Warn if .over_react.g.dart part is missing.

lib/src/builder/builder.dart

@@ -21,10 +21,6 @@ class OverReactBuilder extends Builder {
   @override
   FutureOr<void> build(BuildStep buildStep) async {
     final source = await buildStep.readAsString(buildStep.inputId);
-    if (!_mightContainDeclarations(source)) {
-      return;
-    }
-
     final libraryUnit = _tryParseCompilationUnit(source, buildStep.inputId);
     if (libraryUnit == null) {
       return;
@@ -79,7 +75,18 @@ class OverReactBuilder extends Builder {
     }
 
     if (outputs.isNotEmpty) {
-      await _writePart(buildStep, outputs);
+      final outputId = buildStep.inputId.changeExtension(outputExtension);
+
+      // Verify that the library file has an `.over_react.g.dart` part.
+      final expectedPart = p.basename(outputId.path);
+      final partUris = libraryUnit.directives
+        .whereType<PartDirective>()
+        .map((p) => p.uri.stringValue);
+      if (!partUris.contains(expectedPart)) {
+        log.warning('Missing "part \'$expectedPart\';".');
+      }
+
+      await _writePart(buildStep, outputId, outputs);
     }
   }
 
@@ -111,8 +118,7 @@ class OverReactBuilder extends Builder {
     }
   }
 
-  static FutureOr<void> _writePart(BuildStep buildStep, Iterable<String> outputs) async {
-    final outputId = buildStep.inputId.changeExtension(outputExtension);
+  static FutureOr<void> _writePart(BuildStep buildStep, AssetId outputId, Iterable<String> outputs) async {
     final partOf = "'${p.basename(buildStep.inputId.uri.toString())}'";
 
     final buffer = new StringBuffer()

test/vm_tests/builder/over_react_builder_test.dart

@@ -1,4 +1,5 @@
 @TestOn('vm')
+import 'dart:async';
 import 'dart:io';
 
 import 'package:build/build.dart';
@@ -14,36 +15,37 @@ typedef void LogRecordFunction(LogRecord record);
 main() {
   group('overReactBuilder', () {
     final builder = overReactBuilder(null);
+    final logger = Logger('overReactBuilderTestLogger');
 
     AssetReader reader;
     InMemoryAssetWriter writer;
     AssetWriterSpy writerSpy;
-    List<String> logMsgs = <String>[];
-    List<Level> logLevels = <Level>[];
+    StreamSubscription logSub;
+    List<LogRecord> logs = <LogRecord>[];
+
     setUp(() async {
       reader = await PackageAssetReader.currentIsolate(
         rootPackage: 'over_react',
       );
 
       writer = new InMemoryAssetWriter();
       writerSpy = AssetWriterSpy(writer);
+
+      logSub = logger.onRecord.listen(logs.add);
     });
 
-    tearDown(() {
-      logMsgs.clear();
+    tearDown(() async {
+      await logSub?.cancel();
+      logs.clear();
       reader = null;
       writer = null;
       writerSpy = null;
     });
 
     void verifyNoErrorLogs() {
-      expect(logLevels, isNot(contains(Level.SEVERE)));
-      expect(logLevels, isNot(contains(Level.WARNING)));
-    }
-
-    void recordLogs(LogRecord record) {
-      logMsgs.add(record.message);
-      logLevels.add(record.level);
+      expect(logs.where((log) => log.level >= Level.WARNING), isEmpty,
+        reason: 'Expected no logs at WARNING or SEVERE level, but got:\n'
+          '\t${logs.join('\n\t')}');
     }
 
     void checkBuildForFile(String assetPath, String expectedOutputAssetPath,
@@ -55,32 +57,39 @@ main() {
         expectedOutputAssetPath : expectedContent
       };
 
-      final logger = Logger('overReactBuilderTestLogger');
-      final sub = logger.onRecord.listen(recordLogs);
       await runBuilder(builder, [inputAsset], reader, writerSpy, AnalyzerResolvers(), logger: logger);
       final actual = writerSpy.assetsWritten;
-      sub.cancel();
 
       checkOutputs(expected, actual, writer);
       verifyNoErrorLogs();
     }
 
     test('does not produce a build output for a file with no over_react annotations', () async {
       var basicAsset = makeAssetId('over_react|test_fixtures/source_files/no_annotations.dart');
-      await runBuilder(builder, [basicAsset], reader, writerSpy, AnalyzerResolvers());
+      await runBuilder(builder, [basicAsset], reader, writerSpy, AnalyzerResolvers(), logger: logger);
 
       expect(writerSpy.assetsWritten, isEmpty);
       verifyNoErrorLogs();
     });
 
     test('does not produce a build output for just a part file', () async {
       var basicAsset = makeAssetId('over_react|test_fixtures/source_files/part_of_basic_library.dart');
-      await runBuilder(builder, [basicAsset], reader, writerSpy, AnalyzerResolvers());
+      await runBuilder(builder, [basicAsset], reader, writerSpy, AnalyzerResolvers(), logger: logger);
 
       expect(writerSpy.assetsWritten, isEmpty);
       verifyNoErrorLogs();
     });
 
+    test('warns if the .over_react.g.dart part is missing', () async {
+      var libraryAsset = makeAssetId('over_react|test_fixtures/source_files/missing_over_react_g_part/library.dart');
+      await runBuilder(builder, [libraryAsset], reader, writerSpy, AnalyzerResolvers(), logger: logger);
+      final expectedWarning = logs.firstWhere((log) {
+        return log.level == Level.WARNING && log.message == 'Missing "part \'library.over_react.g.dart\';".';
+      }, orElse: () => null);
+      expect(expectedWarning, isNotNull,
+        reason: 'Expected a WARNING log for the missing over_react part.');
+    });
+
     group('for backwards compatible boilerplate:', () {
       test('builds from basic component file', () async {
         await checkBuildForFile(

test_fixtures/source_files/missing_over_react_g_part/library.dart

@@ -0,0 +1,3 @@
+import 'package:over_react/over_react.dart';
+
+part 'part.dart';

test_fixtures/source_files/missing_over_react_g_part/part.dart

@@ -0,0 +1,17 @@
+part of 'library.dart';
+
+@Factory()
+UiFactory<BasicPartOfLibProps> BasicPartOfLib = _$BasicPartOfLib;
+
+@Props()
+class _$BasicPartOfLibProps extends UiProps {
+  String basicProp;
+}
+
+@Component()
+class BasicPartOfLibComponent extends UiComponent<BasicPartOfLibProps> {
+  @override
+  render() {
+    return Dom.div()('foo');
+  }
+}