Proposal to deprecate webGoldenComparator. (#161196)

Towards https://github.com/flutter/flutter/issues/160261.

This is a PR and proposal all-in-one to deprecate, and remove, pending
the removal of the HTML backend, `WebGoldenComparator` (and related
symbols, `webGoldenComparator` and `DefaultWebGoldenComparator`).

The concept of `WebGoldenComparator` was added because the HTML-based
renderer could not support the `GoldenFileComparator` contract:


7141c2a136/packages/flutter_test/lib/src/_matchers_web.dart (L105-L108)

Once the Skia renderer (and now, SkWasm renderer) were added, it was now
possible to support the same API used by the native engine(s). This PR
conditionally, if _not_ using the HTML renderer, uses
`goldenFileComparator` instead, which re-uses the same code that
previously backed `DefaultWebGoldenFileComparator`.

No _new_ logic has been introduced in this change. This might need an
iteration or two to get right.

Feedback welcome!

packages/flutter_test/lib/src/_goldens_io.dart

@@ -284,8 +284,16 @@ ByteData _invert(ByteData imageBytes) {
 }
 
 /// An unsupported [WebGoldenComparator] that exists for API compatibility.
+@Deprecated(
+  'Use GoldenFileComparator instead. '
+  'This feature was deprecated after v3.28.0-0.1.pre.',
+)
 class DefaultWebGoldenComparator extends WebGoldenComparator {
   /// This is provided to prevent warnings from the analyzer.
+  @Deprecated(
+    'Use a GoldenFileComparator implementation instead. '
+    'This feature was deprecated after v3.28.0-0.1.pre.',
+  )
   DefaultWebGoldenComparator(Uri _) {
     throw UnsupportedError('DefaultWebGoldenComparator is only supported on the web.');
   }

packages/flutter_test/lib/src/_goldens_web.dart

@@ -35,6 +35,48 @@ Future<ComparisonResult> compareLists(List<int> test, List<int> master) async {
   throw UnsupportedError('Golden testing is not supported on the web.');
 }
 
+/// Implements [GoldenFileComparator] by proxying calls to an HTTP service `/flutter_goldens`.
+final class HttpProxyGoldenComparator extends GoldenFileComparator {
+  /// Creates a comparator with the given test file being executed.
+  ///
+  /// Golden file keys will be interpreted as file paths relative to the
+  /// directory in which this file resides.
+  HttpProxyGoldenComparator(this._testUri);
+  final Uri _testUri;
+
+  @override
+  Future<bool> compare(Uint8List bytes, Uri golden) async {
+    final String key = golden.toString();
+    final String bytesEncoded = base64.encode(bytes);
+    final web.Response response =
+        await web.window
+            .fetch(
+              'flutter_goldens'.toJS,
+              web.RequestInit(
+                method: 'POST',
+                body:
+                    json.encode(<String, Object>{
+                      'testUri': _testUri.toString(),
+                      'key': key,
+                      'bytes': bytesEncoded,
+                    }).toJS,
+              ),
+            )
+            .toDart;
+    final String responseText = (await response.text().toDart).toDart;
+    if (responseText == 'true') {
+      return true;
+    }
+    fail(responseText);
+  }
+
+  @override
+  Future<void> update(Uri golden, Uint8List bytes) async {
+    // Update is handled on the server side, just use the same logic here
+    await compare(bytes, golden);
+  }
+}
+
 /// The default [WebGoldenComparator] implementation for `flutter test`.
 ///
 /// This comparator will send a request to the test server for golden comparison
@@ -43,6 +85,10 @@ Future<ComparisonResult> compareLists(List<int> test, List<int> master) async {
 /// See also:
 ///
 ///   * [matchesGoldenFile], the function that invokes the comparator.
+@Deprecated(
+  'Use goldenFileComparator instead. '
+  'This feature was deprecated after v3.28.0-0.1.pre.',
+)
 class DefaultWebGoldenComparator extends WebGoldenComparator {
   /// Creates a new [DefaultWebGoldenComparator] for the specified [testUri].
   ///
@@ -50,13 +96,14 @@ class DefaultWebGoldenComparator extends WebGoldenComparator {
   /// directory in which [testUri] resides.
   ///
   /// The [testUri] must represent a file.
-  DefaultWebGoldenComparator(this.testUri);
+  @Deprecated(
+    'Use an implementation of GoldenFileComparator instead. '
+    'This feature was deprecated after v3.28.0-0.1.pre.',
+  )
+  DefaultWebGoldenComparator(Uri testUri) : _comparatorImpl = HttpProxyGoldenComparator(testUri);
 
-  /// The test file currently being executed.
-  ///
-  /// Golden file keys will be interpreted as file paths relative to the
-  /// directory in which this file resides.
-  Uri testUri;
+  // TODO(matanlurey): Refactor as part of https://github.com/flutter/flutter/issues/160261.
+  final HttpProxyGoldenComparator _comparatorImpl;
 
   @override
   Future<bool> compare(double width, double height, Uri golden) async {
@@ -69,7 +116,7 @@ class DefaultWebGoldenComparator extends WebGoldenComparator {
                 method: 'POST',
                 body:
                     json.encode(<String, Object>{
-                      'testUri': testUri.toString(),
+                      'testUri': _comparatorImpl._testUri.toString(),
                       'key': key,
                       'width': width.round(),
                       'height': height.round(),
@@ -92,33 +139,11 @@ class DefaultWebGoldenComparator extends WebGoldenComparator {
 
   @override
   Future<bool> compareBytes(Uint8List bytes, Uri golden) async {
-    final String key = golden.toString();
-    final String bytesEncoded = base64.encode(bytes);
-    final web.Response response =
-        await web.window
-            .fetch(
-              'flutter_goldens'.toJS,
-              web.RequestInit(
-                method: 'POST',
-                body:
-                    json.encode(<String, Object>{
-                      'testUri': testUri.toString(),
-                      'key': key,
-                      'bytes': bytesEncoded,
-                    }).toJS,
-              ),
-            )
-            .toDart;
-    final String responseText = (await response.text().toDart).toDart;
-    if (responseText == 'true') {
-      return true;
-    }
-    fail(responseText);
+    return _comparatorImpl.compare(bytes, golden);
   }
 
   @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);
+    await _comparatorImpl.update(golden, bytes);
   }
 }

packages/flutter_test/lib/src/_matchers_web.dart

@@ -85,11 +85,11 @@ class MatchesGoldenFile extends AsyncMatcher {
             return 'could not encode screenshot.';
           }
           if (autoUpdateGoldenFiles) {
-            await webGoldenComparator.updateBytes(bytes.buffer.asUint8List(), key);
+            await goldenFileComparator.update(key, bytes.buffer.asUint8List());
             return null;
           }
           try {
-            final bool success = await webGoldenComparator.compareBytes(
+            final bool success = await goldenFileComparator.compare(
               bytes.buffer.asUint8List(),
               key,
             );
@@ -126,7 +126,7 @@ class MatchesGoldenFile extends AsyncMatcher {
 
   @override
   Description describe(Description description) {
-    final Uri testNameUri = webGoldenComparator.getTestUri(key, version);
+    final Uri testNameUri = goldenFileComparator.getTestUri(key, version);
     return description.add('one widget whose rasterized image matches golden image "$testNameUri"');
   }
 }

packages/flutter_test/lib/src/_test_selector_web.dart

@@ -7,6 +7,7 @@ import 'dart:js_interop';
 import 'dart:ui' as ui;
 import 'dart:ui_web' as ui_web;
 
+import 'package:flutter/foundation.dart' show isSkiaWeb;
 import 'package:stream_channel/stream_channel.dart';
 import 'package:test_api/backend.dart';
 
@@ -41,7 +42,14 @@ Future<void> runWebTest(WebTest test) async {
   final Completer<void> completer = Completer<void>();
   await ui_web.bootstrapEngine(runApp: () => completer.complete());
   await completer.future;
-  webGoldenComparator = DefaultWebGoldenComparator(test.goldensUri);
+
+  // TODO(matanlurey): Remove webGoldenComparator when dart:html is deprecated.
+  // See https://github.com/flutter/flutter/issues/145954.
+  if (isSkiaWeb) {
+    goldenFileComparator = HttpProxyGoldenComparator(test.goldensUri);
+  } else {
+    webGoldenComparator = DefaultWebGoldenComparator(test.goldensUri);
+  }
 
   /// This hard-codes the device pixel ratio to 3.0 and a 2400 x 1800 window
   /// size for the purposes of testing.

packages/flutter_test/lib/src/goldens.dart

@@ -216,6 +216,10 @@ GoldenFileComparator goldenFileComparator = const TrivialComparator._();
 ///  * [DefaultWebGoldenComparator] for the default [WebGoldenComparator]
 ///    implementation for `flutter test`.
 ///  * [matchesGoldenFile], the function that invokes the comparator.
+@Deprecated(
+  'Use GoldenFileComparator instead. '
+  'This feature was deprecated after v3.28.0-0.1.pre.',
+)
 abstract class WebGoldenComparator {
   /// Compares the rendered pixels of size [width]x[height] that is being
   /// rendered on the top left of the screen against the golden file identified