clear timeline events prior to starting a new action (#12984)

* clear timeline events prior to starting a new action

* trailing commas

packages/flutter_driver/lib/src/driver.dart

@@ -127,6 +127,7 @@ class FlutterDriver {
   static const String _kFlutterExtensionMethod = 'ext.flutter.driver';
   static const String _kSetVMTimelineFlagsMethod = '_setVMTimelineFlags';
   static const String _kGetVMTimelineMethod = '_getVMTimeline';
+  static const String _kClearVMTimelineMethod = '_clearVMTimeline';
 
   static int _nextDriverId = 0;
 
@@ -184,7 +185,7 @@ class FlutterDriver {
     final FlutterDriver driver = new FlutterDriver.connectedTo(
       client, connection.peer, isolate,
       printCommunication: printCommunication,
-        logCommunicationToFile: logCommunicationToFile
+      logCommunicationToFile: logCommunicationToFile,
     );
 
     // Attempts to resume the isolate, but does not crash if it fails because
@@ -327,13 +328,13 @@ class FlutterDriver {
       throw new DriverError(
         'Failed to fulfill ${command.runtimeType}: Flutter application not responding',
         error,
-        stackTrace
+        stackTrace,
       );
     } catch (error, stackTrace) {
       throw new DriverError(
         'Failed to fulfill ${command.runtimeType} due to remote error',
         error,
-        stackTrace
+        stackTrace,
       );
     }
     if (response['isError'])
@@ -503,7 +504,10 @@ class FlutterDriver {
   }
 
   /// Starts recording performance traces.
-  Future<Null> startTracing({ List<TimelineStream> streams: _defaultStreams, Duration timeout: _kShortTimeout }) async {
+  Future<Null> startTracing({
+    List<TimelineStream> streams: _defaultStreams,
+    Duration timeout: _kShortTimeout,
+  }) async {
     assert(streams != null && streams.isNotEmpty);
     try {
       await _peer.sendRequest(_kSetVMTimelineFlagsMethod, <String, String>{
@@ -514,7 +518,7 @@ class FlutterDriver {
       throw new DriverError(
         'Failed to start tracing due to remote error',
         error,
-        stackTrace
+        stackTrace,
       );
     }
   }
@@ -530,7 +534,7 @@ class FlutterDriver {
       throw new DriverError(
         'Failed to stop tracing due to remote error',
         error,
-        stackTrace
+        stackTrace,
       );
     }
   }
@@ -545,12 +549,38 @@ class FlutterDriver {
   ///
   /// [streams] limits the recorded timeline event streams to only the ones
   /// listed. By default, all streams are recorded.
-  Future<Timeline> traceAction(Future<dynamic> action(), { List<TimelineStream> streams: _defaultStreams }) async {
+  ///
+  /// If [retainPriorEvents] is true, retains events recorded prior to calling
+  /// [action]. Otherwise, prior events are cleared before calling [action]. By
+  /// default, prior events are cleared.
+  Future<Timeline> traceAction(
+    Future<dynamic> action(), {
+    List<TimelineStream> streams: _defaultStreams,
+    bool retainPriorEvents: false,
+  }) async {
+    if (!retainPriorEvents) {
+      await clearTimeline();
+    }
     await startTracing(streams: streams);
     await action();
     return stopTracingAndDownloadTimeline();
   }
 
+  /// Clears all timeline events recorded up until now.
+  Future<Null> clearTimeline({ Duration timeout: _kShortTimeout }) async {
+    try {
+      await _peer
+          .sendRequest(_kClearVMTimelineMethod, <String, String>{})
+          .timeout(timeout);
+    } catch(error, stackTrace) {
+      throw new DriverError(
+        'Failed to clear event timeline due to remote error',
+        error,
+        stackTrace,
+      );
+    }
+  }
+
   /// [action] will be executed with the frame sync mechanism disabled.
   ///
   /// By default, Flutter Driver waits until there is no pending frame scheduled

packages/flutter_driver/test/flutter_driver_test.dart

@@ -230,25 +230,45 @@ void main() {
       });
     });
 
+    group('clearTimeline', () {
+      test('clears timeline', () async {
+        bool clearWasCalled = false;
+        when(mockPeer.sendRequest('_clearVMTimeline', argThat(equals(<String, dynamic>{}))))
+            .thenAnswer((_) async {
+          clearWasCalled = true;
+          return null;
+        });
+        await driver.clearTimeline();
+        expect(clearWasCalled, isTrue);
+      });
+    });
+
     group('traceAction', () {
-      test('traces action', () async {
-        bool actionCalled = false;
-        bool startTracingCalled = false;
-        bool stopTracingCalled = false;
+      List<String> log;
+
+      setUp(() async {
+        log = <String>[];
+
+        when(mockPeer.sendRequest('_clearVMTimeline', argThat(equals(<String, dynamic>{}))))
+            .thenAnswer((_) async {
+          log.add('clear');
+          return null;
+        });
 
         when(mockPeer.sendRequest('_setVMTimelineFlags', argThat(equals(<String, dynamic>{'recordedStreams': '[all]'}))))
             .thenAnswer((_) async {
-            startTracingCalled = true;
+          log.add('startTracing');
           return null;
         });
 
         when(mockPeer.sendRequest('_setVMTimelineFlags', argThat(equals(<String, dynamic>{'recordedStreams': '[]'}))))
             .thenAnswer((_) async {
-            stopTracingCalled = true;
+          log.add('stopTracing');
           return null;
         });
 
         when(mockPeer.sendRequest('_getVMTimeline')).thenAnswer((_) async {
+          log.add('download');
           return <String, dynamic> {
             'traceEvents': <dynamic>[
               <String, String>{
@@ -257,14 +277,34 @@ void main() {
             ],
           };
         });
+      });
 
+      test('without clearing timeline', () async {
         final Timeline timeline = await driver.traceAction(() {
-          actionCalled = true;
+          log.add('action');
+        }, retainPriorEvents: true);
+
+        expect(log, const <String>[
+          'startTracing',
+          'action',
+          'stopTracing',
+          'download',
+        ]);
+        expect(timeline.events.single.name, 'test event');
       });
 
-        expect(actionCalled, isTrue);
-        expect(startTracingCalled, isTrue);
-        expect(stopTracingCalled, isTrue);
+      test('with clearing timeline', () async {
+        final Timeline timeline = await driver.traceAction(() {
+          log.add('action');
+        });
+
+        expect(log, const <String>[
+          'clear',
+          'startTracing',
+          'action',
+          'stopTracing',
+          'download',
+        ]);
         expect(timeline.events.single.name, 'test event');
       });
     });
@@ -304,7 +344,8 @@ void main() {
           TimelineStream.dart,
           TimelineStream.gc,
           TimelineStream.compiler
-        ]);
+        ],
+        retainPriorEvents: true);
 
         expect(actionCalled, isTrue);
         expect(startTracingCalled, isTrue);