Reland "Use Layer.toImage for golden tests on CanvasKit" (#136918)

Relands https://github.com/flutter/flutter/pull/135249

A golden test was failing in post submit in the previous PR

packages/flutter_test/lib/src/_goldens_io.dart

@@ -299,6 +299,16 @@ class DefaultWebGoldenComparator extends WebGoldenComparator {
   Future<void> update(double width, double height, Uri golden) {
     throw UnsupportedError('DefaultWebGoldenComparator is only supported on the web.');
   }
+
+  @override
+  Future<bool> compareBytes(Uint8List bytes, Uri golden) {
+    throw UnsupportedError('DefaultWebGoldenComparator is only supported on the web.');
+  }
+
+  @override
+  Future<void> updateBytes(Uint8List bytes, Uri golden) {
+    throw UnsupportedError('DefaultWebGoldenComparator is only supported on the web.');
+  }
 }
 
 /// Reads the red value out of a 32 bit rgba pixel.

packages/flutter_test/lib/src/_goldens_web.dart

@@ -80,4 +80,30 @@ class DefaultWebGoldenComparator extends WebGoldenComparator {
     // Update is handled on the server side, just use the same logic here
     await compare(width, height, golden);
   }
+
+  @override
+  Future<bool> compareBytes(Uint8List bytes, Uri golden) async {
+    final String key = golden.toString();
+    final String bytesEncoded = base64.encode(bytes);
+    final html.HttpRequest request = await html.HttpRequest.request(
+      'flutter_goldens',
+      method: 'POST',
+      sendData: json.encode(<String, Object>{
+        'testUri': testUri.toString(),
+        'key': key,
+        'bytes': bytesEncoded,
+      }),
+    );
+    final String response = request.response as String;
+    if (response == 'true') {
+      return true;
+    }
+    fail(response);
+  }
+
+  @override
+  Future<void> updateBytes(Uint8List bytes, Uri golden) async {
+    // Update is handled on the server side, just use the same logic here
+    await compareBytes(bytes, golden);
+  }
 }

packages/flutter_test/lib/src/_matchers_web.dart

@@ -4,6 +4,7 @@
 
 import 'dart:ui' as ui;
 
+import 'package:flutter/foundation.dart';
 import 'package:flutter/rendering.dart';
 import 'package:flutter/widgets.dart';
 import 'package:matcher/expect.dart';
@@ -61,7 +62,39 @@ class MatchesGoldenFile extends AsyncMatcher {
     final ui.FlutterView view = binding.platformDispatcher.implicitView!;
     final RenderView renderView = binding.renderViews.firstWhere((RenderView r) => r.flutterView == view);
 
-    // Unlike `flutter_tester`, we don't have the ability to render an element
+    if (isCanvasKit) {
+      // In CanvasKit, use Layer.toImage to generate the screenshot.
+      final TestWidgetsFlutterBinding binding = TestWidgetsFlutterBinding.instance;
+      return binding.runAsync<String?>(() async {
+        assert(element.renderObject != null);
+        RenderObject renderObject = element.renderObject!;
+        while (!renderObject.isRepaintBoundary) {
+          renderObject = renderObject.parent!;
+        }
+        assert(!renderObject.debugNeedsPaint);
+        final OffsetLayer layer = renderObject.debugLayer! as OffsetLayer;
+        final ui.Image image = await layer.toImage(renderObject.paintBounds);
+        try {
+          final ByteData? bytes = await image.toByteData(format: ui.ImageByteFormat.png);
+          if (bytes == null) {
+            return 'could not encode screenshot.';
+          }
+          if (autoUpdateGoldenFiles) {
+            await webGoldenComparator.updateBytes(bytes.buffer.asUint8List(), key);
+            return null;
+          }
+          try {
+            final bool success = await webGoldenComparator.compareBytes(bytes.buffer.asUint8List(), key);
+            return success ? null : 'does not match';
+          } on TestFailure catch (ex) {
+            return ex.message;
+          }
+        } finally {
+          image.dispose();
+        }
+      });
+    } else {
+      // In the HTML renderer, we don't have the ability to render an element
       // to an image directly. Instead, we will use `window.render()` to render
       // only the element being requested, and send a request to the test server
       // requesting it to take a screenshot through the browser's debug interface.
@@ -81,6 +114,7 @@ class MatchesGoldenFile extends AsyncMatcher {
       _renderElement(view, renderView);
       return result;
     }
+  }
 
   @override
   Description describe(Description description) {

packages/flutter_test/lib/src/goldens.dart

@@ -185,6 +185,34 @@ abstract class WebGoldenComparator {
   /// is left up to the implementation class.
   Future<void> update(double width, double height, Uri golden);
 
+  /// Compares the pixels of decoded png [bytes] against the golden file
+  /// identified by [golden].
+  ///
+  /// The returned future completes with a boolean value that indicates whether
+  /// the pixels rendered on screen match the golden file's pixels.
+  ///
+  /// In the case of comparison mismatch, the comparator may choose to throw a
+  /// [TestFailure] if it wants to control the failure message, often in the
+  /// form of a [ComparisonResult] that provides detailed information about the
+  /// mismatch.
+  ///
+  /// The method by which [golden] is located and by which its bytes are loaded
+  /// is left up to the implementation class. For instance, some implementations
+  /// may load files from the local file system, whereas others may load files
+  /// over the network or from a remote repository.
+  Future<bool> compareBytes(Uint8List bytes, Uri golden);
+
+  /// Compares the pixels of decoded png [bytes] against the golden file
+  /// identified by [golden].
+  ///
+  /// This will be invoked in lieu of [compareBytes] when [autoUpdateGoldenFiles]
+  /// is `true` (which gets set automatically by the test framework when the
+  /// user runs `flutter test --update-goldens --platform=chrome`).
+  ///
+  /// The method by which [golden] is located and by which its bytes are written
+  /// is left up to the implementation class.
+  Future<void> updateBytes(Uint8List bytes, Uri golden);
+
   /// Returns a new golden file [Uri] to incorporate any [version] number with
   /// the [key].
   ///
@@ -298,12 +326,7 @@ class _TrivialWebGoldenComparator implements WebGoldenComparator {
 
   @override
   Future<bool> compare(double width, double height, Uri golden) {
-    // Ideally we would use markTestSkipped here but in some situations,
-    // comparators are called outside of tests.
-    // See also: https://github.com/flutter/flutter/issues/91285
-    // ignore: avoid_print
-    print('Golden comparison requested for "$golden"; skipping...');
-    return Future<bool>.value(true);
+    return _warnAboutSkipping(golden);
   }
 
   @override
@@ -315,6 +338,25 @@ class _TrivialWebGoldenComparator implements WebGoldenComparator {
   Uri getTestUri(Uri key, int? version) {
     return key;
   }
+
+  @override
+  Future<bool> compareBytes(Uint8List bytes, Uri golden) {
+    return _warnAboutSkipping(golden);
+  }
+
+  @override
+  Future<void> updateBytes(Uint8List bytes, Uri golden) {
+    throw StateError('webGoldenComparator has not been initialized');
+  }
+
+  Future<bool> _warnAboutSkipping(Uri golden) {
+    // Ideally we would use markTestSkipped here but in some situations,
+    // comparators are called outside of tests.
+    // See also: https://github.com/flutter/flutter/issues/91285
+    // ignore: avoid_print
+    print('Golden comparison requested for "$golden"; skipping...');
+    return Future<bool>.value(true);
+  }
 }
 
 /// The result of a pixel comparison test.

packages/flutter_tools/lib/src/test/flutter_web_platform.dart

@@ -336,10 +336,16 @@ class FlutterWebPlatform extends PlatformPlugin {
       final Map<String, Object?> body = json.decode(await request.readAsString()) as Map<String, Object?>;
       final Uri goldenKey = Uri.parse(body['key']! as String);
       final Uri testUri = Uri.parse(body['testUri']! as String);
-      final num width = body['width']! as num;
-      final num height = body['height']! as num;
+      final num? width = body['width'] as num?;
+      final num? height = body['height'] as num?;
       Uint8List bytes;
 
+      if (body.containsKey('bytes')) {
+        bytes = base64.decode(body['bytes']! as String);
+      } else {
+        // TODO(hterkelsen): Do not use browser screenshots for testing on the
+        // web once we transition off the HTML renderer. See:
+        // https://github.com/flutter/flutter/issues/135700
         try {
           final ChromeTab chromeTab = (await _browserManager!._browser.chromeConnection.getTab((ChromeTab tab) {
             return tab.url.contains(_browserManager!._browser.url!);
@@ -354,8 +360,8 @@ class FlutterWebPlatform extends PlatformPlugin {
             'clip': <String, Object>{
               'x': 0.0,
               'y': 0.0,
-            'width': width.toDouble(),
-            'height': height.toDouble(),
+              'width': width!.toDouble(),
+              'height': height!.toDouble(),
               'scale': 1.0,
             },
           });
@@ -367,7 +373,7 @@ class FlutterWebPlatform extends PlatformPlugin {
           _logger.printError('Caught FormatException: $ex');
           return shelf.Response.ok('Caught exception: $ex');
         }
-
+      }
       final String? errorMessage = await _testGoldenComparator.compareGoldens(testUri, bytes, goldenKey, updateGoldens);
       return shelf.Response.ok(errorMessage ?? 'true');
     } else {