misc: Ensure there is exactly one blank line before and after output

In the case where a generator returns output without a trailing newline,
it would end up right next to the next generator header

Author: kevmoo

source_gen/CHANGELOG.md

@@ -2,6 +2,12 @@
 
 * Simplification to the output of generator names in header sections.
 
+* Update handling of whitespace in generator outputs.
+  * If the output from a generator has wrapping whitespace, it is trimmed.
+  * If the output from a generator is empty or whitespace-only, it is ignored.
+  * These changes will likely have no effect on output, unless you customize
+    the code formatter.
+
 ## 0.8.1+3
 
 * More redundant new lines elimination.

source_gen/lib/src/builder.dart

@@ -115,7 +115,7 @@ class _Builder extends Builder {
             LineSplitter.split(item.toString()).map((line) => '// $line\n'))
         ..writeln(_headerLine)
         ..writeln()
-        ..write(item.output);
+        ..writeln(item.output);
     }
 
     var genPartContent = contentBuffer.toString();
@@ -212,9 +212,16 @@ Stream<GeneratedOutput> _generate(LibraryElement library,
       log.fine(msg);
       var createdUnit = await gen.generate(libraryReader, buildStep);
 
-      if (createdUnit != null && createdUnit.isNotEmpty) {
-        yield new GeneratedOutput(gen, createdUnit);
+      if (createdUnit == null) {
+        continue;
       }
+
+      createdUnit = createdUnit.trim();
+      if (createdUnit.isEmpty) {
+        continue;
+      }
+
+      yield new GeneratedOutput(gen, createdUnit);
     } catch (e, stack) {
       log.severe('Error running $gen', e, stack);
       yield new GeneratedOutput.fromError(gen, e, stack);

source_gen/lib/src/generated_output.dart

@@ -17,7 +17,10 @@ class GeneratedOutput {
   GeneratedOutput(this.generator, this.output)
       : error = null,
         stackTrace = null,
-        assert(output != null && output.isNotEmpty);
+        assert(output != null),
+        assert(output.isNotEmpty),
+        // assuming length check is cheaper than simple string equality
+        assert(output.length == output.trim().length);
 
   GeneratedOutput.fromError(this.generator, this.error, this.stackTrace)
       : this.output = _outputFromError(error);

source_gen/test/builder_test.dart

@@ -65,8 +65,8 @@ void main() {
   test('Expect no error when multiple generators used on nonstandalone builder',
       () async {
     expect(
-        () =>
-            new PartBuilder([const CommentGenerator(), const _NoOpGenerator()]),
+        () => new PartBuilder(
+            [const CommentGenerator(), const _LiteralGenerator()]),
         returnsNormally);
   });
 
@@ -103,9 +103,9 @@ void main() {
           const CommentGenerator(forClasses: true, forLibrary: true),
           _testGenPartContentForClassesAndLibrary));
 
-  test('No-op generator produces no generated parts', () async {
+  test('null result produces no generated parts', () async {
     var srcs = _createPackageStub();
-    var builder = new PartBuilder([const _NoOpGenerator()]);
+    var builder = _unformattedLiteral();
     await testBuilder(builder, srcs, outputs: {});
   });
 
@@ -133,6 +133,38 @@ void main() {
     expect(logs, ['Missing "part \'test_lib.g.dart\';".']);
   });
 
+  test('generator with an empty result creates no outputs', () async {
+    var srcs = _createPackageStub(testLibContent: _testLibContentNoPart);
+    var builder = _unformattedLiteral('');
+    await testBuilder(
+      builder,
+      srcs,
+      outputs: {},
+    );
+  });
+
+  test('generator with whitespace-only result has no outputs', () async {
+    var srcs = _createPackageStub(testLibContent: _testLibContentNoPart);
+    var builder = _unformattedLiteral('\n  \n');
+    await testBuilder(
+      builder,
+      srcs,
+      outputs: {},
+    );
+  });
+
+  test('generator result with wrapping whitespace is trimmed', () async {
+    var srcs = _createPackageStub(testLibContent: _testLibContent);
+    var builder = _unformattedLiteral('\n// hello\n');
+    await testBuilder(
+      builder,
+      srcs,
+      outputs: {
+        '$_pkgName|lib/test_lib.g.dart': _whitespaceTrimmed,
+      },
+    );
+  });
+
   test('defaults to formatting generated code with the DartFormatter',
       () async {
     await testBuilder(new PartBuilder([const UnformattedCodeGenerator()]),
@@ -191,15 +223,15 @@ void main() {
   });
 
   test('Should have a readable toString() message for builders', () {
-    final builder = new LibraryBuilder(const _NoOpGenerator());
-    expect(builder.toString(), 'Generating .g.dart: _NoOpGenerator');
+    final builder = new LibraryBuilder(const _LiteralGenerator());
+    expect(builder.toString(), 'Generating .g.dart: _LiteralGenerator');
 
     final builders = new PartBuilder([
-      const _NoOpGenerator(),
-      const _NoOpGenerator(),
+      const _LiteralGenerator(),
+      const _LiteralGenerator(),
     ]);
     expect(builders.toString(),
-        'Generating .g.dart: _NoOpGenerator, _NoOpGenerator');
+        'Generating .g.dart: _LiteralGenerator, _LiteralGenerator');
   });
 }
 
@@ -223,12 +255,17 @@ Map<String, String> _createPackageStub(
   };
 }
 
-/// Doesn't generate output for any element
-class _NoOpGenerator extends Generator {
-  const _NoOpGenerator();
+PartBuilder _unformattedLiteral([String content]) =>
+    new PartBuilder([new _LiteralGenerator(content)], formatOutput: (s) => s);
+
+/// Returns the [String] provided in the constructor, or `null`.
+class _LiteralGenerator extends Generator {
+  final String _content;
+
+  const _LiteralGenerator([this._content]);
 
   @override
-  Future<String> generate(LibraryReader library, _) => null;
+  FutureOr<String> generate(_, __) => _content;
 }
 
 class _BadOutputGenerator extends Generator {
@@ -341,3 +378,14 @@ part of 'test_lib.dart';
 
 // Code for "class A"
 ''';
+
+const _whitespaceTrimmed = r'''// GENERATED CODE - DO NOT MODIFY BY HAND
+
+part of test_lib;
+
+// **************************************************************************
+// _LiteralGenerator
+// **************************************************************************
+
+// hello
+''';

source_gen/test/src/comment_generator.dart

@@ -15,13 +15,13 @@ class CommentGenerator extends Generator {
 
   @override
   Future<String> generate(LibraryReader library, _) async {
-    var output = new StringBuffer();
+    var output = <String>[];
     if (forLibrary) {
       var name = library.element.name;
       if (name.isEmpty) {
         name = library.element.source.uri.pathSegments.last;
       }
-      output.writeln('// Code for "$name"');
+      output.add('// Code for "$name"');
     }
     if (forClasses) {
       for (var classElement
@@ -32,9 +32,9 @@ class CommentGenerator extends Generator {
               todo: 'Rename ${classElement.displayName} to something else.',
               element: classElement);
         }
-        output.writeln('// Code for "$classElement"');
+        output.add('// Code for "$classElement"');
       }
     }
-    return '$output';
+    return output.join('\n');
   }
 }