add benchmark for ChangeNotifier.notifyListeners (#62715)

dev/benchmarks/microbenchmarks/lib/foundation/change_notifier_bench.dart

@@ -0,0 +1,116 @@
+// Copyright 2014 The Flutter Authors. 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:flutter/foundation.dart';
+
+import '../common.dart';
+
+const int _kNumIterations = 1000;
+const double _scale = 1000.0 / _kNumIterations;
+const int _kNumWarmUp = 100;
+
+void main() {
+  assert(false, "Don't run benchmarks in checked mode! Use 'flutter run --release'.");
+
+  void listener() {}
+  void listener2() {}
+  void listener3() {}
+  void listener4() {}
+  void listener5() {}
+
+  // Warm up lap
+  for (int i = 0; i < _kNumWarmUp; i += 1) {
+    _Notifier()
+      ..addListener(listener)
+      ..addListener(listener2)
+      ..addListener(listener3)
+      ..addListener(listener4)
+      ..addListener(listener5)
+      ..notify()
+      ..removeListener(listener)
+      ..removeListener(listener2)
+      ..removeListener(listener3)
+      ..removeListener(listener4)
+      ..removeListener(listener5);
+  }
+
+  final Stopwatch addListenerWatch = Stopwatch();
+  final Stopwatch removeListenerWatch = Stopwatch();
+  final Stopwatch notifyListenersWatch = Stopwatch();
+  final BenchmarkResultPrinter printer = BenchmarkResultPrinter();
+
+  for (int listenersCount = 0; listenersCount <= 5; listenersCount++) {
+
+    for (int j = 0; j < _kNumIterations; j += 1) {
+      final _Notifier notifier = _Notifier();
+      addListenerWatch.start();
+
+      notifier.addListener(listener);
+      if (listenersCount > 1)
+        notifier.addListener(listener2);
+      if (listenersCount > 2)
+        notifier.addListener(listener3);
+      if (listenersCount > 3)
+        notifier.addListener(listener4);
+      if (listenersCount > 4)
+        notifier.addListener(listener5);
+
+      addListenerWatch.stop();
+      notifyListenersWatch.start();
+
+      notifier.notify();
+
+      notifyListenersWatch.stop();
+      removeListenerWatch.start();
+
+      // Remove listeners in reverse order to evaluate the worse-case scenario:
+      // the listener removed is the last listener
+      if (listenersCount > 4)
+        notifier.removeListener(listener5);
+      if (listenersCount > 3)
+        notifier.removeListener(listener4);
+      if (listenersCount > 2)
+        notifier.removeListener(listener3);
+      if (listenersCount > 1)
+        notifier.removeListener(listener2);
+      notifier.removeListener(listener);
+
+      removeListenerWatch.stop();
+    }
+
+    final int notifyListener = notifyListenersWatch.elapsedMicroseconds;
+    notifyListenersWatch.reset();
+    final int addListenerElapsed = addListenerWatch.elapsedMicroseconds;
+    addListenerWatch.reset();
+    final int removeListenerElapsed = removeListenerWatch.elapsedMicroseconds;
+    removeListenerWatch.reset();
+
+    printer.addResult(
+      description: 'addListener ($listenersCount listeners)',
+      value: addListenerElapsed * _scale,
+      unit: 'ns per iteration',
+      name: 'addListener${listenersCount}_iteration',
+    );
+
+    printer.addResult(
+      description: 'removeListener ($listenersCount listeners)',
+      value: removeListenerElapsed * _scale,
+      unit: 'ns per iteration',
+      name: 'removeListener${listenersCount}_iteration',
+    );
+
+    printer.addResult(
+      description: 'notifyListener ($listenersCount listeners)',
+      value: notifyListener * _scale,
+      unit: 'ns per iteration',
+      name: 'notifyListener${listenersCount}_iteration',
+    );
+  }
+
+  printer.printToStdout();
+}
+
+class _Notifier extends ChangeNotifier {
+  void notify() => notifyListeners();
+}

dev/devicelab/lib/tasks/microbenchmarks.dart

@@ -54,7 +54,8 @@ TaskFunction createMicrobenchmarkTask() {
       ...await _runMicrobench('lib/stocks/animation_bench.dart'),
       ...await _runMicrobench('lib/language/sync_star_bench.dart'),
       ...await _runMicrobench('lib/language/sync_star_semantics_bench.dart'),
-    };
+      ...await _runMicrobench('lib/foundation/change_notifier_bench.dart'),
+ };
 
     return TaskResult.success(allResults, benchmarkScoreKeys: allResults.keys.toList());
   };