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

 // Copyright 2018 The Chromium 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:math' as math;
 import 'dart:typed_data';
 import 'dart:ui' as ui;

 import 'package:flutter/rendering.dart';
 import 'package:flutter/semantics.dart';
 import 'package:flutter/widgets.dart';

 import 'finders.dart';
 import 'widget_tester.dart';

 /// The result of evaluating a semantics node by a [AccessibilityGuideline].
 class Evaluation {
   /// Create a passing evaluation.
   const Evaluation.pass() : passed = true, reason = null;

   /// Create a failing evaluation, with an optional [reason] explaining the
   /// result.
   const Evaluation.fail([this.reason]) : passed = false;

   // private constructor for adding cases together.
   const Evaluation._(this.passed, this.reason);

   /// Whether the given tree or node passed the policy evaluation.
   final bool passed;

   /// If [passed] is false, contains the reason for failure.
   final String reason;

   /// Combines two evaluation results.
   ///
   /// The [reason] will be concatenated with a newline, and [passed] will be
   /// combined with an `&&` operator.
   Evaluation operator +(Evaluation other) {
     if (other == null)
       return this;
     final StringBuffer buffer = StringBuffer();
     if (reason != null) {
       buffer.write(reason);
       buffer.write(' ');
     }
     if (other.reason != null)
       buffer.write(other.reason);
     return Evaluation._(passed && other.passed, buffer.isEmpty ? null : buffer.toString());
   }
 }

 /// An accessibility guideline describes a recommendation an application should
 /// meet to be considered accessible.
 abstract class AccessibilityGuideline {
   /// A const constructor allows subclasses to be const.
   const AccessibilityGuideline();

   /// Evaluate whether the current state of the `tester` conforms to the rule.
   FutureOr<Evaluation> evaluate(WidgetTester tester);

   /// A description of the policy restrictions and criteria.
   String get description;
 }

 /// A guideline which enforces that all tapable semantics nodes have a minimum
 /// size.
 ///
 /// Each platform defines its own guidelines for minimum tap areas.
 @visibleForTesting
 class MinimumTapTargetGuideline extends AccessibilityGuideline {
   const MinimumTapTargetGuideline._(this.size, this.link);

   /// The minimum allowed size of a tapable node.
   final Size size;

   /// A link describing the tap target guidelines for a platform.
   final String link;

   @override
   FutureOr<Evaluation> evaluate(WidgetTester tester) {
     final SemanticsNode root = tester.binding.pipelineOwner.semanticsOwner.rootSemanticsNode;
     Evaluation traverse(SemanticsNode node) {
       Evaluation result = const Evaluation.pass();
       node.visitChildren((SemanticsNode child) {
         result += traverse(child);
         return true;
       });
       if (node.isMergedIntoParent)
         return result;
       final SemanticsData data = node.getSemanticsData();
       // Skip node if it has no actions, or is marked as hidden.
       if ((!data.hasAction(ui.SemanticsAction.longPress)
         && !data.hasAction(ui.SemanticsAction.tap))
         || data.hasFlag(ui.SemanticsFlag.isHidden))
         return result;
       Rect paintBounds = node.rect;
       SemanticsNode current = node;
       while (current != null) {
         if (current.transform != null)
           paintBounds = MatrixUtils.transformRect(current.transform, paintBounds);
         current = current.parent;
       }
       // skip node if it is touching the edge of the screen, since it might
       // be partially scrolled offscreen.
       const double delta = 0.001;
       if (paintBounds.left <= delta
         || paintBounds.top <= delta
         || (paintBounds.bottom - ui.window.physicalSize.height).abs() <= delta
         || (paintBounds.right - ui.window.physicalSize.width).abs() <= delta)
         return result;
       // shrink by device pixel ratio.
       final Size candidateSize = paintBounds.size / ui.window.devicePixelRatio;
       if (candidateSize.width < size.width || candidateSize.height < size.height)
         result += Evaluation.fail(
           '$node: expected tap target size of at least $size, but found $candidateSize\n'
           'See also: $link');
       return result;
     }
     return traverse(root);
   }

   @override
   String get description => 'Tappable objects should be at least $size';
 }

 /// A guideline which verifies that all nodes that contribute semantics via text
 /// meet minimum contrast levels.
 ///
 /// The guidelines are defined by the Web Content Accessibility Guidelines,
 /// http://www.w3.org/TR/UNDERSTANDING-WCAG20/visual-audio-contrast-contrast.html.
 @visibleForTesting
 class MinimumTextContrastGuideline extends AccessibilityGuideline {
   const MinimumTextContrastGuideline._();

   /// The minimum text size considered large for contrast checking.
   ///
   /// Defined by http://www.w3.org/TR/UNDERSTANDING-WCAG20/visual-audio-contrast-contrast.html
   static const int kLargeTextMinimumSize = 18;

   /// The minimum text size for bold text to be considered large for contrast
   /// checking.
   ///
   /// Defined by http://www.w3.org/TR/UNDERSTANDING-WCAG20/visual-audio-contrast-contrast.html
   static const int kBoldTextMinimumSize = 14;

   /// The minimum contrast ratio for normal text.
   ///
   /// Defined by http://www.w3.org/TR/UNDERSTANDING-WCAG20/visual-audio-contrast-contrast.html
   static const double kMinimumRatioNormalText = 4.5;

   /// The minimum contrast ratio for large text.
   ///
   /// Defined by http://www.w3.org/TR/UNDERSTANDING-WCAG20/visual-audio-contrast-contrast.html
   static const double kMinimumRatioLargeText = 3.0;

   @override
   Future<Evaluation> evaluate(WidgetTester tester) async {
     final SemanticsNode root = tester.binding.pipelineOwner.semanticsOwner.rootSemanticsNode;
     final RenderView renderView = tester.binding.renderView;
     final OffsetLayer layer = renderView.layer;
     ui.Image image;
     final ByteData byteData = await tester.binding.runAsync<ByteData>(() async  {
       // Needs to be the same pixel ratio otherwise our dimensions won't match the
       // last transform layer.
       image = await layer.toImage(renderView.paintBounds, pixelRatio: 1.0);
       return image.toByteData();
     });

     Future<Evaluation> evaluateNode(SemanticsNode node) async {
       final SemanticsData data = node.getSemanticsData();
       final List<SemanticsNode> children = <SemanticsNode>[];
       Evaluation result = const Evaluation.pass();
       node.visitChildren((SemanticsNode child) {
         children.add(child);
         return true;
       });
       for (SemanticsNode child in children)
         result += await evaluateNode(child);
       if (_shouldSkipNode(data))
         return result;

       // We need to look up the inherited text properties to determine the
       // contrast ratio based on text size/weight.
       double fontSize;
       bool isBold;
       final String text = (data.label?.isEmpty == true) ? data.value : data.label;
       final List<Element> elements = find.text(text).hitTestable().evaluate().toList();
       if (elements.length == 1) {
         final Element element = elements.single;
         final Widget widget = element.widget;
         final DefaultTextStyle defaultTextStyle = DefaultTextStyle.of(element);
         if (widget is Text) {
           TextStyle effectiveTextStyle = widget.style;
           if (widget.style == null || widget.style.inherit)
             effectiveTextStyle = defaultTextStyle.style.merge(widget.style);
           fontSize = effectiveTextStyle.fontSize;
           isBold = effectiveTextStyle.fontWeight == FontWeight.bold;
         } else if (widget is EditableText) {
           isBold = widget.style.fontWeight == FontWeight.bold;
           fontSize = widget.style.fontSize;
         } else {
           assert(false);
         }
       } else if (elements.length > 1) {
         return Evaluation.fail('Multiple nodes with the same label: ${data.label}\n');
       } else {
         // If we can't find the text node then assume the label does not
         // correspond to actual text.
         return result;
       }

       // Transform local coordinate to screen coordinates.
       Rect paintBounds = node.rect;
       SemanticsNode current = node;
       while (current != null && current.parent != null) {
         if (current.transform != null)
           paintBounds = MatrixUtils.transformRect(current.transform, paintBounds);
         paintBounds = paintBounds.shift(current.parent?.rect?.topLeft ?? Offset.zero);
         current = current.parent;
       }
       if (_isNodeOffScreen(paintBounds))
         return result;
       final List<int> subset = _subsetToRect(byteData, paintBounds, image.width, image.height);
       // Node was too far off screen.
      if (subset.isEmpty)
        return result;
       final _ContrastReport report = _ContrastReport(subset);
       final double contrastRatio = report.contrastRatio();
       const double delta = -0.01;
       double targetContrastRatio;
       if ((isBold && fontSize > kBoldTextMinimumSize) || (fontSize ?? 12.0) > kLargeTextMinimumSize) {
         targetContrastRatio = kMinimumRatioLargeText;
       } else {
         targetContrastRatio = kMinimumRatioNormalText;
       }
       if (contrastRatio - targetContrastRatio >= delta)
         return result + const Evaluation.pass();
       return result + Evaluation.fail(
         '$node:\nExpected contrast ratio of at least '
         '$targetContrastRatio but found ${contrastRatio.toStringAsFixed(2)} for a font size of $fontSize. '
         'The computed foreground color was: ${report.lightColor}, '
         'The computed background color was: ${report.darkColor}\n'
         'See also: https://www.w3.org/TR/UNDERSTANDING-WCAG20/visual-audio-contrast-contrast.html'
       );
     }
     return evaluateNode(root);
   }

   // Skip routes which might have labels, and nodes without any text.
   bool _shouldSkipNode(SemanticsData data) {
     if (data.hasFlag(ui.SemanticsFlag.scopesRoute))
       return true;
     if (data.label?.trim()?.isEmpty == true && data.value?.trim()?.isEmpty == true)
       return true;
     return false;
   }

   // Returns a rect that is entirely on screen, or null if it is too far off.
   //
   // Given an 1800 * 2400 pixel buffer, can we actually get all the data from
   // this node? allow a small delta overlap before culling the node.
   bool _isNodeOffScreen(Rect paintBounds) {
     return paintBounds.top < -50.0
       || paintBounds.left <  -50.0
       || paintBounds.bottom > 2400.0 + 50.0
       || paintBounds.right > 1800.0 + 50.0;
   }

   List<int> _subsetToRect(ByteData data, Rect paintBounds, int width, int height) {
     final int newWidth = paintBounds.size.width.ceil();
     final int newHeight = paintBounds.size.height.ceil();
     final int leftX = paintBounds.topLeft.dx.ceil();
     final int rightX = leftX + newWidth;
     final int topY = paintBounds.topLeft.dy.ceil();
     final int bottomY = topY + newHeight;
     final List<int> buffer = <int>[];

     // Data is stored in row major order.
     for (int i = 0; i < data.lengthInBytes; i+=4) {
       final int index = i ~/ 4;
       final int dx = index % width;
       final int dy = index ~/ width;
       if (dx >= leftX && dx <= rightX && dy >= topY && dy <= bottomY) {
         final int r = data.getUint8(i);
         final int g = data.getUint8(i + 1);
         final int b = data.getUint8(i + 2);
         final int a = data.getUint8(i + 3);
         final int color = (((a & 0xff) << 24) |
           ((r & 0xff) << 16) |
           ((g & 0xff) << 8)  |
           ((b & 0xff) << 0)) & 0xFFFFFFFF;
         buffer.add(color);
       }
     }
     return buffer;
   }

   @override
   String get description => 'Text contrast should follow WCAG guidelines';
 }

 class _ContrastReport {
   factory _ContrastReport(List<int> colors) {
     final Map<int, int> colorHistogram = <int, int>{};
     for (int color in colors)
       colorHistogram[color] = (colorHistogram[color] ?? 0) + 1;
     if (colorHistogram.length == 1) {
       final Color hslColor = Color(colorHistogram.keys.first);
       return _ContrastReport._(hslColor, hslColor);
     }
     // to determine the lighter and darker color, partition the colors
     // by lightness and then choose the mode from each group.
     double averageLightness = 0.0;
     for (int color in colorHistogram.keys) {
       final HSLColor hslColor = HSLColor.fromColor(Color(color));
       averageLightness += hslColor.lightness * colorHistogram[color];
     }
     averageLightness /= colors.length;
     assert(averageLightness != double.nan);
     int lightColor = 0;
     int darkColor = 0;
     int lightCount = 0;
     int darkCount = 0;
     // Find the most frequently occurring light and dark color.
     for (MapEntry<int, int> entry in colorHistogram.entries) {
       final HSLColor color = HSLColor.fromColor(Color(entry.key));
       final int count = entry.value;
       if (color.lightness <= averageLightness && count > darkCount) {
         darkColor = entry.key;
         darkCount = count;
       } else if (color.lightness > averageLightness && count > lightCount) {
         lightColor = entry.key;
         lightCount = count;
       }
     }
     assert (lightColor != 0 && darkColor != 0);
     return _ContrastReport._(Color(lightColor), Color(darkColor));
   }

   const _ContrastReport._(this.lightColor, this.darkColor);

   final Color lightColor;
   final Color darkColor;

   /// Computes the contrast ratio as defined by the WCAG.
   ///
   /// source: https://www.w3.org/TR/UNDERSTANDING-WCAG20/visual-audio-contrast-contrast.html
   double contrastRatio() {
     return (_luminance(lightColor) + 0.05) / (_luminance(darkColor) + 0.05);
   }

   /// Relative luminance calculation.
   ///
   /// Based on https://www.w3.org/TR/2008/REC-WCAG20-20081211/#relativeluminancedef
   static double _luminance(Color color) {
     double r = color.red / 255.0;
     double g = color.green / 255.0;
     double b = color.blue / 255.0;
     if (r <= 0.03928)
       r /= 12.92;
     else
       r = math.pow((r + 0.055)/ 1.055, 2.4);
     if (g <= 0.03928)
       g /= 12.92;
     else
       g = math.pow((g + 0.055)/ 1.055, 2.4);
     if (b <= 0.03928)
       b /= 12.92;
     else
       b = math.pow((b + 0.055)/ 1.055, 2.4);
     return 0.2126 * r + 0.7152 * g + 0.0722 * b;
   }
 }

 /// A guideline which requires tapable semantic nodes a minimum size of 48 by 48.
 ///
 /// See also:
 ///
 ///  * [Android tap target guidelines](https://support.google.com/accessibility/android/answer/7101858?hl=en).
 const AccessibilityGuideline androidTapTargetGuideline = MinimumTapTargetGuideline._(
   Size(48.0, 48.0),
   'https://support.google.com/accessibility/android/answer/7101858?hl=en',
 );

 /// A guideline which requires tapable semantic nodes a minimum size of 44 by 44.
 ///
 /// See also:
 ///
 ///   * [iOS human interface guidelines](https://developer.apple.com/design/human-interface-guidelines/ios/visual-design/adaptivity-and-layout/).
 const AccessibilityGuideline iOSTapTargetGuideline = MinimumTapTargetGuideline._(
   Size(44.0, 44.0),
   'https://developer.apple.com/design/human-interface-guidelines/ios/visual-design/adaptivity-and-layout/',
 );

 /// A guideline which requires text contrast to meet minimum values.
 ///
 /// This guideline traverses the semantics tree looking for nodes with values or
 /// labels that corresponds to a Text or Editable text widget. Given the
 /// background pixels for the area around this widget, it performs a very naive
 /// partitioning of the colors into "light" and "dark" and then chooses the most
 /// frequently occurring color in each partition as a representative of the
 /// foreground and background colors. The contrast ratio is calculated from
 /// these colors according to the [WCAG](https://www.w3.org/TR/UNDERSTANDING-WCAG20/visual-audio-contrast-contrast.html#contrast-ratiodef)
 const AccessibilityGuideline textContrastGuideline = MinimumTextContrastGuideline._();
	// Copyright 2018 The Chromium 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:math' as math;
	import 'dart:typed_data';
	import 'dart:ui' as ui;

	import 'package:flutter/rendering.dart';
	import 'package:flutter/semantics.dart';
	import 'package:flutter/widgets.dart';

	import 'finders.dart';
	import 'widget_tester.dart';

	/// The result of evaluating a semantics node by a [AccessibilityGuideline].
	class Evaluation {
	/// Create a passing evaluation.
	const Evaluation.pass() : passed = true, reason = null;

	/// Create a failing evaluation, with an optional [reason] explaining the
	/// result.
	const Evaluation.fail([this.reason]) : passed = false;

	// private constructor for adding cases together.
	const Evaluation._(this.passed, this.reason);

	/// Whether the given tree or node passed the policy evaluation.
	final bool passed;

	/// If [passed] is false, contains the reason for failure.
	final String reason;

	/// Combines two evaluation results.
	///
	/// The [reason] will be concatenated with a newline, and [passed] will be
	/// combined with an `&&` operator.
	Evaluation operator +(Evaluation other) {
	if (other == null)
	return this;
	final StringBuffer buffer = StringBuffer();
	if (reason != null) {
	buffer.write(reason);
	buffer.write(' ');
	}
	if (other.reason != null)
	buffer.write(other.reason);
	return Evaluation._(passed && other.passed, buffer.isEmpty ? null : buffer.toString());
	}
	}

	/// An accessibility guideline describes a recommendation an application should
	/// meet to be considered accessible.
	abstract class AccessibilityGuideline {
	/// A const constructor allows subclasses to be const.
	const AccessibilityGuideline();

	/// Evaluate whether the current state of the `tester` conforms to the rule.
	FutureOr<Evaluation> evaluate(WidgetTester tester);

	/// A description of the policy restrictions and criteria.
	String get description;
	}

	/// A guideline which enforces that all tapable semantics nodes have a minimum
	/// size.
	///
	/// Each platform defines its own guidelines for minimum tap areas.
	@visibleForTesting
	class MinimumTapTargetGuideline extends AccessibilityGuideline {
	const MinimumTapTargetGuideline._(this.size, this.link);

	/// The minimum allowed size of a tapable node.
	final Size size;

	/// A link describing the tap target guidelines for a platform.
	final String link;

	@override
	FutureOr<Evaluation> evaluate(WidgetTester tester) {
	final SemanticsNode root = tester.binding.pipelineOwner.semanticsOwner.rootSemanticsNode;
	Evaluation traverse(SemanticsNode node) {
	Evaluation result = const Evaluation.pass();
	node.visitChildren((SemanticsNode child) {
	result += traverse(child);
	return true;
	});
	if (node.isMergedIntoParent)
	return result;
	final SemanticsData data = node.getSemanticsData();
	// Skip node if it has no actions, or is marked as hidden.
	if ((!data.hasAction(ui.SemanticsAction.longPress)
	&& !data.hasAction(ui.SemanticsAction.tap))
	\|\| data.hasFlag(ui.SemanticsFlag.isHidden))
	return result;
	Rect paintBounds = node.rect;
	SemanticsNode current = node;
	while (current != null) {
	if (current.transform != null)
	paintBounds = MatrixUtils.transformRect(current.transform, paintBounds);
	current = current.parent;
	}
	// skip node if it is touching the edge of the screen, since it might
	// be partially scrolled offscreen.
	const double delta = 0.001;
	if (paintBounds.left <= delta
	\|\| paintBounds.top <= delta
	\|\| (paintBounds.bottom - ui.window.physicalSize.height).abs() <= delta
	\|\| (paintBounds.right - ui.window.physicalSize.width).abs() <= delta)
	return result;
	// shrink by device pixel ratio.
	final Size candidateSize = paintBounds.size / ui.window.devicePixelRatio;
	if (candidateSize.width < size.width \|\| candidateSize.height < size.height)
	result += Evaluation.fail(
	'$node: expected tap target size of at least $size, but found $candidateSize\n'
	'See also: $link');
	return result;
	}
	return traverse(root);
	}

	@override
	String get description => 'Tappable objects should be at least $size';
	}

	/// A guideline which verifies that all nodes that contribute semantics via text
	/// meet minimum contrast levels.
	///
	/// The guidelines are defined by the Web Content Accessibility Guidelines,
	/// http://www.w3.org/TR/UNDERSTANDING-WCAG20/visual-audio-contrast-contrast.html.
	@visibleForTesting
	class MinimumTextContrastGuideline extends AccessibilityGuideline {
	const MinimumTextContrastGuideline._();

	/// The minimum text size considered large for contrast checking.
	///
	/// Defined by http://www.w3.org/TR/UNDERSTANDING-WCAG20/visual-audio-contrast-contrast.html
	static const int kLargeTextMinimumSize = 18;

	/// The minimum text size for bold text to be considered large for contrast
	/// checking.
	///
	/// Defined by http://www.w3.org/TR/UNDERSTANDING-WCAG20/visual-audio-contrast-contrast.html
	static const int kBoldTextMinimumSize = 14;

	/// The minimum contrast ratio for normal text.
	///
	/// Defined by http://www.w3.org/TR/UNDERSTANDING-WCAG20/visual-audio-contrast-contrast.html
	static const double kMinimumRatioNormalText = 4.5;

	/// The minimum contrast ratio for large text.
	///
	/// Defined by http://www.w3.org/TR/UNDERSTANDING-WCAG20/visual-audio-contrast-contrast.html
	static const double kMinimumRatioLargeText = 3.0;

	@override
	Future<Evaluation> evaluate(WidgetTester tester) async {
	final SemanticsNode root = tester.binding.pipelineOwner.semanticsOwner.rootSemanticsNode;
	final RenderView renderView = tester.binding.renderView;
	final OffsetLayer layer = renderView.layer;
	ui.Image image;
	final ByteData byteData = await tester.binding.runAsync<ByteData>(() async {
	// Needs to be the same pixel ratio otherwise our dimensions won't match the
	// last transform layer.
	image = await layer.toImage(renderView.paintBounds, pixelRatio: 1.0);
	return image.toByteData();
	});

	Future<Evaluation> evaluateNode(SemanticsNode node) async {
	final SemanticsData data = node.getSemanticsData();
	final List<SemanticsNode> children = <SemanticsNode>[];
	Evaluation result = const Evaluation.pass();
	node.visitChildren((SemanticsNode child) {
	children.add(child);
	return true;
	});
	for (SemanticsNode child in children)
	result += await evaluateNode(child);
	if (_shouldSkipNode(data))
	return result;

	// We need to look up the inherited text properties to determine the
	// contrast ratio based on text size/weight.
	double fontSize;
	bool isBold;
	final String text = (data.label?.isEmpty == true) ? data.value : data.label;
	final List<Element> elements = find.text(text).hitTestable().evaluate().toList();
	if (elements.length == 1) {
	final Element element = elements.single;
	final Widget widget = element.widget;
	final DefaultTextStyle defaultTextStyle = DefaultTextStyle.of(element);
	if (widget is Text) {
	TextStyle effectiveTextStyle = widget.style;
	if (widget.style == null \|\| widget.style.inherit)
	effectiveTextStyle = defaultTextStyle.style.merge(widget.style);
	fontSize = effectiveTextStyle.fontSize;
	isBold = effectiveTextStyle.fontWeight == FontWeight.bold;
	} else if (widget is EditableText) {
	isBold = widget.style.fontWeight == FontWeight.bold;
	fontSize = widget.style.fontSize;
	} else {
	assert(false);
	}
	} else if (elements.length > 1) {
	return Evaluation.fail('Multiple nodes with the same label: ${data.label}\n');
	} else {
	// If we can't find the text node then assume the label does not
	// correspond to actual text.
	return result;
	}

	// Transform local coordinate to screen coordinates.
	Rect paintBounds = node.rect;
	SemanticsNode current = node;
	while (current != null && current.parent != null) {
	if (current.transform != null)
	paintBounds = MatrixUtils.transformRect(current.transform, paintBounds);
	paintBounds = paintBounds.shift(current.parent?.rect?.topLeft ?? Offset.zero);
	current = current.parent;
	}
	if (_isNodeOffScreen(paintBounds))
	return result;
	final List<int> subset = _subsetToRect(byteData, paintBounds, image.width, image.height);
	// Node was too far off screen.
	if (subset.isEmpty)
	return result;
	final _ContrastReport report = _ContrastReport(subset);
	final double contrastRatio = report.contrastRatio();
	const double delta = -0.01;
	double targetContrastRatio;
	if ((isBold && fontSize > kBoldTextMinimumSize) \|\| (fontSize ?? 12.0) > kLargeTextMinimumSize) {
	targetContrastRatio = kMinimumRatioLargeText;
	} else {
	targetContrastRatio = kMinimumRatioNormalText;
	}
	if (contrastRatio - targetContrastRatio >= delta)
	return result + const Evaluation.pass();
	return result + Evaluation.fail(
	'$node:\nExpected contrast ratio of at least '
	'$targetContrastRatio but found ${contrastRatio.toStringAsFixed(2)} for a font size of $fontSize. '
	'The computed foreground color was: ${report.lightColor}, '
	'The computed background color was: ${report.darkColor}\n'
	'See also: https://www.w3.org/TR/UNDERSTANDING-WCAG20/visual-audio-contrast-contrast.html'
	);
	}
	return evaluateNode(root);
	}

	// Skip routes which might have labels, and nodes without any text.
	bool _shouldSkipNode(SemanticsData data) {
	if (data.hasFlag(ui.SemanticsFlag.scopesRoute))
	return true;
	if (data.label?.trim()?.isEmpty == true && data.value?.trim()?.isEmpty == true)
	return true;
	return false;
	}

	// Returns a rect that is entirely on screen, or null if it is too far off.
	//
	// Given an 1800 * 2400 pixel buffer, can we actually get all the data from
	// this node? allow a small delta overlap before culling the node.
	bool _isNodeOffScreen(Rect paintBounds) {
	return paintBounds.top < -50.0
	\|\| paintBounds.left < -50.0
	\|\| paintBounds.bottom > 2400.0 + 50.0
	\|\| paintBounds.right > 1800.0 + 50.0;
	}

	List<int> _subsetToRect(ByteData data, Rect paintBounds, int width, int height) {
	final int newWidth = paintBounds.size.width.ceil();
	final int newHeight = paintBounds.size.height.ceil();
	final int leftX = paintBounds.topLeft.dx.ceil();
	final int rightX = leftX + newWidth;
	final int topY = paintBounds.topLeft.dy.ceil();
	final int bottomY = topY + newHeight;
	final List<int> buffer = <int>[];

	// Data is stored in row major order.
	for (int i = 0; i < data.lengthInBytes; i+=4) {
	final int index = i ~/ 4;
	final int dx = index % width;
	final int dy = index ~/ width;
	if (dx >= leftX && dx <= rightX && dy >= topY && dy <= bottomY) {
	final int r = data.getUint8(i);
	final int g = data.getUint8(i + 1);
	final int b = data.getUint8(i + 2);
	final int a = data.getUint8(i + 3);
	final int color = (((a & 0xff) << 24) \|
	((r & 0xff) << 16) \|
	((g & 0xff) << 8) \|
	((b & 0xff) << 0)) & 0xFFFFFFFF;
	buffer.add(color);
	}
	}
	return buffer;
	}

	@override
	String get description => 'Text contrast should follow WCAG guidelines';
	}

	class _ContrastReport {
	factory _ContrastReport(List<int> colors) {
	final Map<int, int> colorHistogram = <int, int>{};
	for (int color in colors)
	colorHistogram[color] = (colorHistogram[color] ?? 0) + 1;
	if (colorHistogram.length == 1) {
	final Color hslColor = Color(colorHistogram.keys.first);
	return _ContrastReport._(hslColor, hslColor);
	}
	// to determine the lighter and darker color, partition the colors
	// by lightness and then choose the mode from each group.
	double averageLightness = 0.0;
	for (int color in colorHistogram.keys) {
	final HSLColor hslColor = HSLColor.fromColor(Color(color));
	averageLightness += hslColor.lightness * colorHistogram[color];
	}
	averageLightness /= colors.length;
	assert(averageLightness != double.nan);
	int lightColor = 0;
	int darkColor = 0;
	int lightCount = 0;
	int darkCount = 0;
	// Find the most frequently occurring light and dark color.
	for (MapEntry<int, int> entry in colorHistogram.entries) {
	final HSLColor color = HSLColor.fromColor(Color(entry.key));
	final int count = entry.value;
	if (color.lightness <= averageLightness && count > darkCount) {
	darkColor = entry.key;
	darkCount = count;
	} else if (color.lightness > averageLightness && count > lightCount) {
	lightColor = entry.key;
	lightCount = count;
	}
	}
	assert (lightColor != 0 && darkColor != 0);
	return _ContrastReport._(Color(lightColor), Color(darkColor));
	}

	const _ContrastReport._(this.lightColor, this.darkColor);

	final Color lightColor;
	final Color darkColor;

	/// Computes the contrast ratio as defined by the WCAG.
	///
	/// source: https://www.w3.org/TR/UNDERSTANDING-WCAG20/visual-audio-contrast-contrast.html
	double contrastRatio() {
	return (_luminance(lightColor) + 0.05) / (_luminance(darkColor) + 0.05);
	}

	/// Relative luminance calculation.
	///
	/// Based on https://www.w3.org/TR/2008/REC-WCAG20-20081211/#relativeluminancedef
	static double _luminance(Color color) {
	double r = color.red / 255.0;
	double g = color.green / 255.0;
	double b = color.blue / 255.0;
	if (r <= 0.03928)
	r /= 12.92;
	else
	r = math.pow((r + 0.055)/ 1.055, 2.4);
	if (g <= 0.03928)
	g /= 12.92;
	else
	g = math.pow((g + 0.055)/ 1.055, 2.4);
	if (b <= 0.03928)
	b /= 12.92;
	else
	b = math.pow((b + 0.055)/ 1.055, 2.4);
	return 0.2126 * r + 0.7152 * g + 0.0722 * b;
	}
	}

	/// A guideline which requires tapable semantic nodes a minimum size of 48 by 48.
	///
	/// See also:
	///
	/// * [Android tap target guidelines](https://support.google.com/accessibility/android/answer/7101858?hl=en).
	const AccessibilityGuideline androidTapTargetGuideline = MinimumTapTargetGuideline._(
	Size(48.0, 48.0),
	'https://support.google.com/accessibility/android/answer/7101858?hl=en',
	);

	/// A guideline which requires tapable semantic nodes a minimum size of 44 by 44.
	///
	/// See also:
	///
	/// * [iOS human interface guidelines](https://developer.apple.com/design/human-interface-guidelines/ios/visual-design/adaptivity-and-layout/).
	const AccessibilityGuideline iOSTapTargetGuideline = MinimumTapTargetGuideline._(
	Size(44.0, 44.0),
	'https://developer.apple.com/design/human-interface-guidelines/ios/visual-design/adaptivity-and-layout/',
	);

	/// A guideline which requires text contrast to meet minimum values.
	///
	/// This guideline traverses the semantics tree looking for nodes with values or
	/// labels that corresponds to a Text or Editable text widget. Given the
	/// background pixels for the area around this widget, it performs a very naive
	/// partitioning of the colors into "light" and "dark" and then chooses the most
	/// frequently occurring color in each partition as a representative of the
	/// foreground and background colors. The contrast ratio is calculated from
	/// these colors according to the [WCAG](https://www.w3.org/TR/UNDERSTANDING-WCAG20/visual-audio-contrast-contrast.html#contrast-ratiodef)
	const AccessibilityGuideline textContrastGuideline = MinimumTextContrastGuideline._();