packages/flutter_test/lib/src/_goldens_io.dart - mirrors/flutter - Git at Google

 // 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 'dart:async';
 import 'dart:io';
 import 'dart:math' as math;
 import 'dart:typed_data';
 import 'dart:ui';

 import 'package:flutter/foundation.dart';
 import 'package:path/path.dart' as path;
 // ignore: deprecated_member_use
 import 'package:test_api/test_api.dart' as test_package show TestFailure;

 import 'goldens.dart';
 import 'test_async_utils.dart';

 /// The default [GoldenFileComparator] implementation for `flutter test`.
 ///
 /// The term __golden file__ refers to a master image that is considered the
 /// true rendering of a given widget, state, application, or other visual
 /// representation you have chosen to capture. This comparator loads golden
 /// files from the local file system, treating the golden key as a relative
 /// path from the test file's directory.
 ///
 /// This comparator performs a pixel-for-pixel comparison of the decoded PNGs,
 /// returning true only if there's an exact match. In cases where the captured
 /// test image does not match the golden file, this comparator will provide
 /// output to illustrate the difference, described in further detail below.
 ///
 /// When using `flutter test --update-goldens`, [LocalFileComparator]
 /// updates the golden files on disk to match the rendering.
 ///
 /// ## Local Output from Golden File Testing
 ///
 /// The [LocalFileComparator] will output test feedback when a golden file test
 /// fails. This output takes the form of differential images contained within a
 /// `failures` directory that will be generated in the same location specified
 /// by the golden key. The differential images include the master and test
 /// images that were compared, as well as an isolated diff of detected pixels,
 /// and a masked diff that overlays these detected pixels over the master image.
 ///
 /// The following images are examples of a test failure output:
 ///
 /// |  File Name                 |  Image Output |
 /// |----------------------------|---------------|
 /// |  testName_masterImage.png  | ![A golden master image](https://flutter.github.io/assets-for-api-docs/assets/flutter-test/goldens/widget_masterImage.png)  |
 /// |  testName_testImage.png    | ![Test image](https://flutter.github.io/assets-for-api-docs/assets/flutter-test/goldens/widget_testImage.png)  |
 /// |  testName_isolatedDiff.png | ![An isolated pixel difference.](https://flutter.github.io/assets-for-api-docs/assets/flutter-test/goldens/widget_isolatedDiff.png) |
 /// |  testName_maskedDiff.png   | ![A masked pixel difference](https://flutter.github.io/assets-for-api-docs/assets/flutter-test/goldens/widget_maskedDiff.png) |
 ///
 /// See also:
 ///
 ///   * [GoldenFileComparator], the abstract class that [LocalFileComparator]
 ///   implements.
 ///   * [matchesGoldenFile], the function from [flutter_test] that invokes the
 ///    comparator.
 class LocalFileComparator extends GoldenFileComparator with LocalComparisonOutput {
   /// Creates a new [LocalFileComparator] for the specified [testFile].
   ///
   /// Golden file keys will be interpreted as file paths relative to the
   /// directory in which [testFile] resides.
   ///
   /// The [testFile] URL must represent a file.
   LocalFileComparator(Uri testFile, {path.Style? pathStyle})
     : basedir = _getBasedir(testFile, pathStyle),
       _path = _getPath(pathStyle);

   static path.Context _getPath(path.Style? style) {
     return path.Context(style: style ?? path.Style.platform);
   }

   static Uri _getBasedir(Uri testFile, path.Style? pathStyle) {
     final path.Context context = _getPath(pathStyle);
     final String testFilePath = context.fromUri(testFile);
     final String testDirectoryPath = context.dirname(testFilePath);
     return context.toUri(testDirectoryPath + context.separator);
   }

   /// The directory in which the test was loaded.
   ///
   /// Golden file keys will be interpreted as file paths relative to this
   /// directory.
   final Uri basedir;

   /// Path context exists as an instance variable rather than just using the
   /// system path context in order to support testing, where we can spoof the
   /// platform to test behaviors with arbitrary path styles.
   final path.Context _path;

   @override
   Future<bool> compare(Uint8List imageBytes, Uri golden) async {
     final ComparisonResult result = await GoldenFileComparator.compareLists(
       imageBytes,
       await getGoldenBytes(golden),
     );

     if (!result.passed) {
       final String error = await generateFailureOutput(result, golden, basedir);
       throw FlutterError(error);
     }
     return result.passed;
   }

   @override
   Future<void> update(Uri golden, Uint8List imageBytes) async {
     final File goldenFile = _getGoldenFile(golden);
     await goldenFile.parent.create(recursive: true);
     await goldenFile.writeAsBytes(imageBytes, flush: true);
   }

   /// Returns the bytes of the given [golden] file.
   ///
   /// If the file cannot be found, an error will be thrown.
   @protected
   Future<List<int>> getGoldenBytes(Uri golden) async {
     final File goldenFile = _getGoldenFile(golden);
     if (!goldenFile.existsSync()) {
       throw test_package.TestFailure(
         'Could not be compared against non-existent file: "$golden"'
       );
     }
     final List<int> goldenBytes = await goldenFile.readAsBytes();
     return goldenBytes;
   }

   File _getGoldenFile(Uri golden) => File(_path.join(_path.fromUri(basedir), _path.fromUri(golden.path)));
 }

 /// A mixin for use in golden file comparators that run locally and provide
 /// output.
 mixin LocalComparisonOutput {
   /// Writes out diffs from the [ComparisonResult] of a golden file test.
   ///
   /// Will throw an error if a null result is provided.
   Future<String> generateFailureOutput(
     ComparisonResult result,
     Uri golden,
     Uri basedir, {
     String key = '',
   }) async => TestAsyncUtils.guard<String>(() async {
     String additionalFeedback = '';
     if (result.diffs != null) {
       additionalFeedback = '\nFailure feedback can be found at ${path.join(basedir.path, 'failures')}';
       final Map<String, Image> diffs = result.diffs!.cast<String, Image>();
       for (final MapEntry<String, Image> entry in diffs.entries) {
         final File output = getFailureFile(
           key.isEmpty ? entry.key : entry.key + '_' + key,
           golden,
           basedir,
         );
         output.parent.createSync(recursive: true);
         final ByteData? pngBytes = await entry.value.toByteData(format: ImageByteFormat.png);
         output.writeAsBytesSync(pngBytes!.buffer.asUint8List());
       }
     }
     return 'Golden "$golden": ${result.error}$additionalFeedback';
   });

   /// Returns the appropriate file for a given diff from a [ComparisonResult].
   File getFailureFile(String failure, Uri golden, Uri basedir) {
     final String fileName = golden.pathSegments.last;
     final String testName = fileName.split(path.extension(fileName))[0]
       + '_'
       + failure
       + '.png';
     return File(path.join(
       path.fromUri(basedir),
       path.fromUri(Uri.parse('failures/$testName')),
     ));
   }
 }

 /// Returns a [ComparisonResult] to describe the pixel differential of the
 /// [test] and [master] image bytes provided.
 Future<ComparisonResult> compareLists(List<int>? test, List<int>? master) async {
   if (identical(test, master))
     return ComparisonResult(
       passed: true,
       diffPercent: 0.0,
     );

   if (test == null || master == null || test.isEmpty || master.isEmpty) {
     return ComparisonResult(
       passed: false,
       diffPercent: 1.0,
       error: 'Pixel test failed, null image provided.',
     );
   }

   final Codec testImageCodec =
       await instantiateImageCodec(Uint8List.fromList(test));
   final Image testImage = (await testImageCodec.getNextFrame()).image;
   final ByteData? testImageRgba = await testImage.toByteData();

   final Codec masterImageCodec =
       await instantiateImageCodec(Uint8List.fromList(master));
   final Image masterImage = (await masterImageCodec.getNextFrame()).image;
   final ByteData? masterImageRgba = await masterImage.toByteData();

   final int width = testImage.width;
   final int height = testImage.height;

   if (width != masterImage.width || height != masterImage.height) {
     return ComparisonResult(
       passed: false,
       diffPercent: 1.0,
       error: 'Pixel test failed, image sizes do not match.\n'
         'Master Image: ${masterImage.width} X ${masterImage.height}\n'
         'Test Image: ${testImage.width} X ${testImage.height}',
     );
   }

   int pixelDiffCount = 0;
   final int totalPixels = width * height;
   final ByteData invertedMasterRgba = _invert(masterImageRgba!);
   final ByteData invertedTestRgba = _invert(testImageRgba!);

   final Uint8List testImageBytes = (await testImage.toByteData())!.buffer.asUint8List();
   final ByteData maskedDiffRgba = ByteData(testImageBytes.length);
   maskedDiffRgba.buffer.asUint8List().setRange(0, testImageBytes.length, testImageBytes);
   final ByteData isolatedDiffRgba = ByteData(width * height * 4);

   for (int x = 0; x < width; x++) {
     for (int y =0; y < height; y++) {
       final int byteOffset = (width * y + x) * 4;
       final int testPixel = testImageRgba.getUint32(byteOffset);
       final int masterPixel = masterImageRgba.getUint32(byteOffset);

       final int diffPixel = (_readRed(testPixel) - _readRed(masterPixel)).abs()
         + (_readGreen(testPixel) - _readGreen(masterPixel)).abs()
         + (_readBlue(testPixel) - _readBlue(masterPixel)).abs()
         + (_readAlpha(testPixel) - _readAlpha(masterPixel)).abs();

       if (diffPixel != 0 ) {
         final int invertedMasterPixel = invertedMasterRgba.getUint32(byteOffset);
         final int invertedTestPixel = invertedTestRgba.getUint32(byteOffset);
         // We grab the max of the 0xAABBGGRR encoded bytes, and then convert
         // back to 0xRRGGBBAA for the actual pixel value, since this is how it
         // was historically done.
         final int maskPixel = _toRGBA(math.max(
           _toABGR(invertedMasterPixel),
           _toABGR(invertedTestPixel),
         ));
         maskedDiffRgba.setUint32(byteOffset, maskPixel);
         isolatedDiffRgba.setUint32(byteOffset, maskPixel);
         pixelDiffCount++;
       }
     }
   }

   if (pixelDiffCount > 0) {
     final double diffPercent = pixelDiffCount / totalPixels;
     return ComparisonResult(
       passed: false,
       diffPercent: diffPercent,
       error: 'Pixel test failed, '
         '${(diffPercent * 100).toStringAsFixed(2)}% '
         'diff detected.',
       diffs:  <String, Image>{
         'masterImage' : masterImage,
         'testImage' : testImage,
         'maskedDiff' : await _createImage(maskedDiffRgba, width, height),
         'isolatedDiff' : await _createImage(isolatedDiffRgba, width, height),
       },
     );
   }
   return ComparisonResult(passed: true, diffPercent: 0.0);
 }

 /// Inverts [imageBytes], returning a new [ByteData] object.
 ByteData _invert(ByteData imageBytes) {
   final ByteData bytes = ByteData(imageBytes.lengthInBytes);
   // Invert the RGB data (but not A).
   for (int i = 0; i < imageBytes.lengthInBytes; i += 4) {
     bytes.setUint8(i, 255 - imageBytes.getUint8(i));
     bytes.setUint8(i + 1, 255 - imageBytes.getUint8(i + 1));
     bytes.setUint8(i + 2, 255 - imageBytes.getUint8(i + 2));
     bytes.setUint8(i + 3, imageBytes.getUint8(i + 3));
   }
   return bytes;
 }

 /// An unsupported [WebGoldenComparator] that exists for API compatibility.
 class DefaultWebGoldenComparator extends WebGoldenComparator {
   @override
   Future<bool> compare(double width, double height, Uri golden) {
     throw UnsupportedError('DefaultWebGoldenComparator is only supported on the web.');
   }

   @override
   Future<void> update(double width, double height, Uri golden) {
     throw UnsupportedError('DefaultWebGoldenComparator is only supported on the web.');
   }
 }

 /// Reads the red value out of a 32 bit rgba pixel.
 int _readRed(int pixel) => (pixel >> 24) & 0xff;

 /// Reads the green value out of a 32 bit rgba pixel.
 int _readGreen(int pixel) => (pixel >> 16) & 0xff;

 /// Reads the blue value out of a 32 bit rgba pixel.
 int _readBlue(int pixel) => (pixel >> 8) & 0xff;

 /// Reads the alpha value out of a 32 bit rgba pixel.
 int _readAlpha(int pixel) => pixel & 0xff;

 /// Convenience wrapper around [decodeImageFromPixels].
 Future<Image> _createImage(ByteData bytes, int width, int height) {
   final Completer<Image> completer = Completer<Image>();
   decodeImageFromPixels(
     bytes.buffer.asUint8List(),
     width,
     height,
     PixelFormat.rgba8888,
     completer.complete,
   );
   return completer.future;
 }

 // Converts a 32 bit rgba pixel to a 32 bit abgr pixel
 int _toABGR(int rgba) =>
     (_readAlpha(rgba) << 24) |
     (_readBlue(rgba) << 16) |
     (_readGreen(rgba) << 8) |
     _readRed(rgba);

 // Converts a 32 bit abgr pixel to a 32 bit rgba pixel
 int _toRGBA(int abgr) =>
   // This is just a mirror of the other conversion.
   _toABGR(abgr);
	// 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 'dart:async';
	import 'dart:io';
	import 'dart:math' as math;
	import 'dart:typed_data';
	import 'dart:ui';

	import 'package:flutter/foundation.dart';
	import 'package:path/path.dart' as path;
	// ignore: deprecated_member_use
	import 'package:test_api/test_api.dart' as test_package show TestFailure;

	import 'goldens.dart';
	import 'test_async_utils.dart';

	/// The default [GoldenFileComparator] implementation for `flutter test`.
	///
	/// The term __golden file__ refers to a master image that is considered the
	/// true rendering of a given widget, state, application, or other visual
	/// representation you have chosen to capture. This comparator loads golden
	/// files from the local file system, treating the golden key as a relative
	/// path from the test file's directory.
	///
	/// This comparator performs a pixel-for-pixel comparison of the decoded PNGs,
	/// returning true only if there's an exact match. In cases where the captured
	/// test image does not match the golden file, this comparator will provide
	/// output to illustrate the difference, described in further detail below.
	///
	/// When using `flutter test --update-goldens`, [LocalFileComparator]
	/// updates the golden files on disk to match the rendering.
	///
	/// ## Local Output from Golden File Testing
	///
	/// The [LocalFileComparator] will output test feedback when a golden file test
	/// fails. This output takes the form of differential images contained within a
	/// `failures` directory that will be generated in the same location specified
	/// by the golden key. The differential images include the master and test
	/// images that were compared, as well as an isolated diff of detected pixels,
	/// and a masked diff that overlays these detected pixels over the master image.
	///
	/// The following images are examples of a test failure output:
	///
	/// \| File Name \| Image Output \|
	/// \|----------------------------\|---------------\|
	/// \| testName_masterImage.png \| ![A golden master image](https://flutter.github.io/assets-for-api-docs/assets/flutter-test/goldens/widget_masterImage.png) \|
	/// \| testName_testImage.png \| ![Test image](https://flutter.github.io/assets-for-api-docs/assets/flutter-test/goldens/widget_testImage.png) \|
	/// \| testName_isolatedDiff.png \| ![An isolated pixel difference.](https://flutter.github.io/assets-for-api-docs/assets/flutter-test/goldens/widget_isolatedDiff.png) \|
	/// \| testName_maskedDiff.png \| ![A masked pixel difference](https://flutter.github.io/assets-for-api-docs/assets/flutter-test/goldens/widget_maskedDiff.png) \|
	///
	/// See also:
	///
	/// * [GoldenFileComparator], the abstract class that [LocalFileComparator]
	/// implements.
	/// * [matchesGoldenFile], the function from [flutter_test] that invokes the
	/// comparator.
	class LocalFileComparator extends GoldenFileComparator with LocalComparisonOutput {
	/// Creates a new [LocalFileComparator] for the specified [testFile].
	///
	/// Golden file keys will be interpreted as file paths relative to the
	/// directory in which [testFile] resides.
	///
	/// The [testFile] URL must represent a file.
	LocalFileComparator(Uri testFile, {path.Style? pathStyle})
	: basedir = _getBasedir(testFile, pathStyle),
	_path = _getPath(pathStyle);

	static path.Context _getPath(path.Style? style) {
	return path.Context(style: style ?? path.Style.platform);
	}

	static Uri _getBasedir(Uri testFile, path.Style? pathStyle) {
	final path.Context context = _getPath(pathStyle);
	final String testFilePath = context.fromUri(testFile);
	final String testDirectoryPath = context.dirname(testFilePath);
	return context.toUri(testDirectoryPath + context.separator);
	}

	/// The directory in which the test was loaded.
	///
	/// Golden file keys will be interpreted as file paths relative to this
	/// directory.
	final Uri basedir;

	/// Path context exists as an instance variable rather than just using the
	/// system path context in order to support testing, where we can spoof the
	/// platform to test behaviors with arbitrary path styles.
	final path.Context _path;

	@override
	Future<bool> compare(Uint8List imageBytes, Uri golden) async {
	final ComparisonResult result = await GoldenFileComparator.compareLists(
	imageBytes,
	await getGoldenBytes(golden),
	);

	if (!result.passed) {
	final String error = await generateFailureOutput(result, golden, basedir);
	throw FlutterError(error);
	}
	return result.passed;
	}

	@override
	Future<void> update(Uri golden, Uint8List imageBytes) async {
	final File goldenFile = _getGoldenFile(golden);
	await goldenFile.parent.create(recursive: true);
	await goldenFile.writeAsBytes(imageBytes, flush: true);
	}

	/// Returns the bytes of the given [golden] file.
	///
	/// If the file cannot be found, an error will be thrown.
	@protected
	Future<List<int>> getGoldenBytes(Uri golden) async {
	final File goldenFile = _getGoldenFile(golden);
	if (!goldenFile.existsSync()) {
	throw test_package.TestFailure(
	'Could not be compared against non-existent file: "$golden"'
	);
	}
	final List<int> goldenBytes = await goldenFile.readAsBytes();
	return goldenBytes;
	}

	File _getGoldenFile(Uri golden) => File(_path.join(_path.fromUri(basedir), _path.fromUri(golden.path)));
	}

	/// A mixin for use in golden file comparators that run locally and provide
	/// output.
	mixin LocalComparisonOutput {
	/// Writes out diffs from the [ComparisonResult] of a golden file test.
	///
	/// Will throw an error if a null result is provided.
	Future<String> generateFailureOutput(
	ComparisonResult result,
	Uri golden,
	Uri basedir, {
	String key = '',
	}) async => TestAsyncUtils.guard<String>(() async {
	String additionalFeedback = '';
	if (result.diffs != null) {
	additionalFeedback = '\nFailure feedback can be found at ${path.join(basedir.path, 'failures')}';
	final Map<String, Image> diffs = result.diffs!.cast<String, Image>();
	for (final MapEntry<String, Image> entry in diffs.entries) {
	final File output = getFailureFile(
	key.isEmpty ? entry.key : entry.key + '_' + key,
	golden,
	basedir,
	);
	output.parent.createSync(recursive: true);
	final ByteData? pngBytes = await entry.value.toByteData(format: ImageByteFormat.png);
	output.writeAsBytesSync(pngBytes!.buffer.asUint8List());
	}
	}
	return 'Golden "$golden": ${result.error}$additionalFeedback';
	});

	/// Returns the appropriate file for a given diff from a [ComparisonResult].
	File getFailureFile(String failure, Uri golden, Uri basedir) {
	final String fileName = golden.pathSegments.last;
	final String testName = fileName.split(path.extension(fileName))[0]
	+ '_'
	+ failure
	+ '.png';
	return File(path.join(
	path.fromUri(basedir),
	path.fromUri(Uri.parse('failures/$testName')),
	));
	}
	}

	/// Returns a [ComparisonResult] to describe the pixel differential of the
	/// [test] and [master] image bytes provided.
	Future<ComparisonResult> compareLists(List<int>? test, List<int>? master) async {
	if (identical(test, master))
	return ComparisonResult(
	passed: true,
	diffPercent: 0.0,
	);

	if (test == null \|\| master == null \|\| test.isEmpty \|\| master.isEmpty) {
	return ComparisonResult(
	passed: false,
	diffPercent: 1.0,
	error: 'Pixel test failed, null image provided.',
	);
	}

	final Codec testImageCodec =
	await instantiateImageCodec(Uint8List.fromList(test));
	final Image testImage = (await testImageCodec.getNextFrame()).image;
	final ByteData? testImageRgba = await testImage.toByteData();

	final Codec masterImageCodec =
	await instantiateImageCodec(Uint8List.fromList(master));
	final Image masterImage = (await masterImageCodec.getNextFrame()).image;
	final ByteData? masterImageRgba = await masterImage.toByteData();

	final int width = testImage.width;
	final int height = testImage.height;

	if (width != masterImage.width \|\| height != masterImage.height) {
	return ComparisonResult(
	passed: false,
	diffPercent: 1.0,
	error: 'Pixel test failed, image sizes do not match.\n'
	'Master Image: ${masterImage.width} X ${masterImage.height}\n'
	'Test Image: ${testImage.width} X ${testImage.height}',
	);
	}

	int pixelDiffCount = 0;
	final int totalPixels = width * height;
	final ByteData invertedMasterRgba = _invert(masterImageRgba!);
	final ByteData invertedTestRgba = _invert(testImageRgba!);

	final Uint8List testImageBytes = (await testImage.toByteData())!.buffer.asUint8List();
	final ByteData maskedDiffRgba = ByteData(testImageBytes.length);
	maskedDiffRgba.buffer.asUint8List().setRange(0, testImageBytes.length, testImageBytes);
	final ByteData isolatedDiffRgba = ByteData(width * height * 4);

	for (int x = 0; x < width; x++) {
	for (int y =0; y < height; y++) {
	final int byteOffset = (width * y + x) * 4;
	final int testPixel = testImageRgba.getUint32(byteOffset);
	final int masterPixel = masterImageRgba.getUint32(byteOffset);

	final int diffPixel = (_readRed(testPixel) - _readRed(masterPixel)).abs()
	+ (_readGreen(testPixel) - _readGreen(masterPixel)).abs()
	+ (_readBlue(testPixel) - _readBlue(masterPixel)).abs()
	+ (_readAlpha(testPixel) - _readAlpha(masterPixel)).abs();

	if (diffPixel != 0 ) {
	final int invertedMasterPixel = invertedMasterRgba.getUint32(byteOffset);
	final int invertedTestPixel = invertedTestRgba.getUint32(byteOffset);
	// We grab the max of the 0xAABBGGRR encoded bytes, and then convert
	// back to 0xRRGGBBAA for the actual pixel value, since this is how it
	// was historically done.
	final int maskPixel = _toRGBA(math.max(
	_toABGR(invertedMasterPixel),
	_toABGR(invertedTestPixel),
	));
	maskedDiffRgba.setUint32(byteOffset, maskPixel);
	isolatedDiffRgba.setUint32(byteOffset, maskPixel);
	pixelDiffCount++;
	}
	}
	}

	if (pixelDiffCount > 0) {
	final double diffPercent = pixelDiffCount / totalPixels;
	return ComparisonResult(
	passed: false,
	diffPercent: diffPercent,
	error: 'Pixel test failed, '
	'${(diffPercent * 100).toStringAsFixed(2)}% '
	'diff detected.',
	diffs: <String, Image>{
	'masterImage' : masterImage,
	'testImage' : testImage,
	'maskedDiff' : await _createImage(maskedDiffRgba, width, height),
	'isolatedDiff' : await _createImage(isolatedDiffRgba, width, height),
	},
	);
	}
	return ComparisonResult(passed: true, diffPercent: 0.0);
	}

	/// Inverts [imageBytes], returning a new [ByteData] object.
	ByteData _invert(ByteData imageBytes) {
	final ByteData bytes = ByteData(imageBytes.lengthInBytes);
	// Invert the RGB data (but not A).
	for (int i = 0; i < imageBytes.lengthInBytes; i += 4) {
	bytes.setUint8(i, 255 - imageBytes.getUint8(i));
	bytes.setUint8(i + 1, 255 - imageBytes.getUint8(i + 1));
	bytes.setUint8(i + 2, 255 - imageBytes.getUint8(i + 2));
	bytes.setUint8(i + 3, imageBytes.getUint8(i + 3));
	}
	return bytes;
	}

	/// An unsupported [WebGoldenComparator] that exists for API compatibility.
	class DefaultWebGoldenComparator extends WebGoldenComparator {
	@override
	Future<bool> compare(double width, double height, Uri golden) {
	throw UnsupportedError('DefaultWebGoldenComparator is only supported on the web.');
	}

	@override
	Future<void> update(double width, double height, Uri golden) {
	throw UnsupportedError('DefaultWebGoldenComparator is only supported on the web.');
	}
	}

	/// Reads the red value out of a 32 bit rgba pixel.
	int _readRed(int pixel) => (pixel >> 24) & 0xff;

	/// Reads the green value out of a 32 bit rgba pixel.
	int _readGreen(int pixel) => (pixel >> 16) & 0xff;

	/// Reads the blue value out of a 32 bit rgba pixel.
	int _readBlue(int pixel) => (pixel >> 8) & 0xff;

	/// Reads the alpha value out of a 32 bit rgba pixel.
	int _readAlpha(int pixel) => pixel & 0xff;

	/// Convenience wrapper around [decodeImageFromPixels].
	Future<Image> _createImage(ByteData bytes, int width, int height) {
	final Completer<Image> completer = Completer<Image>();
	decodeImageFromPixels(
	bytes.buffer.asUint8List(),
	width,
	height,
	PixelFormat.rgba8888,
	completer.complete,
	);
	return completer.future;
	}

	// Converts a 32 bit rgba pixel to a 32 bit abgr pixel
	int _toABGR(int rgba) =>
	(_readAlpha(rgba) << 24) \|
	(_readBlue(rgba) << 16) \|
	(_readGreen(rgba) << 8) \|
	_readRed(rgba);

	// Converts a 32 bit abgr pixel to a 32 bit rgba pixel
	int _toRGBA(int abgr) =>
	// This is just a mirror of the other conversion.
	_toABGR(abgr);