refactor: remove redundant color attributes and improve color contrast handling

CHANGELOG.md

@@ -13,6 +13,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 - improve accessibility globally in the app
 - add scrollable support for subtitle preferences bottom sheet
+- improve color contrast handling
 
 ### Fixed
 

feature/onboarding/src/commonMain/kotlin/bzh/ajaury/chombev/onboarding/ui/components/OnboardingInfoPage.kt

@@ -46,7 +46,6 @@ fun OnboardingInfoPage(
         Text(
             text = description,
             style = MaterialTheme.typography.bodyLarge,
-            color = MaterialTheme.colorScheme.onSurfaceVariant,
             textAlign = TextAlign.Center,
         )
     }

feature/onboarding/src/commonMain/kotlin/bzh/ajaury/chombev/onboarding/ui/components/OnboardingLevelPage.kt

@@ -51,7 +51,6 @@ fun OnboardingLevelPage(
         Text(
             text = stringResource(Res.string.onboarding_level_description),
             style = MaterialTheme.typography.bodyMedium,
-            color = MaterialTheme.colorScheme.onSurfaceVariant,
             textAlign = TextAlign.Center,
         )
         LevelOption(
@@ -110,7 +109,6 @@ private fun LevelOption(
                 Text(
                     text = description,
                     style = MaterialTheme.typography.bodySmall,
-                    color = MaterialTheme.colorScheme.onSurfaceVariant,
                 )
             }
         }

feature/onboarding/src/commonMain/kotlin/bzh/ajaury/chombev/onboarding/ui/components/OnboardingPageIndicator.kt

@@ -32,7 +32,7 @@ fun OnboardingPageIndicator(
                         if (index == currentPage) {
                             MaterialTheme.colorScheme.primary
                         } else {
-                            MaterialTheme.colorScheme.surfaceVariant
+                            MaterialTheme.colorScheme.onBackground
                         },
                     ),
             )

feature/records/src/commonMain/kotlin/bzh/ajaury/chombev/player/ui/PlayerScreen.kt

@@ -241,7 +241,6 @@ private fun PlayerTopBar(
                             Text(
                                 text = translation,
                                 style = MaterialTheme.typography.titleSmall,
-                                color = MaterialTheme.colorScheme.onSurfaceVariant,
                                 maxLines = 1,
                                 overflow = TextOverflow.Ellipsis,
                             )

feature/records/src/commonMain/kotlin/bzh/ajaury/chombev/player/ui/components/LabeledSlider.kt

@@ -10,6 +10,7 @@ import androidx.compose.material3.Text
 import androidx.compose.runtime.Composable
 import androidx.compose.ui.Alignment
 import androidx.compose.ui.Modifier
+import androidx.compose.ui.draw.alpha
 import androidx.compose.ui.semantics.ProgressBarRangeInfo
 import androidx.compose.ui.semantics.clearAndSetSemantics
 import androidx.compose.ui.semantics.contentDescription
@@ -69,13 +70,12 @@ fun LabeledSlider(
             Text(
                 text = title,
                 style = MaterialTheme.typography.titleMedium,
-                color = MaterialTheme.colorScheme.onSurface.copy(alpha = contentAlpha),
+                modifier = Modifier.weight(1f).alpha(contentAlpha),
-                modifier = Modifier.weight(1f),
             )
             Text(
+                modifier = Modifier.alpha(contentAlpha),
                 text = valueLabel,
-                style = MaterialTheme.typography.titleMedium,
+                style = MaterialTheme.typography.labelLarge,
-                color = MaterialTheme.colorScheme.primary.copy(alpha = contentAlpha),
                 maxLines = 1,
                 softWrap = false,
             )

feature/records/src/commonMain/kotlin/bzh/ajaury/chombev/preferences/ui/PreferenceToggle.kt

@@ -51,7 +51,6 @@ fun PreferenceToggle(
             Text(
                 text = subtitle,
                 style = MaterialTheme.typography.bodySmall,
-                color = MaterialTheme.colorScheme.onSurfaceVariant,
             )
         }
 

feature/records/src/commonMain/kotlin/bzh/ajaury/chombev/records/ui/RecordView.kt

@@ -60,7 +60,6 @@ fun RecordView(
                 Text(
                     text = translation,
                     style = MaterialTheme.typography.bodySmall,
-                    color = MaterialTheme.colorScheme.onSurfaceVariant,
                 )
             }
         }

shared/src/commonMain/kotlin/bzh/ajaury/chombev/theme/Color.kt

@@ -4,7 +4,7 @@ import androidx.compose.ui.graphics.Color
 
 // region Light
 internal val md_light_primary = Color(0xFFFF9201)
-internal val md_light_onPrimary = Color(0xFFFFFFFF)
+internal val md_light_onPrimary = Color(0xFF000000)
 internal val md_light_primaryContainer = Color(0xFFFFEBD1)
 internal val md_light_onPrimaryContainer = Color(0xFF1C1613)
 
@@ -25,10 +25,10 @@ internal val md_light_onErrorContainer = Color(0xFF410002)
 
 internal val md_light_background = Color(0xFFF5FAFB)
 internal val md_light_onBackground = Color(0xFF171D1E)
-internal val md_light_surface = Color(0xFFF5FAFB)
+internal val md_light_surface = Color(0xFFFFFFFF)
 internal val md_light_onSurface = Color(0xFF171D1E)
-internal val md_light_surfaceVariant = Color(0xFF91949C)
+internal val md_light_surfaceVariant = Color(0xFFFAFEFF)
-internal val md_light_onSurfaceVariant = Color(0xFF3F484A)
+internal val md_light_onSurfaceVariant = Color(0xFF000000)
 
 internal val md_light_outline = Color(0xFF6F797B)
 internal val md_light_outlineVariant = Color(0xFFBFC8CA)
@@ -64,7 +64,7 @@ internal val md_dark_onBackground = Color(0xFFDEE3E4)
 internal val md_dark_surface = Color(0xFF0E1415)
 internal val md_dark_onSurface = Color(0xFFDEE3E4)
 internal val md_dark_surfaceVariant = Color(0xFF3F484A)
-internal val md_dark_onSurfaceVariant = Color(0xFFBFC8CA)
+internal val md_dark_onSurfaceVariant = Color(0xFFFFFFFF)
 
 internal val md_dark_outline = Color(0xFF899294)
 internal val md_dark_outlineVariant = Color(0xFF3F484A)

shared/src/jvmTest/kotlin/bzh/ajaury/chombev/theme/ColorContrastTest.kt

@@ -0,0 +1,139 @@
+package bzh.ajaury.chombev.theme
+
+import androidx.compose.material3.ColorScheme
+import androidx.compose.ui.graphics.Color
+import androidx.compose.ui.graphics.luminance
+import kotlin.math.round
+import kotlin.test.Test
+import kotlin.test.assertEquals
+import kotlin.test.fail
+
+/**
+ * Checks the color pairs of both themes against the WCAG 2.2 AA contrast ratios: 4.5:1 for text
+ * (1.4.3) and 3:1 for meaningful non-text elements such as borders (1.4.11).
+ *
+ * Each failure lists every offending pair at once, with its ratio, so a palette change can be fixed
+ * in one go. The palette lives in commonMain, so checking it here covers every platform.
+ */
+class ColorContrastTest {
+
+    @Test
+    fun lightColorSchemeMeetsWcagAa() {
+        assertMeetsWcagAa(ChomBevLightColorScheme, themeName = "light")
+    }
+
+    @Test
+    fun darkColorSchemeMeetsWcagAa() {
+        assertMeetsWcagAa(ChomBevDarkColorScheme, themeName = "dark")
+    }
+
+    @Test
+    fun contrastRatioMatchesWcagReferenceValues() {
+        assertEquals(21.0, contrastRatio(Color.Black, Color.White), absoluteTolerance = 0.01)
+        assertEquals(1.0, contrastRatio(Color.White, Color.White), absoluteTolerance = 0.01)
+        // Reference value from the WebAIM contrast checker: #767676 is the lightest gray passing
+        // 4.5:1 on white.
+        assertEquals(4.54, contrastRatio(Color(0xFF767676), Color.White), absoluteTolerance = 0.01)
+        // The ratio does not depend on which color is the foreground.
+        assertEquals(contrastRatio(Color.Red, Color.White), contrastRatio(Color.White, Color.Red))
+    }
+
+    private fun assertMeetsWcagAa(scheme: ColorScheme, themeName: String) {
+        val failures = contrastPairs(scheme).mapNotNull { pair ->
+            val ratio = contrastRatio(pair.foreground, pair.background)
+            if (ratio >= pair.minRatio) {
+                null
+            } else {
+                "${pair.name}: ${ratio.format()}:1 (minimum ${pair.minRatio}:1)"
+            }
+        }
+        if (failures.isNotEmpty()) {
+            fail(
+                "The $themeName color scheme fails WCAG AA contrast for:\n" +
+                    failures.joinToString(separator = "\n") { "  - $it" },
+            )
+        }
+    }
+
+    private fun contrastPairs(scheme: ColorScheme): List<ContrastPair> = with(scheme) {
+        val surfaces = listOf(
+            "surface" to surface,
+            "surfaceDim" to surfaceDim,
+            "surfaceBright" to surfaceBright,
+            "surfaceContainerLowest" to surfaceContainerLowest,
+            "surfaceContainerLow" to surfaceContainerLow,
+            "surfaceContainer" to surfaceContainer,
+            "surfaceContainerHigh" to surfaceContainerHigh,
+            "surfaceContainerHighest" to surfaceContainerHighest,
+        )
+        buildList {
+            // Content colors on the color they are paired with.
+            text("onPrimary", onPrimary, "primary", primary)
+            text("onPrimaryContainer", onPrimaryContainer, "primaryContainer", primaryContainer)
+            text("onSecondary", onSecondary, "secondary", secondary)
+            text("onSecondaryContainer", onSecondaryContainer, "secondaryContainer", secondaryContainer)
+            text("onTertiary", onTertiary, "tertiary", tertiary)
+            text("onTertiaryContainer", onTertiaryContainer, "tertiaryContainer", tertiaryContainer)
+            text("onError", onError, "error", error)
+            text("onErrorContainer", onErrorContainer, "errorContainer", errorContainer)
+            text("onBackground", onBackground, "background", background)
+            text("onSurfaceVariant", onSurfaceVariant, "surfaceVariant", surfaceVariant)
+            text("inverseOnSurface", inverseOnSurface, "inverseSurface", inverseSurface)
+            // Snackbar action.
+            text("inversePrimary", inversePrimary, "inverseSurface", inverseSurface)
+
+            surfaces.forEach { (surfaceName, surfaceColor) ->
+                text("onSurface", onSurface, surfaceName, surfaceColor)
+                text("onSurfaceVariant", onSurfaceVariant, surfaceName, surfaceColor)
+                // Colored text: text buttons, slider values, error messages.
+                text("primary", primary, surfaceName, surfaceColor)
+                text("error", error, surfaceName, surfaceColor)
+                // Borders of text fields and outlined components.
+                nonText("outline", outline, surfaceName, surfaceColor)
+            }
+        }
+    }
+
+    private fun MutableList<ContrastPair>.text(
+        foregroundName: String,
+        foreground: Color,
+        backgroundName: String,
+        background: Color,
+    ) {
+        add(ContrastPair("$foregroundName on $backgroundName", foreground, background, TEXT_MIN_RATIO))
+    }
+
+    private fun MutableList<ContrastPair>.nonText(
+        foregroundName: String,
+        foreground: Color,
+        backgroundName: String,
+        background: Color,
+    ) {
+        add(ContrastPair("$foregroundName on $backgroundName", foreground, background, NON_TEXT_MIN_RATIO))
+    }
+
+    private data class ContrastPair(
+        val name: String,
+        val foreground: Color,
+        val background: Color,
+        val minRatio: Double,
+    )
+
+    private companion object {
+        const val TEXT_MIN_RATIO = 4.5
+        const val NON_TEXT_MIN_RATIO = 3.0
+
+        /** WCAG contrast ratio, from 1 (identical) to 21 (black on white). */
+        fun contrastRatio(first: Color, second: Color): Double {
+            // Color.luminance() is the WCAG relative luminance of the sRGB color.
+            val lighter = maxOf(first.luminance(), second.luminance())
+            val darker = minOf(first.luminance(), second.luminance())
+            return (lighter + 0.05) / (darker + 0.05)
+        }
+
+        fun Double.format(): String {
+            val hundredths = round(this * 100).toInt()
+            return "${hundredths / 100}.${(hundredths % 100).toString().padStart(2, '0')}"
+        }
+    }
+}