistsError(String builder, List<String> files) {
+  return "error: Builder `Instance of '$builder'` declared outputs "
+      "`$files` but those files already exist. That build step has been "
+      "skipped.";
+}