Delete via AssetWriter in AssetGraphLoader so it's testable.

_test_common/lib/runner_asset_writer_spy.dart

@@ -16,11 +16,14 @@ class RunnerAssetWriterSpy extends AssetWriterSpy implements RunnerAssetWriter {
   RunnerAssetWriterSpy(this._delegate) : super(_delegate);
 
   @override
-  Future delete(AssetId id) {
+  Future<void> delete(AssetId id) {
     _assetsDeleted.add(id);
     return _delegate.delete(id);
   }
 
+  @override
+  Future<void> deleteDirectory(AssetId id) => _delegate.deleteDirectory(id);
+
   @override
   Future<void> completeBuild() async {}
 }

build/lib/src/state/filesystem.dart

@@ -41,6 +41,11 @@ abstract interface class Filesystem {
   /// If the file does not exist, does nothing.
   void deleteSync(String path);
 
+  /// Deletes a directory recursively.
+  ///
+  /// If the directory does not exist, does nothing.
+  Future<void> deleteDirectory(String path);
+
   /// Writes a file.
   ///
   /// Creates enclosing directories as needed if they don't exist.
@@ -110,6 +115,14 @@ class IoFilesystem implements Filesystem {
     });
   }
 
+  @override
+  Future<void> deleteDirectory(String path) {
+    return _pool.withResource(() async {
+      final directory = Directory(path);
+      if (await directory.exists()) await directory.delete(recursive: true);
+    });
+  }
+
   @override
   void writeAsBytesSync(
     String path,
@@ -191,6 +204,13 @@ class InMemoryFilesystem implements Filesystem {
   @override
   void deleteSync(String path) => _files.remove(path);
 
+  @override
+  Future<void> deleteDirectory(String path) {
+    final prefix = '$path/';
+    _files.removeWhere((filePath, _) => filePath.startsWith(prefix));
+    return Future.value();
+  }
+
   @override
   void writeAsBytesSync(String path, List<int> contents) {
     _files[path] = Uint8List.fromList(contents);

build_runner_core/CHANGELOG.md

@@ -2,6 +2,8 @@
 
 - Breaking: refactor `OverridableEnvironment` and `IOEnvironment` into
   `BuildEnvironment`
+- Breaking: add `deleteDirectory` to `RunnerAssetWriter`, make `delete`
+  return `Future<void>`, remove deprecated `OnDelete`.
 - Bump the min SDK to 3.7.0.
 - Fix crash when running on assets ending in a dot.
 - Start using `package:build/src/internal.dart'.

build_runner_core/lib/src/asset/batch.dart

@@ -154,4 +154,7 @@ final class BatchWriter extends RunnerAssetWriter {
   Future<void> completeBuild() async {
     await _batch.completeWrites(_inner);
   }
+
+  @override
+  Future<void> deleteDirectory(AssetId id) => _inner.deleteDirectory(id);
 }

build_runner_core/lib/src/asset/reader_writer.dart

@@ -163,6 +163,12 @@ class ReaderWriter extends AssetReader
     await filesystem.delete(path);
   }
 
+  @override
+  Future<void> deleteDirectory(AssetId id) async {
+    final path = _pathFor(id);
+    await filesystem.deleteDirectory(path);
+  }
+
   @override
   Future<void> completeBuild() async {
     // TODO(davidmorgan): add back write caching, "batching".

build_runner_core/lib/src/asset/writer.dart

@@ -6,11 +6,17 @@ import 'dart:async';
 
 import 'package:build/build.dart';
 
-@Deprecated('No longer used')
-typedef OnDelete = void Function(AssetId id);
-
 abstract class RunnerAssetWriter implements AssetWriter {
-  Future delete(AssetId id);
+  /// Delete [id].
+  Future<void> delete(AssetId id);
+
+  /// Delete the directory [id] recursively.
+  ///
+  /// Usually an `AssetId` points to a file, but here its `path` is a directory.
+  ///
+  /// Delete unconditionally and recursively: if the directory does not exist,
+  /// do nothing.
+  Future<void> deleteDirectory(AssetId id);
 
   /// Called after each completed build.
   ///

build_runner_core/lib/src/asset_graph/graph_loader.dart

@@ -10,7 +10,6 @@ import 'package:build/experiments.dart';
 import 'package:built_collection/built_collection.dart';
 // ignore: implementation_imports
 import 'package:logging/logging.dart';
-import 'package:path/path.dart' as p;
 
 import '../asset/writer.dart';
 import '../asset_graph/exceptions.dart';
@@ -66,7 +65,7 @@ class AssetGraphLoader {
           'version mismatch or corrupted asset graph.',
         );
         await Future.wait([
-          _deleteGeneratedDir(),
+          writer.deleteDirectory(_generatedOutputDirectoryId),
           FailureReporter.cleanErrorCache(),
         ]);
         return null;
@@ -109,9 +108,9 @@ class AssetGraphLoader {
         );
       }
       await Future.wait([
-        _deleteAssetGraph(packageGraph),
+        writer.delete(assetGraphId),
         cachedGraph.deleteOutputs(packageGraph, writer),
-        _deleteGeneratedDir(),
+        writer.deleteDirectory(_generatedOutputDirectoryId),
         FailureReporter.cleanErrorCache(),
       ]);
       if (_runningFromSnapshot) {
@@ -124,9 +123,9 @@ class AssetGraphLoader {
         'Throwing away cached asset graph due to Dart SDK update.',
       );
       await Future.wait([
-        _deleteAssetGraph(packageGraph),
+        writer.delete(assetGraphId),
         cachedGraph.deleteOutputs(packageGraph, writer),
-        _deleteGeneratedDir(),
+        writer.deleteDirectory(_generatedOutputDirectoryId),
         FailureReporter.cleanErrorCache(),
       ]);
       if (_runningFromSnapshot) {
@@ -136,16 +135,9 @@ class AssetGraphLoader {
     }
     return cachedGraph;
   }
-}
-
-Future<void> _deleteAssetGraph(PackageGraph packageGraph) =>
-    File(p.join(packageGraph.root.path, assetGraphPath)).delete();
 
-Future<void> _deleteGeneratedDir() async {
-  var generatedDir = Directory(generatedOutputDirectory);
-  if (await generatedDir.exists()) {
-    await generatedDir.delete(recursive: true);
-  }
+  AssetId get _generatedOutputDirectoryId =>
+      AssetId(packageGraph.root.name, generatedOutputDirectory);
 }
 
 bool get _runningFromSnapshot => !Platform.script.path.endsWith('.dart');