Use builders to generate showcase test files

`source_gen` parses all classese annotated with `@Showcased` and generates test files with cases to generate screenshots of those annotated widgets.

build.yaml

@@ -0,0 +1,7 @@
+builders:
+  showcase:
+    import: 'package:showcase/showcase_generator.dart'
+    builder_factories: ['showcaseBuilder']
+    # build_to: 'source'
+    build_extensions: {'.dart': ['.showcased.dart']}
+    auto_apply: dependents

example/README.md

@@ -0,0 +1,8 @@
+# showcase_example
+
+A new Flutter project.
+
+## Getting Started
+
+For help getting started with Flutter, view our online
+[documentation](https://flutter.io/).

example/lib/my_widget.dart

@@ -0,0 +1,49 @@
+import 'package:flutter/material.dart';
+import 'package:showcase/showcase.dart';
+
+@Showcased()
+class MyWidget extends StatelessWidget {
+  final bool anotherBuild;
+  final double sliderValue;
+
+  MyWidget({
+    this.anotherBuild = false,
+    this.sliderValue = 2.0,
+  });
+
+  @override
+  Widget build(BuildContext context) {
+    if (anotherBuild) {
+      return Row(
+        mainAxisAlignment: MainAxisAlignment.center,
+        children: [
+          Container(color: Colors.red, width: 40.0, height: 40.0),
+          Container(color: Colors.green, width: 40.0, height: 40.0),
+          Container(color: Colors.blue, width: 40.0, height: 40.0),
+        ],
+      );
+    }
+
+    return Column(
+      mainAxisAlignment: MainAxisAlignment.center,
+      children: [
+        Text('This is my text'),
+        Text('This is another text'),
+        RaisedButton(
+          onPressed: () {},
+          child: Text('This should be a button'),
+          color: Colors.amber,
+        ),
+        Switch(
+          onChanged: (_) {},
+          value: true,
+        ),
+        Slider(
+          value: sliderValue,
+          max: 7.0,
+          onChanged: (_) {},
+        ),
+      ],
+    );
+  }
+}

example/pubspec.yaml

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+name: showcase_example
+description: A new Flutter project.
+
+version: 1.0.0+1
+
+environment:
+  sdk: ">=2.0.0-dev.68.0 <3.0.0"
+
+dependencies:
+  flutter:
+    sdk: flutter
+
+dev_dependencies:
+  flutter_test:
+    sdk: flutter
+  build_runner: ^0.10.0
+  showcase:
+    path: ../.
+
+flutter:
+  uses-material-design: true

lib/showcase.dart

@@ -1 +1,2 @@
-export './showcase_widgets.dart' show showcaseWidgets;
+export 'package:showcase/showcase_widgets.dart' show showcaseWidgets;
+export 'package:showcase/showcased.dart' show Showcased;

lib/showcase_generator.dart

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+import 'dart:async';
+import 'dart:io';
+
+import 'package:analyzer/dart/element/element.dart';
+import 'package:source_gen/source_gen.dart';
+import 'package:build/build.dart';
+import 'package:showcase/showcased.dart';
+
+class ShowcaseGenerator extends Generator {
+  TypeChecker get _typeChecker => const TypeChecker.fromRuntime(Showcased);
+
+  File generatedFilePath(Uri originalFile) {
+    var butFirst = originalFile.pathSegments.skip(1).toList()..insert(0, 'test');
+    var filename = butFirst.removeLast();
+    butFirst.add(filename.replaceAllMapped(RegExp(r'^(.*)\.dart$'), (match) {
+      return '${match[1]}_test.showcased.dart';
+    }));
+    var newPath = originalFile.replace(pathSegments: butFirst);
+    return File(newPath.path);
+  }
+
+  @override
+  FutureOr<String> generate(LibraryReader library, BuildStep buildStep) {
+    for (var annotatedElement in library.annotatedWith(_typeChecker)) {
+      // TODO(igor): filter Widgets only
+      var generatedValue = generateForAnnotatedElement(
+          annotatedElement.element, annotatedElement.annotation, buildStep, library);
+      return generatedValue;
+    }
+  }
+
+  Future<String> generateForAnnotatedElement(
+      Element element, ConstantReader annotation, BuildStep buildStep, LibraryReader library) async {
+    var buffer = StringBuffer();
+
+    Uri assetUri = library.pathToAsset(buildStep.inputId);
+
+    TypeChecker a = const TypeChecker.fromUrl('package:flutter/widgets.dart#Widget');
+    TypeChecker b = const TypeChecker.fromUrl('package:showcase/showcase.dart#Showcased');
+
+    buffer.writeln('''
+// GENERATED CODE - DO NOT MODIFY BY HAND
+
+import 'package:flutter_test/flutter_test.dart';
+import 'package:showcase/showcase.dart';
+import '${assetUri.toString()}';
+
+void main() {
+  group('Showcase ${element.name}', () {
+    showcaseWidgets([${element.name}()]);
+  });
+}
+''');
+
+    File file = generatedFilePath(assetUri);
+    await file.create(recursive: true);
+
+    await file.writeAsString(buffer.toString());
+
+    return null;
+  }
+}
+
+Builder showcaseBuilder(BuilderOptions options) {
+  return LibraryBuilder(ShowcaseGenerator(), generatedExtension: '.showcased.dart');
+}

lib/showcased.dart

@@ -0,0 +1,3 @@
+class Showcased {
+  const Showcased();
+}