Add communication logging to Flutter Driver (#7485)

This is handy for debugging a test. Communication is logged to:
* `flutter_driver_commands_{x}.log`, where {x} is an integer, and
* (if requested by user) to stdout

fixes #7473

packages/flutter_driver/lib/src/driver.dart

@@ -4,20 +4,23 @@
 
 import 'dart:async';
 import 'dart:convert';
+import 'package:file/file.dart' as f;
 import 'dart:io';
 import 'package:json_rpc_2/json_rpc_2.dart' as rpc;
 import 'package:meta/meta.dart';
+import 'package:path/path.dart' as p;
 import 'package:vm_service_client/vm_service_client.dart';
 import 'package:web_socket_channel/io.dart';
 
+import 'common.dart';
 import 'error.dart';
 import 'find.dart';
+import 'frame_sync.dart';
 import 'gesture.dart';
 import 'health.dart';
 import 'input.dart';
 import 'message.dart';
 import 'render_tree.dart';
-import 'frame_sync.dart';
 import 'timeline.dart';
 
 /// Timeline stream identifier.
@@ -94,13 +97,19 @@ class FlutterDriver {
   /// Creates a driver that uses a connection provided by the given
   /// [_serviceClient], [_peer] and [_appIsolate].
   @visibleForTesting
-  FlutterDriver.connectedTo(this._serviceClient, this._peer, this._appIsolate);
+  FlutterDriver.connectedTo(this._serviceClient, this._peer, this._appIsolate,
+      { bool printCommunication: false, bool logCommunicationToFile: true })
+      : _printCommunication = printCommunication,
+        _logCommunicationToFile = logCommunicationToFile,
+        _driverId = _nextDriverId++;
 
   static const String _kFlutterExtensionMethod = 'ext.flutter.driver';
   static const String _kSetVMTimelineFlagsMethod = '_setVMTimelineFlags';
   static const String _kGetVMTimelineMethod = '_getVMTimeline';
   static const Duration _kRpcGraceTime = const Duration(seconds: 2);
 
+  static int _nextDriverId = 0;
+
   /// Connects to a Flutter application.
   ///
   /// Resumes the application if it is currently paused (e.g. at a breakpoint).
@@ -108,7 +117,15 @@ class FlutterDriver {
   /// [dartVmServiceUrl] is the URL to Dart observatory (a.k.a. VM service). If
   /// not specified, the URL specified by the `VM_SERVICE_URL` environment
   /// variable is used, or 'http://localhost:8183'.
-  static Future<FlutterDriver> connect({String dartVmServiceUrl}) async {
+  ///
+  /// [printCommunication] determines whether the command communication between
+  /// the test and the app should be printed to stdout.
+  ///
+  /// [logCommunicationToFile] determines whether the command communication
+  /// between the test and the app should be logged to `flutter_driver_commands.log`.
+  static Future<FlutterDriver> connect({ String dartVmServiceUrl,
+                                         bool printCommunication: false,
+                                         bool logCommunicationToFile: true }) async {
     dartVmServiceUrl ??= Platform.environment['VM_SERVICE_URL'];
     dartVmServiceUrl ??= 'http://localhost:8183';
 
@@ -137,7 +154,11 @@ class FlutterDriver {
       isolate = await vm.isolates.first.loadRunnable();
     }
 
-    FlutterDriver driver = new FlutterDriver.connectedTo(client, connection.peer, isolate);
+    FlutterDriver driver = new FlutterDriver.connectedTo(
+        client, connection.peer, isolate,
+        printCommunication: printCommunication,
+        logCommunicationToFile: logCommunicationToFile
+    );
 
     // Attempts to resume the isolate, but does not crash if it fails because
     // the isolate is already resumed. There could be a race with other tools,
@@ -220,19 +241,28 @@ class FlutterDriver {
     return driver;
   }
 
+  /// The unique ID of this driver instance.
+  final int _driverId;
   /// Client connected to the Dart VM running the Flutter application
   final VMServiceClient _serviceClient;
   /// JSON-RPC client useful for sending raw JSON requests.
   final rpc.Peer _peer;
   /// The main isolate hosting the Flutter application
   final VMIsolateRef _appIsolate;
+  /// Whether to print communication between host and app to `stdout`.
+  final bool _printCommunication;
+  /// Whether to log communication between host and app to `flutter_driver_commands.log`.
+  final bool _logCommunicationToFile;
 
   Future<Map<String, dynamic>> _sendCommand(Command command) async {
     Map<String, dynamic> response;
     try {
-       response = await _appIsolate
-          .invokeExtension(_kFlutterExtensionMethod, command.serialize())
+      Map<String, String> serialized = command.serialize();
+      _logCommunication('>>> $serialized');
+      response = await _appIsolate
+          .invokeExtension(_kFlutterExtensionMethod, serialized)
           .timeout(command.timeout + _kRpcGraceTime);
+      _logCommunication('<<< $response');
     } on TimeoutException catch (error, stackTrace) {
       throw new DriverError(
         'Failed to fulfill ${command.runtimeType}: Flutter application not responding',
@@ -251,6 +281,16 @@ class FlutterDriver {
     return response['response'];
   }
 
+  void _logCommunication(String message)  {
+    if (_printCommunication)
+      _log.info(message);
+    if (_logCommunicationToFile) {
+      f.File file = fs.file(p.join(testOutputsDirectory, 'flutter_driver_commands_${_driverId}.log'));
+      file.createSync(recursive: true); // no-op if file exists
+      file.writeAsStringSync('${new DateTime.now()} $message\n', mode: f.FileMode.APPEND, flush: true);
+    }
+  }
+
   /// Checks the status of the Flutter Driver extension.
   Future<Health> checkHealth() async {
     return Health.fromJson(await _sendCommand(new GetHealth()));