flutter/packages/flutter_tools/lib/src/tracing.dart

// Copyright 2015 The Chromium 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 'dart:async';

import 'base/file_system.dart';
import 'base/utils.dart';
import 'build_info.dart';
import 'globals.dart';
import 'vmservice.dart';

// Names of some of the Timeline events we care about.
const String _kFlutterEngineMainEnterEventName = 'FlutterEngineMainEnter';
const String _kFrameworkInitEventName = 'Framework initialization';
const String _kFirstUsefulFrameEventName = 'Widgets completed first useful frame';

class Tracing {
  Tracing(this.vmService);

  static Future<Tracing> connect(Uri uri) async {
    final VMService observatory = await VMService.connect(uri);
    return new Tracing(observatory);
  }

  final VMService vmService;

  Future<Null> startTracing() async {
    await vmService.vm.setVMTimelineFlags(<String>['Compiler', 'Dart', 'Embedder', 'GC']);
    await vmService.vm.clearVMTimeline();
  }

  /// Stops tracing; optionally wait for first frame.
  Future<Map<String, dynamic>> stopTracingAndDownloadTimeline({
    bool waitForFirstFrame: false
  }) async {
    Map<String, dynamic> timeline;

    if (!waitForFirstFrame) {
      // Stop tracing immediately and get the timeline
      await vmService.vm.setVMTimelineFlags(<String>[]);
      timeline = await vmService.vm.getVMTimeline();
    } else {
      final Completer<Null> whenFirstFrameRendered = new Completer<Null>();

      vmService.onTimelineEvent.listen((ServiceEvent timelineEvent) {
        final List<Map<String, dynamic>> events = timelineEvent.timelineEvents;
        for (Map<String, dynamic> event in events) {
          if (event['name'] == _kFirstUsefulFrameEventName)
            whenFirstFrameRendered.complete();
        }
      });

      await whenFirstFrameRendered.future.timeout(
          const Duration(seconds: 10),
          onTimeout: () {
            printError(
                'Timed out waiting for the first frame event. Either the '
                    'application failed to start, or the event was missed because '
                    '"flutter run" took too long to subscribe to timeline events.'
            );
            return null;
          }
      );

      timeline = await vmService.vm.getVMTimeline();

      await vmService.vm.setVMTimelineFlags(<String>[]);
    }

    return timeline;
  }
}

/// Download the startup trace information from the given observatory client and
/// store it to build/start_up_info.json.
Future<Null> downloadStartupTrace(VMService observatory) async {
  final String traceInfoFilePath = fs.path.join(getBuildDirectory(), 'start_up_info.json');
  final File traceInfoFile = fs.file(traceInfoFilePath);

  // Delete old startup data, if any.
  if (await traceInfoFile.exists())
    await traceInfoFile.delete();

  // Create "build" directory, if missing.
  if (!(await traceInfoFile.parent.exists()))
    await traceInfoFile.parent.create();

  final Tracing tracing = new Tracing(observatory);

  final Map<String, dynamic> timeline = await tracing.stopTracingAndDownloadTimeline(
      waitForFirstFrame: true
  );

  int extractInstantEventTimestamp(String eventName) {
    final List<Map<String, dynamic>> events = timeline['traceEvents'];
    final Map<String, dynamic> event = events.firstWhere(
            (Map<String, dynamic> event) => event['name'] == eventName, orElse: () => null
    );
    return event == null ? null : event['ts'];
  }

  final int engineEnterTimestampMicros = extractInstantEventTimestamp(_kFlutterEngineMainEnterEventName);
  final int frameworkInitTimestampMicros = extractInstantEventTimestamp(_kFrameworkInitEventName);
  final int firstFrameTimestampMicros = extractInstantEventTimestamp(_kFirstUsefulFrameEventName);

  if (engineEnterTimestampMicros == null) {
    printTrace('Engine start event is missing in the timeline: $timeline');
    throw 'Engine start event is missing in the timeline. Cannot compute startup time.';
  }

  if (firstFrameTimestampMicros == null) {
    printTrace('First frame event is missing in the timeline: $timeline');
    throw 'First frame event is missing in the timeline. Cannot compute startup time.';
  }

  final int timeToFirstFrameMicros = firstFrameTimestampMicros - engineEnterTimestampMicros;
  final Map<String, dynamic> traceInfo = <String, dynamic>{
    'engineEnterTimestampMicros': engineEnterTimestampMicros,
    'timeToFirstFrameMicros': timeToFirstFrameMicros,
  };

  if (frameworkInitTimestampMicros != null) {
    traceInfo['timeToFrameworkInitMicros'] = frameworkInitTimestampMicros - engineEnterTimestampMicros;
    traceInfo['timeAfterFrameworkInitMicros'] = firstFrameTimestampMicros - frameworkInitTimestampMicros;
  }

  traceInfoFile.writeAsStringSync(toPrettyJson(traceInfo));

  printStatus('Time to first frame: ${timeToFirstFrameMicros ~/ 1000}ms.');
  printStatus('Saved startup trace info in ${traceInfoFile.path}.');
}