Refactor PathProvidingAssetReader to AssetPathProvider.

Author: davidmorgan

build/lib/src/builder/build_step_impl.dart

@@ -18,6 +18,7 @@ import '../asset/id.dart';
 import '../asset/reader.dart';
 import '../asset/writer.dart';
 import '../resource/resource.dart';
+import '../state/asset_path_provider.dart';
 import '../state/input_tracker.dart';
 import '../state/reader_state.dart';
 import 'build_step.dart';
@@ -80,6 +81,9 @@ class BuildStepImpl implements BuildStep, AssetReaderState {
         _stageTracker = stageTracker ?? NoOpStageTracker.instance,
         _reportUnusedAssets = reportUnusedAssets;
 
+  @override
+  AssetPathProvider? get assetPathProvider => _reader.assetPathProvider;
+
   @override
   InputTracker? get inputTracker => _reader.inputTracker;
 

build/lib/src/internal.dart

@@ -6,5 +6,6 @@
 /// `build_test` only.
 library;
 
+export 'state/asset_path_provider.dart';
 export 'state/input_tracker.dart';
 export 'state/reader_state.dart';

build/lib/src/state/asset_path_provider.dart

@@ -0,0 +1,21 @@
+// 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';
+
+/// Converts [AssetId] to paths.
+abstract interface class AssetPathProvider {
+  String pathFor(AssetId id);
+}
+
+/// Applies a function to an existing [AssetPathProvider].
+class OverlayAssetPathProvider implements AssetPathProvider {
+  AssetPathProvider delegate;
+  AssetId Function(AssetId) overlay;
+
+  OverlayAssetPathProvider({required this.delegate, required this.overlay});
+
+  @override
+  String pathFor(AssetId id) => delegate.pathFor(overlay(id));
+}

build/lib/src/state/reader_state.dart

@@ -3,6 +3,7 @@
 // BSD-style license that can be found in the LICENSE file.
 
 import '../asset/reader.dart';
+import 'asset_path_provider.dart';
 import 'input_tracker.dart';
 
 /// Provides access to the state backing an [AssetReader].
@@ -21,6 +22,10 @@ extension AssetReaderStateExtension on AssetReader {
     return result;
   }
 
+  AssetPathProvider? get assetPathProvider => this is AssetReaderState
+      ? (this as AssetReaderState).assetPathProvider
+      : null;
+
   /// Throws if `this` is not an [AssetReaderState].
   void _requireIsAssetReaderState() {
     if (this is! AssetReaderState) {
@@ -35,4 +40,8 @@ abstract interface class AssetReaderState {
   /// The [InputTracker] that this reader records reads to; or `null` if it does
   /// not have one.
   InputTracker? get inputTracker;
+
+  /// The [AssetPathProvider] associated with this reader, or `null` if it does
+  /// not have one.
+  AssetPathProvider? get assetPathProvider;
 }

build_runner_core/lib/src/asset/batch.dart

@@ -53,13 +53,12 @@ final class _FileSystemWriteBatch {
 /// they are flushed to the file system.
 (RunnerAssetReader, RunnerAssetWriter) wrapInBatch({
   required RunnerAssetReader reader,
-  required PathProvidingAssetReader pathProvidingReader,
   required RunnerAssetWriter writer,
 }) {
   final batch = _FileSystemWriteBatch._();
 
   return (
-    BatchReader(reader, pathProvidingReader, batch),
+    BatchReader(reader, batch),
     BatchWriter(writer, batch),
   );
 }
@@ -73,13 +72,11 @@ final class _PendingFileState {
 }
 
 @internal
-final class BatchReader extends AssetReader
-    implements RunnerAssetReader, PathProvidingAssetReader {
+final class BatchReader extends AssetReader implements RunnerAssetReader {
   final RunnerAssetReader _inner;
-  final PathProvidingAssetReader _innerPathProviding;
   final _FileSystemWriteBatch _batch;
 
-  BatchReader(this._inner, this._innerPathProviding, this._batch);
+  BatchReader(this._inner, this._batch);
 
   _PendingFileState? _stateFor(AssetId id) {
     return _batch._pendingWrites[id];
@@ -101,11 +98,6 @@ final class BatchReader extends AssetReader
         .where((asset) => _stateFor(asset)?.isDeleted != true);
   }
 
-  @override
-  String pathTo(AssetId id) {
-    return _innerPathProviding.pathTo(id);
-  }
-
   @override
   Future<List<int>> readAsBytes(AssetId id) async {
     if (_stateFor(id) case final state?) {

build_runner_core/lib/src/asset/build_cache.dart

@@ -2,29 +2,39 @@ import 'dart:async';
 import 'dart:convert';
 
 import 'package:build/build.dart';
+// ignore: implementation_imports
+import 'package:build/src/internal.dart';
 import 'package:crypto/crypto.dart';
 import 'package:glob/glob.dart';
 
 import '../asset_graph/graph.dart';
 import '../asset_graph/node.dart';
 import '../util/constants.dart';
-import 'reader.dart';
 import 'writer.dart';
 
 /// Wraps an [AssetReader] and translates reads for generated files into reads
 /// from the build cache directory
-class BuildCacheReader implements AssetReader {
-  final AssetGraph _assetGraph;
+class BuildCacheReader implements AssetReader, AssetReaderState {
+  @override
+  final AssetPathProvider? assetPathProvider;
+
   final AssetReader _delegate;
+  final AssetGraph _assetGraph;
   final String _rootPackage;
 
-  BuildCacheReader._(this._delegate, this._assetGraph, this._rootPackage);
+  BuildCacheReader(
+      AssetReader delegate, AssetGraph assetGraph, String rootPackage)
+      : _delegate = delegate,
+        _assetGraph = assetGraph,
+        _rootPackage = rootPackage,
+        assetPathProvider = delegate.assetPathProvider == null
+            ? null
+            : OverlayAssetPathProvider(
+                delegate: delegate.assetPathProvider!,
+                overlay: (id) => _cacheLocation(id, assetGraph, rootPackage));
 
-  factory BuildCacheReader(
-          AssetReader delegate, AssetGraph assetGraph, String rootPackage) =>
-      delegate is PathProvidingAssetReader
-          ? _PathProvidingBuildCacheReader._(delegate, assetGraph, rootPackage)
-          : BuildCacheReader._(delegate, assetGraph, rootPackage);
+  @override
+  InputTracker? get inputTracker => _delegate.inputTracker;
 
   @override
   Future<bool> canRead(AssetId id) =>
@@ -48,21 +58,6 @@ class BuildCacheReader implements AssetReader {
       'Asset globbing should be done per phase with the AssetGraph');
 }
 
-class _PathProvidingBuildCacheReader extends BuildCacheReader
-    implements PathProvidingAssetReader {
-  @override
-  PathProvidingAssetReader get _delegate =>
-      super._delegate as PathProvidingAssetReader;
-
-  _PathProvidingBuildCacheReader._(PathProvidingAssetReader super.delegate,
-      super.assetGraph, super.rootPackage)
-      : super._();
-
-  @override
-  String pathTo(AssetId id) =>
-      _delegate.pathTo(_cacheLocation(id, _assetGraph, _rootPackage));
-}
-
 class BuildCacheWriter implements RunnerAssetWriter {
   final AssetGraph _assetGraph;
   final RunnerAssetWriter _delegate;

build_runner_core/lib/src/asset/cache.dart

@@ -11,7 +11,6 @@ import 'package:crypto/crypto.dart';
 import 'package:glob/glob.dart';
 
 import 'lru_cache.dart';
-import 'reader.dart';
 
 /// An [AssetReader] that caches all results from the delegate.
 ///
@@ -46,12 +45,7 @@ class CachingAssetReader implements AssetReader {
 
   final AssetReader _delegate;
 
-  CachingAssetReader._(this._delegate);
-
-  factory CachingAssetReader(AssetReader delegate) =>
-      delegate is PathProvidingAssetReader
-          ? _PathProvidingCachingAssetReader._(delegate)
-          : CachingAssetReader._(delegate);
+  CachingAssetReader(this._delegate);
 
   @override
   Future<bool> canRead(AssetId id) =>
@@ -111,17 +105,3 @@ class CachingAssetReader implements AssetReader {
     }
   }
 }
-
-/// A version of a [CachingAssetReader] that implements
-/// [PathProvidingAssetReader].
-class _PathProvidingCachingAssetReader extends CachingAssetReader
-    implements PathProvidingAssetReader {
-  @override
-  PathProvidingAssetReader get _delegate =>
-      super._delegate as PathProvidingAssetReader;
-
-  _PathProvidingCachingAssetReader._(super.delegate) : super._();
-
-  @override
-  String pathTo(AssetId id) => _delegate.pathTo(id);
-}

build_runner_core/lib/src/asset/file_based.dart

@@ -20,8 +20,7 @@ final _descriptorPool = Pool(32);
 
 /// Basic [AssetReader] which uses a [PackageGraph] to look up where to read
 /// files from disk.
-class FileBasedAssetReader extends AssetReader
-    implements RunnerAssetReader, PathProvidingAssetReader {
+class FileBasedAssetReader extends AssetReader implements RunnerAssetReader {
   final PackageGraph packageGraph;
 
   FileBasedAssetReader(this.packageGraph);
@@ -55,9 +54,6 @@ class FileBasedAssetReader extends AssetReader
         .cast<File>()
         .map((file) => _fileToAssetId(file, packageNode));
   }
-
-  @override
-  String pathTo(AssetId id) => _filePathFor(id, packageGraph);
 }
 
 /// Creates an [AssetId] for [file], which is a part of [packageNode].
@@ -112,18 +108,9 @@ class FileBasedAssetWriter implements RunnerAssetWriter {
   Future<void> completeBuild() async {}
 }
 
-/// Returns the path to [id] for a given [packageGraph].
-String _filePathFor(AssetId id, PackageGraph packageGraph) {
-  var package = packageGraph[id.package];
-  if (package == null) {
-    throw PackageNotFoundException(id.package);
-  }
-  return path.join(package.path, id.path);
-}
-
 /// Returns a [File] for [id] given [packageGraph].
 File _fileFor(AssetId id, PackageGraph packageGraph) {
-  return File(_filePathFor(id, packageGraph));
+  return File(packageGraph.pathFor(id));
 }
 
 /// Returns a `File` for the asset reference by [id] given [packageGraph].

build_runner_core/lib/src/asset/reader.dart

@@ -19,11 +19,6 @@ import '../util/async.dart';
 /// A [RunnerAssetReader] must implement [MultiPackageAssetReader].
 abstract class RunnerAssetReader implements MultiPackageAssetReader {}
 
-/// An [AssetReader] that can provide actual paths to assets on disk.
-abstract class PathProvidingAssetReader implements AssetReader {
-  String pathTo(AssetId id);
-}
-
 /// Describes if and how a [SingleStepReader] should read an [AssetId].
 class Readability {
   final bool canRead;
@@ -80,6 +75,9 @@ class SingleStepReader implements AssetReader, AssetReaderState {
       this._primaryPackage, this._isReadableNode, this._checkInvalidInput,
       [this._getGlobNode, this._writtenAssets]);
 
+  @override
+  AssetPathProvider? get assetPathProvider => _delegate.assetPathProvider;
+
   /// Checks whether [id] can be read by this step - attempting to build the
   /// asset if necessary.
   ///

build_runner_core/lib/src/environment/create_merged_dir.dart

@@ -7,11 +7,12 @@ import 'dart:convert';
 import 'dart:io';
 
 import 'package:build/build.dart';
+// ignore: implementation_imports
+import 'package:build/src/internal.dart';
 import 'package:logging/logging.dart';
 import 'package:path/path.dart' as p;
 import 'package:pool/pool.dart';
 
-import '../asset/reader.dart';
 import '../environment/build_environment.dart';
 import '../generate/build_directory.dart';
 import '../generate/finalized_assets_view.dart';
@@ -35,7 +36,7 @@ Future<bool> createMergedOutputDirectories(
     AssetReader reader,
     FinalizedAssetsView finalizedAssetsView,
     bool outputSymlinksOnly) async {
-  if (outputSymlinksOnly && reader is! PathProvidingAssetReader) {
+  if (outputSymlinksOnly && reader.assetPathProvider == null) {
     _logger.severe(
         'The current environment does not support symlinks, but symlinks were '
         'requested.');
@@ -229,7 +230,7 @@ Future<AssetId> _writeAsset(
         await Link(_filePathFor(outputDir, outputId)).create(
             // We assert at the top of `createMergedOutputDirectories` that the
             // reader implements this type when requesting symlinks.
-            (reader as PathProvidingAssetReader).pathTo(id),
+            reader.assetPathProvider!.pathFor(id),
             recursive: true);
       } else {
         await _writeAsBytes(outputDir, outputId, await reader.readAsBytes(id));

build_runner_core/lib/src/environment/io_environment.dart

@@ -55,10 +55,7 @@ class IOEnvironment implements BuildEnvironment {
 
     var (reader, writer) = lowResourcesMode
         ? (fileReader, fileWriter)
-        : wrapInBatch(
-            reader: fileReader,
-            pathProvidingReader: fileReader,
-            writer: fileWriter);
+        : wrapInBatch(reader: fileReader, writer: fileWriter);
 
     return IOEnvironment._(reader, writer, assumeTty == true || _canPrompt(),
         outputSymlinksOnly, packageGraph);

build_runner_core/lib/src/package_graph/package_graph.dart

@@ -4,6 +4,9 @@
 
 import 'dart:io';
 
+import 'package:build/build.dart';
+// ignore: implementation_imports
+import 'package:build/src/internal.dart';
 import 'package:package_config/package_config.dart';
 import 'package:path/path.dart' as p;
 import 'package:yaml/yaml.dart';
@@ -16,7 +19,7 @@ final _sdkPackageNode =
     PackageNode(r'$sdk', sdkPath, DependencyType.hosted, null);
 
 /// A graph of the package dependencies for an application.
-class PackageGraph {
+class PackageGraph implements AssetPathProvider {
   /// The root application package.
   final PackageNode root;
 
@@ -167,6 +170,15 @@ class PackageGraph {
   /// Shorthand to get a package by name.
   PackageNode? operator [](String packageName) => allPackages[packageName];
 
+  @override
+  String pathFor(AssetId id) {
+    var package = this[id.package];
+    if (package == null) {
+      throw PackageNotFoundException(id.package);
+    }
+    return p.join(package.path, id.path);
+  }
+
   @override
   String toString() {
     var buffer = StringBuffer();

build_runner_core/test/asset/batch_test.dart

@@ -8,7 +8,6 @@ import 'dart:io';
 
 import 'package:build/build.dart';
 import 'package:build_runner_core/build_runner_core.dart';
-
 import 'package:glob/glob.dart';
 import 'package:package_config/package_config_types.dart';
 import 'package:test/test.dart';
@@ -24,10 +23,7 @@ void main() {
     final fileReader = FileBasedAssetReader(packageGraph);
     final fileWriter = FileBasedAssetWriter(packageGraph);
 
-    (reader, writer) = wrapInBatch(
-        reader: fileReader,
-        pathProvidingReader: fileReader,
-        writer: fileWriter);
+    (reader, writer) = wrapInBatch(reader: fileReader, writer: fileWriter);
   });
 
   test('delays writes until end', () async {