46 lines
1.6 KiB
Dart
46 lines
1.6 KiB
Dart
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import 'dart:math' as math;
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import 'package:flutter/material.dart';
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/// Parses a hex colour string (`#RRGGBB` or `RRGGBB`). Returns `null` when
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/// invalid so callers can skip the pair instead of throwing.
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Color? parseHexColor(String? value) {
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if (value == null || value.trim().isEmpty) return null;
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var hex = value.trim();
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if (!hex.startsWith('#')) hex = '#$hex';
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if (!RegExp(r'^#[0-9A-Fa-f]{6}$').hasMatch(hex)) return null;
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return Color(int.parse(hex.substring(1), radix: 16) + 0xFF000000);
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}
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/// WCAG 2.1 relative luminance contrast ratio between two sRGB colours.
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double contrastRatio(Color foreground, Color background) {
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final l1 = foreground.computeLuminance();
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final l2 = background.computeLuminance();
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final lighter = math.max(l1, l2);
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final darker = math.min(l1, l2);
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return (lighter + 0.05) / (darker + 0.05);
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}
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/// Returns the contrast ratio for a hex pair, or `null` when either colour is
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/// invalid.
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double? hexContrastRatio(String foreground, String background) {
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final fg = parseHexColor(foreground);
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final bg = parseHexColor(background);
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if (fg == null || bg == null) return null;
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return contrastRatio(fg, bg);
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}
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/// WCAG 2.1 level AA thresholds.
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const double kWcagAaNormalText = 4.5;
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const double kWcagAaLargeText = 3.0;
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/// Body text below this ratio is treated as a hard quality error.
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const double kWcagCriticalBodyText = 3.0;
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bool meetsWcagAa(String foreground, String background, {bool largeText = false}) {
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final ratio = hexContrastRatio(foreground, background);
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if (ratio == null) return true;
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final threshold = largeText ? kWcagAaLargeText : kWcagAaNormalText;
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return ratio >= threshold;
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}
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