OK so I got to know how this usually works, on both newest version of Android (Q) and before.
It seems that when the OS creates the WallpaperColors , it also generates color-hints. In the function WallpaperColors.fromBitmap , there is a call to int hints = calculateDarkHints(bitmap); , and this is the code of calculateDarkHints :
/**
* Checks if image is bright and clean enough to support light text.
*
* @param source What to read.
* @return Whether image supports dark text or not.
*/
private static int calculateDarkHints(Bitmap source) {
if (source == null) {
return 0;
}
int[] pixels = new int[source.getWidth() * source.getHeight()];
double totalLuminance = 0;
final int maxDarkPixels = (int) (pixels.length * MAX_DARK_AREA);
int darkPixels = 0;
source.getPixels(pixels, 0 /* offset */, source.getWidth(), 0 /* x */, 0 /* y */,
source.getWidth(), source.getHeight());
// This bitmap was already resized to fit the maximum allowed area.
// Let's just loop through the pixels, no sweat!
float[] tmpHsl = new float[3];
for (int i = 0; i < pixels.length; i++) {
ColorUtils.colorToHSL(pixels[i], tmpHsl);
final float luminance = tmpHsl[2];
final int alpha = Color.alpha(pixels[i]);
// Make sure we don't have a dark pixel mass that will
// make text illegible.
if (luminance < DARK_PIXEL_LUMINANCE && alpha != 0) {
darkPixels++;
}
totalLuminance += luminance;
}
int hints = 0;
double meanLuminance = totalLuminance / pixels.length;
if (meanLuminance > BRIGHT_IMAGE_MEAN_LUMINANCE && darkPixels < maxDarkPixels) {
hints |= HINT_SUPPORTS_DARK_TEXT;
}
if (meanLuminance < DARK_THEME_MEAN_LUMINANCE) {
hints |= HINT_SUPPORTS_DARK_THEME;
}
return hints;
}
Then searching for getColorHints that the WallpaperColors.java has, I've found updateTheme function in StatusBar.java :
WallpaperColors systemColors = mColorExtractor
.getWallpaperColors(WallpaperManager.FLAG_SYSTEM);
final boolean useDarkTheme = systemColors != null
&& (systemColors.getColorHints() & WallpaperColors.HINT_SUPPORTS_DARK_THEME) != 0;
This would work only on Android 8.1 , because then the theme was based on the colors of the wallpaper alone. On Android 9.0 , the user can set it without any connection to the wallpaper.
Here's what I've made, according to what I've seen on Android :
enum class DarkThemeCheckResult {
DEFAULT_BEFORE_THEMES, LIGHT, DARK, PROBABLY_DARK, PROBABLY_LIGHT, USER_CHOSEN
}
@JvmStatic
fun getIsOsDarkTheme(context: Context): DarkThemeCheckResult {
when {
Build.VERSION.SDK_INT <= Build.VERSION_CODES.O -> return DarkThemeCheckResult.DEFAULT_BEFORE_THEMES
Build.VERSION.SDK_INT <= Build.VERSION_CODES.P -> {
val wallpaperManager = WallpaperManager.getInstance(context)
val wallpaperColors = wallpaperManager.getWallpaperColors(WallpaperManager.FLAG_SYSTEM)
?: return DarkThemeCheckResult.UNKNOWN
val primaryColor = wallpaperColors.primaryColor.toArgb()
val secondaryColor = wallpaperColors.secondaryColor?.toArgb() ?: primaryColor
val tertiaryColor = wallpaperColors.tertiaryColor?.toArgb() ?: secondaryColor
val bitmap = generateBitmapFromColors(primaryColor, secondaryColor, tertiaryColor)
val darkHints = calculateDarkHints(bitmap)
//taken from StatusBar.java , in updateTheme :
val HINT_SUPPORTS_DARK_THEME = 1 shl 1
val useDarkTheme = darkHints and HINT_SUPPORTS_DARK_THEME != 0
if (Build.VERSION.SDK_INT == VERSION_CODES.O_MR1)
return if (useDarkTheme)
DarkThemeCheckResult.UNKNOWN_MAYBE_DARK
else DarkThemeCheckResult.UNKNOWN_MAYBE_LIGHT
return if (useDarkTheme)
DarkThemeCheckResult.MOST_PROBABLY_DARK
else DarkThemeCheckResult.MOST_PROBABLY_LIGHT
}
else -> {
return when (context.resources.configuration.uiMode and Configuration.UI_MODE_NIGHT_MASK) {
Configuration.UI_MODE_NIGHT_YES -> DarkThemeCheckResult.DARK
Configuration.UI_MODE_NIGHT_NO -> DarkThemeCheckResult.LIGHT
else -> DarkThemeCheckResult.MOST_PROBABLY_LIGHT
}
}
}
}
fun generateBitmapFromColors(@ColorInt primaryColor: Int, @ColorInt secondaryColor: Int, @ColorInt tertiaryColor: Int): Bitmap {
val colors = intArrayOf(primaryColor, secondaryColor, tertiaryColor)
val imageSize = 6
val bitmap = Bitmap.createBitmap(imageSize, 1, Bitmap.Config.ARGB_8888)
for (i in 0 until imageSize / 2)
bitmap.setPixel(i, 0, colors[0])
for (i in imageSize / 2 until imageSize / 2 + imageSize / 3)
bitmap.setPixel(i, 0, colors[1])
for (i in imageSize / 2 + imageSize / 3 until imageSize)
bitmap.setPixel(i, 0, colors[2])
return bitmap
}
I've set the various possible values, because in most of those cases nothing is guaranteed.
