Add support for getting a string representation of the LayerTree (#47014)

packages/flutter_driver/lib/src/common/layer_tree.dart

@@ -0,0 +1,37 @@
+// 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 'message.dart';
+
+/// A Flutter Driver command that requests a string representation of the layer tree.
+class GetLayerTree extends Command {
+  /// Create a command to request a string representation of the layer tree.
+  const GetLayerTree({ Duration timeout }) : super(timeout: timeout);
+
+  /// Deserializes this command from the value generated by [serialize].
+  GetLayerTree.deserialize(Map<String, String> json) : super.deserialize(json);
+
+  @override
+  String get kind => 'get_layer_tree';
+}
+
+/// A string representation of the layer tree, the result of a
+/// [FlutterDriver.getLayerTree] method.
+class LayerTree extends Result {
+  /// Creates a [LayerTree] object with the given string representation.
+  const LayerTree(this.tree);
+
+  /// String representation of the layer tree.
+  final String tree;
+
+  /// Deserializes the result from JSON.
+  static LayerTree fromJson(Map<String, dynamic> json) {
+    return LayerTree(json['tree'] as String);
+  }
+
+  @override
+  Map<String, dynamic> toJson() => <String, dynamic>{
+    'tree': tree,
+  };
+}

packages/flutter_driver/lib/src/driver/driver.dart

@@ -23,6 +23,7 @@ import '../common/fuchsia_compat.dart';
 import '../common/geometry.dart';
 import '../common/gesture.dart';
 import '../common/health.dart';
+import '../common/layer_tree.dart';
 import '../common/message.dart';
 import '../common/render_tree.dart';
 import '../common/request_data.dart';
@@ -475,6 +476,11 @@ class FlutterDriver {
     return RenderTree.fromJson(await _sendCommand(GetRenderTree(timeout: timeout)));
   }
 
+  /// Returns a dump of the layer tree.
+  Future<LayerTree> getLayerTree({ Duration timeout }) async {
+    return LayerTree.fromJson(await _sendCommand(GetLayerTree(timeout: timeout)));
+  }
+
   /// Taps at the center of the widget located by [finder].
   Future<void> tap(SerializableFinder finder, { Duration timeout }) async {
     await _sendCommand(Tap(finder, timeout: timeout));

packages/flutter_driver/lib/src/extension/extension.dart

@@ -24,6 +24,7 @@ import '../common/frame_sync.dart';
 import '../common/geometry.dart';
 import '../common/gesture.dart';
 import '../common/health.dart';
+import '../common/layer_tree.dart';
 import '../common/message.dart';
 import '../common/render_tree.dart';
 import '../common/request_data.dart';
@@ -107,6 +108,7 @@ class FlutterDriverExtension {
 
     _commandHandlers.addAll(<String, CommandHandlerCallback>{
       'get_health': _getHealth,
+      'get_layer_tree': _getLayerTree,
       'get_render_tree': _getRenderTree,
       'enter_text': _enterText,
       'get_text': _getText,
@@ -130,6 +132,7 @@ class FlutterDriverExtension {
 
     _commandDeserializers.addAll(<String, CommandDeserializerCallback>{
       'get_health': (Map<String, String> params) => GetHealth.deserialize(params),
+      'get_layer_tree': (Map<String, String> params) => GetLayerTree.deserialize(params),
       'get_render_tree': (Map<String, String> params) => GetRenderTree.deserialize(params),
       'enter_text': (Map<String, String> params) => EnterText.deserialize(params),
       'get_text': (Map<String, String> params) => GetText.deserialize(params),
@@ -229,6 +232,10 @@ class FlutterDriverExtension {
 
   Future<Health> _getHealth(Command command) async => const Health(HealthStatus.ok);
 
+  Future<LayerTree> _getLayerTree(Command command) async {
+    return LayerTree(RendererBinding.instance?.renderView?.debugLayer?.toStringDeep());
+  }
+
   Future<RenderTree> _getRenderTree(Command command) async {
     return RenderTree(RendererBinding.instance?.renderView?.toStringDeep());
   }

packages/flutter_driver/test/flutter_driver_test.dart

@@ -6,6 +6,7 @@ import 'dart:async';
 
 import 'package:flutter_driver/src/common/error.dart';
 import 'package:flutter_driver/src/common/health.dart';
+import 'package:flutter_driver/src/common/layer_tree.dart';
 import 'package:flutter_driver/src/common/wait.dart';
 import 'package:flutter_driver/src/driver/driver.dart';
 import 'package:flutter_driver/src/driver/timeline.dart';
@@ -231,6 +232,25 @@ void main() {
       });
     });
 
+    group('getLayerTree', () {
+      test('sends the getLayerTree command', () async {
+        when(mockIsolate.invokeExtension(any, any)).thenAnswer((Invocation i) {
+          expect(i.positionalArguments[1], <String, dynamic>{
+            'command': 'get_layer_tree',
+            'timeout': _kSerializedTestTimeout,
+          });
+          return makeMockResponse(<String, String>{
+            'tree': 'hello',
+          });
+        });
+        final LayerTree result = await driver.getLayerTree(timeout: _kTestTimeout);
+        final LayerTree referenceTree = LayerTree.fromJson(<String, String>{
+            'tree': 'hello',
+          });
+        expect(result.tree, referenceTree.tree);
+      });
+    });
+
     group('waitFor', () {
       test('requires a target reference', () async {
         expect(driver.waitFor(null), throwsA(isInstanceOf<DriverError>()));