Merge AssetWriterSpy functionality into SingleStepReader. (#3892)

* Merge `AssetWriterSpy` functionality into `SingleStepReader`, make it `SingleStepReaderWriter`.

* Address review comments.

build/CHANGELOG.md

@@ -1,5 +1,6 @@
 ## 2.4.3-wip
 
+- `runBuilder` supports passing `fakeStartingAssets` for testing.
 - `AssetNotFoundException` now also reports the missing `path`.
 - Bump the min sdk to 3.7.0.
 - Use `build_test` 3.0.0.

build/lib/src/generate/run_builder.dart

@@ -5,6 +5,8 @@ import 'dart:isolate';
 
 // ignore: implementation_imports
 import 'package:build_runner_core/src/generate/build_step_impl.dart';
+// ignore: implementation_imports
+import 'package:build_runner_core/src/generate/single_step_reader_writer.dart';
 import 'package:logging/logging.dart';
 import 'package:package_config/package_config.dart';
 
@@ -32,18 +34,23 @@ import 'expected_outputs.dart';
 /// If [reportUnusedAssetsForInput] is provided then all calls to
 /// `BuildStep.reportUnusedAssets` in [builder] will be forwarded to this
 /// function with the associated primary input.
+///
+/// Optionally pass [fakeStartingAssets] for testing: the builder will behave as
+/// if exactly those assets are available from previous phases, and will not be
+/// able to access other files except for its own output.
 Future<void> runBuilder(
   Builder builder,
   Iterable<AssetId> inputs,
-  AssetReader reader,
-  AssetWriter writer,
+  AssetReader assetReader,
+  AssetWriter assetWriter,
   Resolvers? resolvers, {
   Logger? logger,
   ResourceManager? resourceManager,
   StageTracker stageTracker = NoOpStageTracker.instance,
   void Function(AssetId input, Iterable<AssetId> assets)?
   reportUnusedAssetsForInput,
   PackageConfig? packageConfig,
+  Set<AssetId>? fakeStartingAssets,
 }) async {
   var shouldDisposeResourceManager = resourceManager == null;
   final resources = resourceManager ?? ResourceManager();
@@ -78,8 +85,17 @@ Future<void> runBuilder(
     var buildStep = BuildStepImpl(
       input,
       outputs,
-      reader,
-      writer,
+      // If there a build running, `assetReader` and `assetWriter` are already a
+      // `SingleStepReaderWriter` instance integrated with the build; the `from`
+      // factory just passes it through.
+      //
+      // If there is no build running, this creates a fake build step,
+      // optionally with `fakeStartingAssets`.
+      SingleStepReaderWriter.from(
+        reader: assetReader,
+        writer: assetWriter,
+        fakeStartingAssets: fakeStartingAssets,
+      ),
       resolvers,
       resources,
       loadPackageConfig,

build/lib/src/internal.dart

@@ -11,3 +11,4 @@ export 'state/asset_path_provider.dart';
 export 'state/filesystem.dart';
 export 'state/filesystem_cache.dart';
 export 'state/reader_state.dart';
+export 'state/reader_writer.dart';

build/lib/src/state/reader_state.dart

@@ -7,11 +7,12 @@ import 'asset_finder.dart';
 import 'asset_path_provider.dart';
 import 'filesystem.dart';
 import 'filesystem_cache.dart';
+import 'reader_writer.dart';
 
 /// Provides access to the state backing an [AssetReader].
 extension AssetReaderStateExtension on AssetReader {
   /// Returns a new instance with optionally updated [cache].
-  AssetReader copyWith({
+  AssetReaderWriter copyWith({
     AssetPathProvider? assetPathProvider,
     FilesystemCache? cache,
   }) {
@@ -55,7 +56,7 @@ extension AssetReaderStateExtension on AssetReader {
 /// The state backing an [AssetReader].
 abstract interface class AssetReaderState {
   /// Returns a new instance with optionally updated [assetPathProvider] and/or [cache].
-  AssetReader copyWith({
+  AssetReaderWriter copyWith({
     AssetPathProvider? assetPathProvider,
     FilesystemCache? cache,
   });

build/lib/src/state/reader_writer.dart

@@ -0,0 +1,83 @@
+// Copyright (c) 2025, 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:convert';
+
+import 'package:crypto/crypto.dart';
+import 'package:glob/glob.dart';
+
+import '../asset/id.dart';
+import '../asset/reader.dart';
+import '../asset/writer.dart';
+import 'asset_finder.dart';
+import 'asset_path_provider.dart';
+import 'filesystem.dart';
+import 'filesystem_cache.dart';
+import 'reader_state.dart';
+
+/// An [AssetReader] and [AssetWriter].
+abstract interface class AssetReaderWriter
+    implements AssetReader, AssetWriter {}
+
+/// An [AssetReaderWriter] that delegates to an [AssetReader] and an
+/// [AssetWriter].
+///
+/// The main `package:build` readers and writers already implement
+/// `AssetReaderWriter`, this is used to support nonstandard readers/writers for
+/// testing.
+class DelegatingAssetReaderWriter
+    implements AssetReaderWriter, AssetReaderState {
+  final AssetReader reader;
+  final AssetWriter writer;
+
+  DelegatingAssetReaderWriter({required this.reader, required this.writer});
+
+  @override
+  AssetFinder get assetFinder => reader.assetFinder;
+
+  @override
+  AssetPathProvider get assetPathProvider => reader.assetPathProvider;
+
+  @override
+  FilesystemCache get cache => reader.cache;
+
+  @override
+  AssetReaderWriter copyWith({
+    AssetPathProvider? assetPathProvider,
+    FilesystemCache? cache,
+  }) => DelegatingAssetReaderWriter(
+    reader: reader.copyWith(assetPathProvider: assetPathProvider, cache: cache),
+    writer: writer,
+  );
+
+  @override
+  Filesystem get filesystem => reader.filesystem;
+
+  @override
+  Future<bool> canRead(AssetId id) => reader.canRead(id);
+
+  @override
+  Future<Digest> digest(AssetId id) => reader.digest(id);
+
+  @override
+  Stream<AssetId> findAssets(Glob glob) => reader.findAssets(glob);
+
+  @override
+  Future<List<int>> readAsBytes(AssetId id) => reader.readAsBytes(id);
+
+  @override
+  Future<String> readAsString(AssetId id, {Encoding encoding = utf8}) =>
+      reader.readAsString(id, encoding: encoding);
+
+  @override
+  Future<void> writeAsBytes(AssetId id, List<int> bytes) =>
+      writer.writeAsBytes(id, bytes);
+
+  @override
+  Future<void> writeAsString(
+    AssetId id,
+    String contents, {
+    Encoding encoding = utf8,
+  }) => writer.writeAsString(id, contents);
+}

build_runner_core/CHANGELOG.md

@@ -14,6 +14,8 @@
 - Refactor `FileBasedAssetReader` and `FileBasedAssetWriter` to `ReaderWriter`.
 - Move `BuildStepImpl` to `build_runner_core`, use `SingleStepReader` directly.
 - Remove `BuildCacheWriter`, functionality is handled by `AssetPathProvider`.
+- Refactor `SingleStepReader` to `SingleStepReaderWriter`, incorporating
+  `AssetWriterSpy` functionality.
 
 ## 8.0.0
 

build_runner_core/lib/src/asset/reader_writer.dart

@@ -18,7 +18,7 @@ import 'writer.dart';
 
 /// Pluggable [AssetReader] and [AssetWriter].
 class ReaderWriter extends AssetReader
-    implements AssetReaderState, RunnerAssetWriter {
+    implements AssetReaderState, RunnerAssetWriter, AssetReaderWriter {
   /// The package the generator is running for.
   ///
   /// Deletes are only allowed within this package.