1、安卓每个版本的更新， 其核心代码也会随之更新，本次为Android 12 版本默认屏幕亮度的修改。

其中涉及的核心代码主要有：

packages\apps\Settings\src\com\android\settings\display\BrightnessLevelPreferenceController.java
frameworks\base\services\core\java\com\android\server\power\PowerManagerService.java
frameworks/base/res/res/values/config.xml

2、设置默认亮度的主要方法代码

在BrightnessLevelPreferenceController中处理：

 public class BrightnessLevelPreferenceController extends AbstractPreferenceController implementsPreferenceControllerMixin, LifecycleObserver, OnStart, OnStop {private ContentObserver mBrightnessObserver =new ContentObserver(new Handler(Looper.getMainLooper())) {@Overridepublic void onChange(boolean selfChange) {updatedSummary(mPreference);}};public BrightnessLevelPreferenceController(Context context, Lifecycle lifecycle) {super(context);if (lifecycle != null) {lifecycle.addObserver(this);}PowerManager powerManager = (PowerManager) context.getSystemService(Context.POWER_SERVICE);mMinBrightness = powerManager.getBrightnessConstraint(PowerManager.BRIGHTNESS_CONSTRAINT_TYPE_MINIMUM);mMaxBrightness = powerManager.getBrightnessConstraint(PowerManager.BRIGHTNESS_CONSTRAINT_TYPE_MAXIMUM);mMinVrBrightness = powerManager.getBrightnessConstraint(PowerManager.BRIGHTNESS_CONSTRAINT_TYPE_MINIMUM_VR);mMaxVrBrightness = powerManager.getBrightnessConstraint(PowerManager.BRIGHTNESS_CONSTRAINT_TYPE_MAXIMUM_VR);mContentResolver = mContext.getContentResolver();mDefaultBacklight = powerManager.getBrightnessConstraint(PowerManager.BRIGHTNESS_CONSTRAINT_TYPE_DEFAULT);}

在PowerManager得到相应的值：

  private void updatedSummary(Preference preference) {if (preference != null) {preference.setSummary(NumberFormat.getPercentInstance().format(getCurrentBrightness()));}}private double getCurrentBrightness() {final int value;if (isInVrMode()) {value = convertLinearToGammaFloat(System.getFloat(mContentResolver,System.SCREEN_BRIGHTNESS_FOR_VR_FLOAT, mMaxBrightness),mMinVrBrightness, mMaxVrBrightness);} else {value = convertLinearToGammaFloat(Settings.System.getFloat(mContentResolver,System.SCREEN_BRIGHTNESS_FLOAT, mDefaultBacklight),mMinBrightness, mMaxBrightness);}return getPercentage(value, GAMMA_SPACE_MIN, GAMMA_SPACE_MAX);}private double getPercentage(double value, int min, int max) {if (value > max) {return 1.0;}if (value < min) {return 0.0;}return (value - min) / (max - min);}

…

public final class PowerManagerService extends SystemServiceimplements Watchdog.Monitor {@VisibleForTestingPowerManagerService(Context context, Injector injector) {super(context);mContext = context;......// Save brightness values:// Get float values from config.// Store float if valid// Otherwise, get int values and convert to float and then store.final float min = mContext.getResources().getFloat(com.android.internal.R.dimen.config_screenBrightnessSettingMinimumFloat);final float max = mContext.getResources().getFloat(com.android.internal.R.dimen.config_screenBrightnessSettingMaximumFloat);final float def = mContext.getResources().getFloat(com.android.internal.R.dimen.config_screenBrightnessSettingDefaultFloat);final float doze = mContext.getResources().getFloat(com.android.internal.R.dimen.config_screenBrightnessDozeFloat);final float dim = mContext.getResources().getFloat(com.android.internal.R.dimen.config_screenBrightnessDimFloat);if (min == INVALID_BRIGHTNESS_IN_CONFIG || max == INVALID_BRIGHTNESS_IN_CONFIG|| def == INVALID_BRIGHTNESS_IN_CONFIG) {mScreenBrightnessMinimum = BrightnessSynchronizer.brightnessIntToFloat(mContext.getResources().getInteger(com.android.internal.R.integer.config_screenBrightnessSettingMinimum),PowerManager.BRIGHTNESS_OFF + 1, PowerManager.BRIGHTNESS_ON,PowerManager.BRIGHTNESS_MIN, PowerManager.BRIGHTNESS_MAX);mScreenBrightnessMaximum = BrightnessSynchronizer.brightnessIntToFloat(mContext.getResources().getInteger(com.android.internal.R.integer.config_screenBrightnessSettingMaximum),PowerManager.BRIGHTNESS_OFF + 1, PowerManager.BRIGHTNESS_ON,PowerManager.BRIGHTNESS_MIN, PowerManager.BRIGHTNESS_MAX);mScreenBrightnessDefault = BrightnessSynchronizer.brightnessIntToFloat(mContext.getResources().getInteger(com.android.internal.R.integer.config_screenBrightnessSettingDefault),PowerManager.BRIGHTNESS_OFF + 1, PowerManager.BRIGHTNESS_ON,PowerManager.BRIGHTNESS_MIN, PowerManager.BRIGHTNESS_MAX);} else {mScreenBrightnessMinimum = min;mScreenBrightnessMaximum = max;mScreenBrightnessDefault = def;}..}

其中它是通过config中实现的：

 public float getBrightnessConstraint(int constraint) {switch (constraint) {case PowerManager.BRIGHTNESS_CONSTRAINT_TYPE_MINIMUM:return mScreenBrightnessMinimum;case PowerManager.BRIGHTNESS_CONSTRAINT_TYPE_MAXIMUM:return mScreenBrightnessMaximum;case PowerManager.BRIGHTNESS_CONSTRAINT_TYPE_DEFAULT:return mScreenBrightnessDefault;case PowerManager.BRIGHTNESS_CONSTRAINT_TYPE_DIM:return mScreenBrightnessDim;case PowerManager.BRIGHTNESS_CONSTRAINT_TYPE_DOZE:return mScreenBrightnessDoze;case PowerManager.BRIGHTNESS_CONSTRAINT_TYPE_MINIMUM_VR:return mScreenBrightnessMinimumVr;case PowerManager.BRIGHTNESS_CONSTRAINT_TYPE_MAXIMUM_VR:return mScreenBrightnessMaximumVr;case PowerManager.BRIGHTNESS_CONSTRAINT_TYPE_DEFAULT_VR:return mScreenBrightnessDefaultVr;default:return PowerManager.BRIGHTNESS_INVALID_FLOAT;}}

3、修改代码的位置：

   --- a/frameworks/base/res/res/values/config.xml+++ b//frameworks/base/res/res/values/config.xml@@ -28,5 +28,5 @@<item name="config_screenBrightnessSettingMaximumFloat" format="float" type="dimen">1.0</item><!-- Default screen brightness setting set.Set this to 0.4 for Default brightness Float.-->-    <item name="config_screenBrightnessSettingDefaultFloat" format="float" type="dimen">0.3</item>+    <item name="config_screenBrightnessSettingDefaultFloat" format="float" type="dimen">0.4</item></resources>