PowerUI是SystemUI控制电量提醒的模块，包括低电提醒和高低温关机提醒。

初始化

PowerUI继承SystemUI，故在SystemUI启动时，PowerUI也会随之调用start()，其代码如下：

public void start() {
        // 获取PowerManager
        mPowerManager = (PowerManager) mContext.getSystemService(Context.POWER_SERVICE);
        // 获取HardwarePorpertiesManger 以便获取当前手机温度
        mHardwarePropertiesManager = (HardwarePropertiesManager)
                mContext.getSystemService(Context.HARDWARE_PROPERTIES_SERVICE);
        mScreenOffTime = mPowerManager.isScreenOn() ? -1 : SystemClock.elapsedRealtime();
        // 获取WarningsUI实例
        mWarnings = Dependency.get(WarningsUI.class);
        mLastConfiguration.setTo(mContext.getResources().getConfiguration());
        // 注册低电系统阈值变化观察者
        ContentObserver obs = new ContentObserver(mHandler) {
            @Override
            public void onChange(boolean selfChange) {
                updateBatteryWarningLevels();
            }
        };
        final ContentResolver resolver = mContext.getContentResolver();
        resolver.registerContentObserver(Settings.Global.getUriFor(
                Settings.Global.LOW_POWER_MODE_TRIGGER_LEVEL),
                false, obs, UserHandle.USER_ALL);
        // 初始化低电警告阈值
        updateBatteryWarningLevels();
        // 初始化电量相关信息广播接收者
        mReceiver.init();

        // Check to see if we need to let the user know that the phone previously shut down due
        // to the temperature being too high.
        showThermalShutdownDialog();
        // 初始化温度警告阈值
        initTemperatureWarning();
    }

从上面不难看出，PowerUI两个功能：
一、监听电量变化，发出低电警告；
二、监听电池温度变化，发出温度警告。

低电警告

在start()时，会进行updateBatteryWarningLevels()，其代码如下:

void updateBatteryWarningLevels() {
        int critLevel = mContext.getResources().getInteger(
                com.android.internal.R.integer.config_criticalBatteryWarningLevel);

        final ContentResolver resolver = mContext.getContentResolver();
        int defWarnLevel = mContext.getResources().getInteger(
                com.android.internal.R.integer.config_lowBatteryWarningLevel);
        int warnLevel =  Settings.Global.getInt(resolver,
                    Settings.Global.LOW_POWER_MODE_TRIGGER_LEVEL, defWarnLevel);
       
        if (warnLevel == 0) {
            warnLevel = defWarnLevel;
        }
        if (warnLevel < critLevel) {
            warnLevel = critLevel;
        }

        mLowBatteryReminderLevels[0] = warnLevel;
        mLowBatteryReminderLevels[1] = critLevel;
        mLowBatteryAlertCloseLevel = mLowBatteryReminderLevels[0]
                + mContext.getResources().getInteger(
                        com.android.internal.R.integer.config_lowBatteryCloseWarningBump);
    }

这里有两个值需要关注：warnLevel和critLevel，warnLevel默认值是15，可以随系统数据库字段LOW_POWER_MODE_TRIGGER_LEVEL对应值而改变，其主要作用为进入低电警告的阈值。critLevel默认值是5，其主要作用是打开低电保护的阈值。这两个值都被写入数组mLowBatteryReminderLevels[]，用来计算是否需要显示或者更新低电提醒。
那什么时候触发低电警告呢？可以看到，在初始化的时候，PowerUI会去注册一个广播接收者，代码如下：

private final class Receiver extends BroadcastReceiver {

        public void init() {
            // Register for Intent broadcasts for...
            IntentFilter filter = new IntentFilter();
            filter.addAction(Intent.ACTION_BATTERY_CHANGED);
            filter.addAction(Intent.ACTION_SCREEN_OFF);
            filter.addAction(Intent.ACTION_SCREEN_ON);
            filter.addAction(Intent.ACTION_USER_SWITCHED);
            mContext.registerReceiver(this, filter, null, mHandler);
        }

        @Override
        public void onReceive(Context context, Intent intent) {
            String action = intent.getAction();
            if (action.equals(Intent.ACTION_BATTERY_CHANGED)) {
                final int oldBatteryLevel = mBatteryLevel;
                mBatteryLevel = intent.getIntExtra(BatteryManager.EXTRA_LEVEL, 100);
                final int oldBatteryStatus = mBatteryStatus;
                mBatteryStatus = intent.getIntExtra(BatteryManager.EXTRA_STATUS,
                        BatteryManager.BATTERY_STATUS_UNKNOWN);
                final int oldPlugType = mPlugType;
                mPlugType = intent.getIntExtra(BatteryManager.EXTRA_PLUGGED, 1);
                final int oldInvalidCharger = mInvalidCharger;
                mInvalidCharger = intent.getIntExtra(BatteryManager.EXTRA_INVALID_CHARGER, 0);

                final boolean plugged = mPlugType != 0;
                final boolean oldPlugged = oldPlugType != 0;

                int oldBucket = findBatteryLevelBucket(oldBatteryLevel);
                int bucket = findBatteryLevelBucket(mBatteryLevel);

                if (DEBUG) {
                    Slog.d(TAG, "buckets   ....." + mLowBatteryAlertCloseLevel
                            + " .. " + mLowBatteryReminderLevels[0]
                            + " .. " + mLowBatteryReminderLevels[1]);
                    Slog.d(TAG, "level          " + oldBatteryLevel + " --> " + mBatteryLevel);
                    Slog.d(TAG, "status         " + oldBatteryStatus + " --> " + mBatteryStatus);
                    Slog.d(TAG, "plugType       " + oldPlugType + " --> " + mPlugType);
                    Slog.d(TAG, "invalidCharger " + oldInvalidCharger + " --> " + mInvalidCharger);
                    Slog.d(TAG, "bucket         " + oldBucket + " --> " + bucket);
                    Slog.d(TAG, "plugged        " + oldPlugged + " --> " + plugged);
                }

                mWarnings.update(mBatteryLevel, bucket, mScreenOffTime);
                if (oldInvalidCharger == 0 && mInvalidCharger != 0) {
                    Slog.d(TAG, "showing invalid charger warning");
                    mWarnings.showInvalidChargerWarning();
                    return;
                } else if (oldInvalidCharger != 0 && mInvalidCharger == 0) {
                    mWarnings.dismissInvalidChargerWarning();
                } else if (mWarnings.isInvalidChargerWarningShowing()) {
                    // if invalid charger is showing, don't show low battery
                    return;
                }

                boolean isPowerSaver = mPowerManager.isPowerSaveMode();
                if (!plugged
                        && !isPowerSaver
                        && (bucket < oldBucket || oldPlugged)
                        && mBatteryStatus != BatteryManager.BATTERY_STATUS_UNKNOWN
                        && bucket < 0) {

                    // only play SFX when the dialog comes up or the bucket changes
                    final boolean playSound = bucket != oldBucket || oldPlugged;
                    mWarnings.showLowBatteryWarning(playSound);
                } else if (isPowerSaver || plugged || (bucket > oldBucket && bucket > 0)) {
                    mWarnings.dismissLowBatteryWarning();
                } else {
                    mWarnings.updateLowBatteryWarning();
                }
            } else if (Intent.ACTION_SCREEN_OFF.equals(action)) {
                mScreenOffTime = SystemClock.elapsedRealtime();
            } else if (Intent.ACTION_SCREEN_ON.equals(action)) {
                mScreenOffTime = -1;
            } else if (Intent.ACTION_USER_SWITCHED.equals(action)) {
                mWarnings.userSwitched();
            } else {
                Slog.w(TAG, "unknown intent: " + intent);
            }
        }
    };

可以看到当收到ACTION_BATTERY_CHANGED的广播时，将进行一系列的flag判断，如是否开启电量保护、是否usb插入以及对当前电量的判断，这里电量判断通过一个bucket值来进行判断是否属于健康电量，其判断逻辑如下：

/**
     * Buckets the battery level.
     *
     * The code in this function is a little weird because I couldn't comprehend
     * the bucket going up when the battery level was going down. --joeo
     *
     * 1 means that the battery is "ok"
     * 0 means that the battery is between "ok" and what we should warn about.
     * less than 0 means that the battery is low
     */
    private int findBatteryLevelBucket(int level) {
        if (level >= mLowBatteryAlertCloseLevel) {
            return 1;
        }
        if (level > mLowBatteryReminderLevels[0]) {
            return 0;
        }
        final int N = mLowBatteryReminderLevels.length;
        for (int i=N-1; i>=0; i--) {
            if (level <= mLowBatteryReminderLevels[i]) {
                return -1-i;
            }
        }
        throw new RuntimeException("not possible!");
    }

这里用到了之前介绍的mLowBatteryReminderLevels[]判断，即warnLevel和critLevel两个值进行判断，当return值小于0则认为当前电量不健康。从而触发低电警告。

高温警告

高温警告触发于电池温度过高时，会提示用户电池温度过高，将设备关机。初始化逻辑如下:

private void initTemperatureWarning() {
        ContentResolver resolver = mContext.getContentResolver();
        Resources resources = mContext.getResources();
        if (Settings.Global.getInt(resolver, Settings.Global.SHOW_TEMPERATURE_WARNING,
                resources.getInteger(R.integer.config_showTemperatureWarning)) == 0) {
            return;
        }

        mThresholdTemp = Settings.Global.getFloat(resolver, Settings.Global.WARNING_TEMPERATURE,
                resources.getInteger(R.integer.config_warningTemperature));

        if (mThresholdTemp < 0f) {
            // Get the throttling temperature. No need to check if we're not throttling.
            float[] throttlingTemps = mHardwarePropertiesManager.getDeviceTemperatures(
                    HardwarePropertiesManager.DEVICE_TEMPERATURE_SKIN,
                    HardwarePropertiesManager.TEMPERATURE_SHUTDOWN);
            if (throttlingTemps == null
                    || throttlingTemps.length == 0
                    || throttlingTemps[0] == HardwarePropertiesManager.UNDEFINED_TEMPERATURE) {
                return;
            }
            mThresholdTemp = throttlingTemps[0] -
                    resources.getInteger(R.integer.config_warningTemperatureTolerance);
        }

        setNextLogTime();

        // This initialization method may be called on a configuration change. Only one set of
        // ongoing callbacks should be occurring, so remove any now. updateTemperatureWarning will
        // schedule an ongoing callback.
        mHandler.removeCallbacks(mUpdateTempCallback);

        // We have passed all of the checks, start checking the temp
        updateTemperatureWarning();
    }

通过这个获取高温警告阈值，供其在updateTemperatureWarning()中进行判断，其代码如下：

 // We create a method reference here so that we are guaranteed that we can remove a callback
 // by using the same instance (method references are not guaranteed to be the same object
 // each time they are created).
 private final Runnable mUpdateTempCallback = this::updateTemperatureWarning;
 protected void updateTemperatureWarning() {
        float[] temps = mHardwarePropertiesManager.getDeviceTemperatures(
                HardwarePropertiesManager.DEVICE_TEMPERATURE_SKIN,
                HardwarePropertiesManager.TEMPERATURE_CURRENT);
        if (temps.length != 0) {
            float temp = temps[0];
            mRecentTemps[mNumTemps++] = temp;

            StatusBar statusBar = getComponent(StatusBar.class);
            if (statusBar != null && !statusBar.isDeviceInVrMode()
                    && temp >= mThresholdTemp) {
                logAtTemperatureThreshold(temp);
                mWarnings.showHighTemperatureWarning();
            } else {
                mWarnings.dismissHighTemperatureWarning();
            }
        }

        logTemperatureStats();

        mHandler.postDelayed(mUpdateTempCallback, TEMPERATURE_INTERVAL);
    }

并且将该方法加入Looper中，这里的TEMPERATURE_INTERVAL=30s，即表示每30s进行一次设备电池温度的获取，并判断是否需要发出高温警告。

小结

PowerUI是SystemUI中监听系统电池模块状态，并以UI形式展现出来的一种框架。如果在定制修改中需要监听系统的某些模块的状态，同样可以采取类似框架进行客制化处理。