fix: support reading environment files as build assets

packages/envied_generator/lib/src/generator.dart

@@ -112,6 +112,7 @@ final class EnviedGenerator extends GeneratorForAnnotation<Envied> {
           throw InvalidGenerationSourceError(error, element: element);
         }
       },
+      buildStep: buildStep,
     );
 
     final DartEmitter emitter = DartEmitter(useNullSafetySyntax: true);

packages/envied_generator/lib/src/load_envs.dart

@@ -1,5 +1,7 @@
+import 'dart:convert' show LineSplitter;
 import 'dart:io' show File;
 
+import 'package:build/build.dart';
 import 'package:envied_generator/src/env_val.dart';
 import 'package:envied_generator/src/parser.dart';
 
@@ -10,21 +12,23 @@ import 'package:envied_generator/src/parser.dart';
 /// [onError] function.
 Future<Map<String, EnvVal>> loadEnvs(
   String path,
-  void Function(String) onError,
-) => loadEnvsFromPaths(<String>[path], onError);
+  void Function(String) onError, {
+  BuildStep? buildStep,
+}) => loadEnvsFromPaths(<String>[path], onError, buildStep: buildStep);
 
 /// Load and merge environment variables from [paths].
 ///
 /// Files are parsed in order. Later files override earlier files, while
 /// duplicate keys within a single file keep the first parsed value.
 Future<Map<String, EnvVal>> loadEnvsFromPaths(
   Iterable<String> paths,
-  void Function(String) onError,
-) async {
+  void Function(String) onError, {
+  BuildStep? buildStep,
+}) async {
   final Map<String, EnvVal> envs = {};
 
   for (final String path in paths) {
-    envs.addAll(await _loadEnv(path, onError, env: envs));
+    envs.addAll(await _loadEnv(path, onError, env: envs, buildStep: buildStep));
   }
 
   return envs;
@@ -34,7 +38,23 @@ Future<Map<String, EnvVal>> _loadEnv(
   String path,
   void Function(String) onError, {
   required Map<String, EnvVal> env,
+  BuildStep? buildStep,
 }) async {
+  final AssetId? assetId = _assetIdForPath(path, buildStep);
+
+  if (assetId != null) {
+    final List<String> lines = [];
+    if (await buildStep!.canRead(assetId)) {
+      lines.addAll(
+        const LineSplitter().convert(await buildStep.readAsString(assetId)),
+      );
+    } else {
+      onError("Environment variable file doesn't exist at `$path`.");
+    }
+
+    return Parser.parse(lines, env: env);
+  }
+
   final File file = File.fromUri(Uri.file(path));
 
   final List<String> lines = [];
@@ -46,3 +66,18 @@ Future<Map<String, EnvVal>> _loadEnv(
 
   return Parser.parse(lines, env: env);
 }
+
+AssetId? _assetIdForPath(String path, BuildStep? buildStep) {
+  if (buildStep == null || File(path).isAbsolute) {
+    return null;
+  }
+
+  try {
+    return AssetId(buildStep.inputId.package, path);
+  } on ArgumentError {
+    return null;
+  } on NoSuchMethodError {
+    // source_gen_test passes a mock BuildStep that does not expose inputId.
+    return null;
+  }
+}

packages/envied_generator/pubspec.yaml

@@ -18,6 +18,7 @@ dependencies:
   source_gen: ^4.1.1
 
 dev_dependencies:
+  build_test: ^3.5.15
   lints: ^6.0.0
   source_gen_test: ^1.3.3
   test: ^1.28.0

packages/envied_generator/test/workspace_resolution_test.dart

@@ -0,0 +1,94 @@
+import 'package:build/build.dart';
+import 'package:build_test/build_test.dart';
+import 'package:envied_generator/builder.dart';
+import 'package:test/test.dart';
+
+void main() {
+  test('loads the default env file from the input package', () async {
+    final AssetId input = AssetId('envied_generator', 'lib/default_env.dart');
+    final AssetId env = AssetId('envied_generator', '.env');
+
+    final TestBuilderResult result = await _build(input, <String, Object>{
+      '$input': '''
+import 'package:envied/envied.dart';
+
+@Envied(requireEnvFile: true)
+abstract class DefaultEnv {
+  @EnviedField()
+  static const String? apiUrl = null;
+}
+''',
+      '$env': 'apiUrl=https://package.example',
+    });
+
+    expect(result.succeeded, isTrue);
+    expect(result.readerWriter.testing.inputsTracked, contains(env));
+    expect(
+      result.readerWriter.testing.readString(
+        input.changeExtension('.envied.g.part'),
+      ),
+      contains("static const String apiUrl = 'https://package.example';"),
+    );
+  });
+
+  test('loads inherited env files from the input package', () async {
+    final AssetId input = AssetId('envied_generator', 'lib/inherited_env.dart');
+    final AssetId defaults = AssetId('envied_generator', 'config/defaults.env');
+    final AssetId env = AssetId('envied_generator', 'config/local.env');
+
+    final TestBuilderResult result = await _build(input, <String, Object>{
+      '$input': '''
+import 'package:envied/envied.dart';
+
+@Envied(
+  path: 'config/local.env',
+  inheritFrom: ['config/defaults.env'],
+  requireEnvFile: true,
+)
+abstract class InheritedEnv {
+  @EnviedField()
+  static const String? apiUrl = null;
+
+  @EnviedField()
+  static const String? feature = null;
+}
+''',
+      '$defaults': '''
+apiUrl=https://default.example
+feature=off
+''',
+      '$env': 'apiUrl=https://local.example',
+    });
+
+    expect(result.succeeded, isTrue);
+    expect(result.readerWriter.testing.inputsTracked, contains(defaults));
+    expect(result.readerWriter.testing.inputsTracked, contains(env));
+    expect(
+      result.readerWriter.testing.readString(
+        input.changeExtension('.envied.g.part'),
+      ),
+      allOf(
+        contains("static const String apiUrl = 'https://local.example';"),
+        contains("static const String feature = 'off';"),
+      ),
+    );
+  });
+}
+
+Future<TestBuilderResult> _build(
+  AssetId input,
+  Map<String, Object> sourceAssets,
+) async {
+  final TestReaderWriter readerWriter = TestReaderWriter(
+    rootPackage: input.package,
+  );
+  await readerWriter.testing.loadIsolateSources();
+
+  return testBuilder(
+    enviedBuilder(BuilderOptions.empty),
+    sourceAssets,
+    rootPackage: input.package,
+    readerWriter: readerWriter,
+    flattenOutput: true,
+  );
+}