Contrast logic

ContrastExtensions.kt

// Kotlin, Android min 19 SDK 

private enum class Adjustment {
    DARKEN,
    LIGHTEN,
    NONE
}

fun Context.adjustTextColor(color: Int, isHighContrastEnabled: Boolean): Int {
    if (isHighContrastEnabled) {
        return color
    }
    return when(checkAdjustment(color)) {
        Adjustment.DARKEN -> Color.BLACK
        Adjustment.LIGHTEN -> Color.WHITE
        Adjustment.NONE -> color
    }
}

fun Context.adjustTextColor(color: ColorStateList, isHighContrastEnabled: Boolean): ColorStateList {
    return when (checkAdjustment(color.defaultColor)) {
        Adjustment.DARKEN -> ColorStateList.valueOf(Color.BLACK)
        Adjustment.LIGHTEN -> ColorStateList.valueOf(Color.WHITE)
        Adjustment.NONE -> color
    }
}

private fun isContrastTooSmall(color: Int, bgColor: Int): Boolean {
    // WCAG 2.0 level AA requires a contrast ratio of 4.5:1
    return ColorUtils.calculateContrast(color, bgColor) < 4.7f
}

private fun checkAdjustment(color: Int): Adjustment {
    val luminance = ColorUtils.calculateLuminance(color)
    if (luminance == 1.0 || luminance == 0.0) {
        return Adjustment.NONE
    }

    if (luminance >= 0.5 && isContrastTooSmall(color, Color.BLACK)) {
        return Adjustment.LIGHTEN
    }

    if (luminance < 0.5 && isContrastTooSmall(color, Color.WHITE)) {
        return Adjustment.DARKEN
    }
    return Adjustment.NONE
}