Hacking.

Author: davidmorgan

build_runner/lib/src/compiler/external_build_step.dart

@@ -0,0 +1,50 @@
+// 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 'dart:io';
+
+import 'package:convert/convert.dart';
+import 'package:crypto/crypto.dart';
+
+class ExternalBuildStep {
+  List<String> inputPaths;
+  String outputPath;
+
+  ExternalBuildStep({required this.inputPaths, required this.outputPath});
+
+  String get depsPath => '$outputPath.deps';
+  String get digestsPath => '$outputPath.digests';
+
+  bool needsToRun() {
+    if (!File(depsPath).existsSync()) return true;
+    if (!File(outputPath).existsSync()) return true;
+    if (!File(digestsPath).existsSync()) return true;
+
+    final digests = computeDigests();
+    final oldDigests = File(digestsPath).readAsStringSync();
+    return digests != oldDigests;
+  }
+
+  void stamp() {
+    // TODO(davidmorgan): spaces.
+    File(depsPath).writeAsStringSync('$outputPath: ${inputPaths.join(' ')}');
+    File(digestsPath).writeAsStringSync(computeDigests());
+  }
+
+  String computeDigests() => '''
+inputs digest: ${_computeDigest(inputPaths)}
+output digest: ${_computeDigest([outputPath])}
+''';
+
+  String _computeDigest(Iterable<String> deps) {
+    final digestSink = AccumulatorSink<Digest>();
+    final result = md5.startChunkedConversion(digestSink);
+    for (final dep in deps) {
+      result.add(File(dep).readAsBytesSync());
+    }
+    result.close();
+    return base64.encode(digestSink.events.first.bytes);
+  }
+}