Stop passing reader and writer via BuildDefinition. (#3922)

Refactor `BuildCacheAssetProvider` to `GeneratedFileHider`.

Author: davidmorgan

build/lib/src/internal.dart

@@ -10,5 +10,6 @@ export 'state/asset_finder.dart';
 export 'state/asset_path_provider.dart';
 export 'state/filesystem.dart';
 export 'state/filesystem_cache.dart';
+export 'state/generated_asset_hider.dart';
 export 'state/reader_state.dart';
 export 'state/reader_writer.dart';

build/lib/src/state/generated_asset_hider.dart

@@ -0,0 +1,22 @@
+// 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 '../asset/id.dart';
+
+/// Optionally modifies the storage location of assets.
+///
+/// `AssetGraph` provides an implementation of this interface that moves
+/// "hidden" generated files to the "build cache" path under `rootPackage`.
+abstract interface class GeneratedAssetHider {
+  /// Returns [id] or an updated [id].
+  AssetId maybeHide(AssetId id, String rootPackage);
+}
+
+/// [GeneratedAssetHider] that does nothing.
+class NoopGeneratedAssetHider implements GeneratedAssetHider {
+  const NoopGeneratedAssetHider();
+
+  @override
+  AssetId maybeHide(AssetId id, String _) => id;
+}

build/lib/src/state/reader_state.dart

@@ -2,27 +2,46 @@
 // 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 'package:build_runner_core/build_runner_core.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 'generated_asset_hider.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].
+  /// Returns a new instance with optionally updated [cache] and/or [generatedAssetHider].
   AssetReaderWriter copyWith({
-    AssetPathProvider? assetPathProvider,
     FilesystemCache? cache,
+    GeneratedAssetHider? generatedAssetHider,
   }) {
     _requireIsAssetReaderState();
     return (this as AssetReaderState).copyWith(
-      assetPathProvider: assetPathProvider,
       cache: cache,
+      generatedAssetHider: generatedAssetHider,
     );
   }
 
+  AssetFinder get assetFinder {
+    _requireIsAssetReaderState();
+    return (this as AssetReaderState).assetFinder;
+  }
+
+  AssetPathProvider get assetPathProvider {
+    _requireIsAssetReaderState();
+    return (this as AssetReaderState).assetPathProvider;
+  }
+
+  GeneratedAssetHider get generatedAssetHider {
+    _requireIsAssetReaderState();
+    return (this as AssetReaderState).generatedAssetHider;
+  }
+
   Filesystem get filesystem {
     _requireIsAssetReaderState();
     return (this as AssetReaderState).filesystem;
@@ -33,14 +52,25 @@ extension AssetReaderStateExtension on AssetReader {
     return (this as AssetReaderState).cache;
   }
 
-  AssetFinder get assetFinder {
-    _requireIsAssetReaderState();
-    return (this as AssetReaderState).assetFinder;
+  /// Throws if `this` is not an [AssetReaderState].
+  void _requireIsAssetReaderState() {
+    if (this is! AssetReaderState) {
+      throw StateError(
+        '`AssetReader` must implement `AssetReaderState`: $this',
+      );
+    }
   }
+}
 
-  AssetPathProvider get assetPathProvider {
+/// Provides access to the state backing an [AssetWriter].
+extension AssetWriterStateExtension on RunnerAssetWriter {
+  /// Returns a new instance with optionally updated [generatedAssetHider].
+  RunnerAssetWriter copyWith({GeneratedAssetHider? generatedAssetHider}) {
     _requireIsAssetReaderState();
-    return (this as AssetReaderState).assetPathProvider;
+    return (this as AssetReaderState).copyWith(
+          generatedAssetHider: generatedAssetHider,
+        )
+        as RunnerAssetWriter;
   }
 
   /// Throws if `this` is not an [AssetReaderState].
@@ -55,21 +85,12 @@ extension AssetReaderStateExtension on AssetReader {
 
 /// The state backing an [AssetReader].
 abstract interface class AssetReaderState {
-  /// Returns a new instance with optionally updated [assetPathProvider] and/or [cache].
+  /// Returns a new instance with optionally updated [cache] and/or [generatedAssetHider].
   AssetReaderWriter copyWith({
-    AssetPathProvider? assetPathProvider,
     FilesystemCache? cache,
+    GeneratedAssetHider? generatedAssetHider,
   });
 
-  /// The [Filesystem] that this reader reads from.
-  ///
-  /// Warning: this access to the filesystem bypasses reader functionality
-  /// such as read tracking, caching and visibility restriction.
-  Filesystem get filesystem;
-
-  /// The [FilesystemCache] that this reader uses for caching.
-  FilesystemCache get cache;
-
   /// The [AssetFinder] associated with this reader.
   ///
   /// All readers have an [AssetFinder], but the functionality it provides,
@@ -78,4 +99,16 @@ abstract interface class AssetReaderState {
 
   /// The [AssetPathProvider] associated with this reader.
   AssetPathProvider get assetPathProvider;
+
+  /// The [GeneratedAssetHider] associated with this reader.
+  GeneratedAssetHider get generatedAssetHider;
+
+  /// The [Filesystem] that this reader reads from.
+  ///
+  /// Warning: this access to the filesystem bypasses reader functionality
+  /// such as read tracking, caching and visibility restriction.
+  Filesystem get filesystem;
+
+  /// The [FilesystemCache] that this reader uses for caching.
+  FilesystemCache get cache;
 }

build/lib/src/state/reader_writer.dart

@@ -14,6 +14,7 @@ import 'asset_finder.dart';
 import 'asset_path_provider.dart';
 import 'filesystem.dart';
 import 'filesystem_cache.dart';
+import 'generated_asset_hider.dart';
 import 'reader_state.dart';
 
 /// An [AssetReader] and [AssetWriter].
@@ -39,15 +40,21 @@ class DelegatingAssetReaderWriter
   @override
   AssetPathProvider get assetPathProvider => reader.assetPathProvider;
 
+  @override
+  GeneratedAssetHider get generatedAssetHider => reader.generatedAssetHider;
+
   @override
   FilesystemCache get cache => reader.cache;
 
   @override
   AssetReaderWriter copyWith({
-    AssetPathProvider? assetPathProvider,
     FilesystemCache? cache,
+    GeneratedAssetHider? generatedAssetHider,
   }) => DelegatingAssetReaderWriter(
-    reader: reader.copyWith(assetPathProvider: assetPathProvider, cache: cache),
+    reader: reader.copyWith(
+      cache: cache,
+      generatedAssetHider: generatedAssetHider,
+    ),
     writer: writer,
   );
 
@@ -80,4 +87,7 @@ class DelegatingAssetReaderWriter
     String contents, {
     Encoding encoding = utf8,
   }) => writer.writeAsString(id, contents);
+
+  // TODO(davidmorgan): fix interfaces/dependencies to improve this.
+  Future delete(AssetId id) => throw UnimplementedError();
 }

build_runner_core/CHANGELOG.md

@@ -12,7 +12,7 @@
 - Add internal `Filesystem` that backs `AssetReader` and `AssetWriter`
   implementations.
 - Refactor `CachingAssetReader` to `FilesystemCache`.
-- Refactor `BuildCacheReader` to `BuildCacheAssetPathProvider`.
+- Refactor `BuildCacheReader` to `GeneratedAssetHider`.
 - Refactor `FileBasedAssetReader` and `FileBasedAssetWriter` to `ReaderWriter`.
 - Move `BuildStepImpl` to `build_runner_core`, use `SingleStepReader` directly.
 - Remove `BuildCacheWriter`, functionality is handled by `AssetPathProvider`.

build_runner_core/lib/src/asset/build_cache.dart

@@ -24,12 +24,14 @@ class ReaderWriter extends AssetReader
   /// Deletes are only allowed within this package.
   final String rootPackage;
 
+  @override
+  final AssetFinder assetFinder;
   @override
   final AssetPathProvider assetPathProvider;
   @override
-  final Filesystem filesystem;
+  final GeneratedAssetHider generatedAssetHider;
   @override
-  final AssetFinder assetFinder;
+  final Filesystem filesystem;
   @override
   final FilesystemCache cache;
 
@@ -45,6 +47,7 @@ class ReaderWriter extends AssetReader
     rootPackage: packageGraph.root.name,
     assetFinder: PackageGraphAssetFinder(packageGraph),
     assetPathProvider: packageGraph,
+    generatedAssetHider: const NoopGeneratedAssetHider(),
     filesystem: IoFilesystem(),
     cache: const PassthroughFilesystemCache(),
     onDelete: null,
@@ -54,31 +57,36 @@ class ReaderWriter extends AssetReader
     required this.rootPackage,
     required this.assetFinder,
     required this.assetPathProvider,
+    required this.generatedAssetHider,
     required this.filesystem,
     required this.cache,
     required this.onDelete,
   });
 
   @override
   ReaderWriter copyWith({
-    AssetPathProvider? assetPathProvider,
     FilesystemCache? cache,
+    GeneratedAssetHider? generatedAssetHider,
     void Function(AssetId)? onDelete,
   }) => ReaderWriter.using(
     rootPackage: rootPackage,
     assetFinder: assetFinder,
-    assetPathProvider: assetPathProvider ?? this.assetPathProvider,
+    assetPathProvider: assetPathProvider,
+    generatedAssetHider: generatedAssetHider ?? this.generatedAssetHider,
     filesystem: filesystem,
     cache: cache ?? this.cache,
     onDelete: onDelete ?? this.onDelete,
   );
 
+  String _pathFor(AssetId id) =>
+      assetPathProvider.pathFor(generatedAssetHider.maybeHide(id, rootPackage));
+
   @override
   Future<bool> canRead(AssetId id) {
     return cache.exists(
       id,
       ifAbsent: () async {
-        final path = assetPathProvider.pathFor(id);
+        final path = _pathFor(id);
         return filesystem.exists(path);
       },
     );
@@ -89,7 +97,7 @@ class ReaderWriter extends AssetReader
     return cache.readAsBytes(
       id,
       ifAbsent: () async {
-        final path = assetPathProvider.pathFor(id);
+        final path = _pathFor(id);
         if (!await filesystem.exists(path)) {
           throw AssetNotFoundException(id, path: path);
         }
@@ -104,7 +112,7 @@ class ReaderWriter extends AssetReader
       id,
       encoding: encoding,
       ifAbsent: () async {
-        final path = assetPathProvider.pathFor(id);
+        final path = _pathFor(id);
         if (!await filesystem.exists(path)) {
           throw AssetNotFoundException(id, path: path);
         }
@@ -121,7 +129,7 @@ class ReaderWriter extends AssetReader
 
   @override
   Future<void> writeAsBytes(AssetId id, List<int> bytes) async {
-    final path = assetPathProvider.pathFor(id);
+    final path = _pathFor(id);
     await filesystem.writeAsBytes(path, bytes);
   }
 
@@ -131,14 +139,14 @@ class ReaderWriter extends AssetReader
     String contents, {
     Encoding encoding = utf8,
   }) async {
-    final path = assetPathProvider.pathFor(id);
+    final path = _pathFor(id);
     await filesystem.writeAsString(path, contents, encoding: encoding);
   }
 
   @override
-  Future delete(AssetId id) async {
+  Future<void> delete(AssetId id) async {
     onDelete?.call(id);
-    final path = assetPathProvider.pathFor(id);
+    final path = _pathFor(id);
     // Hidden generated files are moved by `assetPathProvider` under the root
     // package folder, and it's allowed to delete them. So for assets in a
     // different package, check if the path has mapped onto the generated output