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文章出处：http://www.limodev.cn/blog
作者联系方式：李先静 <xianjimli at hotmail dot com>
BatteryService作为电池及充电相关的服务，它的实现非常简单：
o 监听UEvent，读取sysfs里中的状态。
实现了一个UEvent的观察者。uevent是Linux内核用来向用户空间主动上报事件的机制，对于JAVA程序来说，只实现UEventObserver的虚函数onUEvent，然后注册即可。
private UEventObserver mUEventObserver = new UEventObserver() {
@Override
public void onUEvent(UEventObserver.UEvent event) {
update();
}
};
这里只关注power_supply的事件：
mUEventObserver.startObserving("SUBSYSTEM=power_supply");
当有power_supply相关的事件上报时，就会调用update函数。
update先调用native_update从sysfs中读取相关状态（com_android_server_BatteryService.cpp）：
Linux驱动提供了下列文件，供应用程序获取电源相关状态：
#define AC_ONLINE_PATH "/sys/class/power_supply/ac/online"
#define USB_ONLINE_PATH "/sys/class/power_supply/usb/online"
#define BATTERY_STATUS_PATH "/sys/class/power_supply/battery/status"
#define BATTERY_HEALTH_PATH "/sys/class/power_supply/battery/health"
#define BATTERY_PRESENT_PATH "/sys/class/power_supply/battery/present"
#define BATTERY_CAPACITY_PATH "/sys/class/power_supply/battery/capacity"
#define BATTERY_VOLTAGE_PATH "/sys/class/power_supply/battery/batt_vol"
#define BATTERY_TEMPERATURE_PATH "/sys/class/power_supply/battery/batt_temp"
#define BATTERY_TECHNOLOGY_PATH "/sys/class/power_supply/battery/technology"
在＜DA9034驱动程序阅读笔记(6)＞一文中，我已经提到drivers/power/micco_power.c里注册了充电器(ac)、 usb和电池(battery)三个power_supply。各个power_supply提供的属性和上述文件是对应的，从这些文件中可以读到充电器 (ac)、usb和电池(battery)三个power_supply的相应状态。
update然后根据读到的状态更新BatteryService的成员变量，并广播一个Intent来通知其它关注电源状态的组件。
private final void sendIntent() {
// Pack up the values and broadcast them to everyone
Intent intent = new Intent(Intent.ACTION_BATTERY_CHANGED);
intent.addFlags(Intent.FLAG_RECEIVER_REGISTERED_ONLY);
try {
mBatteryStats.setOnBattery(mPlugType == BATTERY_PLUGGED_NONE, mBatteryLevel);
} catch (RemoteException e) {
// Should never happen.
}
int icon = getIcon(mBatteryLevel);
intent.putExtra("status", mBatteryStatus);
intent.putExtra("health", mBatteryHealth);
intent.putExtra("present", mBatteryPresent);
intent.putExtra("level", mBatteryLevel);
intent.putExtra("scale", BATTERY_SCALE);
intent.putExtra("icon-small", icon);
intent.putExtra("plugged", mPlugType);
intent.putExtra("voltage", mBatteryVoltage);
intent.putExtra("temperature", mBatteryTemperature);
intent.putExtra("technology", mBatteryTechnology);
ActivityManagerNative.broadcastStickyIntent(intent, null);
}
关注ACTION_BATTERY_CHANGED的地方有好几个：
o KeyguardUpdateMonitor 这里主要是用来更新锁屏界面下的电池状态。还有低电警告和关机也是在这里做的。
private void handleBatteryUpdate(int pluggedInStatus, int batteryLevel) {
if (DEBUG) Log.d(TAG, "handleBatteryUpdate");
final boolean pluggedIn = isPluggedIn(pluggedInStatus);
if (isBatteryUpdateInteresting(pluggedIn, batteryLevel)) {
mBatteryLevel = batteryLevel;
mDevicePluggedIn = pluggedIn;
for (int i = 0; i < mInfoCallbacks.size(); i++) {
mInfoCallbacks.get(i).onRefreshBatteryInfo(
shouldShowBatteryInfo(), pluggedIn, batteryLevel);
}
}
// shut down gracefully if our battery is critically low and we are not powered
if (batteryLevel == 0 &&
pluggedInStatus != BATTERY_STATUS_CHARGING &&
pluggedInStatus != BATTERY_STATUS_UNKNOWN) {
ShutdownThread.shutdownAfterDisablingRadio(mContext, false);
}
}
o NotificationManagerService 用来更新充电状态（LED)
if (action.equals(Intent.ACTION_BATTERY_CHANGED)) {
boolean batteryCharging = (intent.getIntExtra("plugged", 0) != 0);
int level = intent.getIntExtra("level", -1);
boolean batteryLow = (level >= 0 && level <= Power.LOW_BATTERY_THRESHOLD);
int status = intent.getIntExtra("status", BatteryManager.BATTERY_STATUS_UNKNOWN);
boolean batteryFull = (status == BatteryManager.BATTERY_STATUS_FULL || level >= 90);
if (batteryCharging != mBatteryCharging ||
batteryLow != mBatteryLow ||
batteryFull != mBatteryFull) {
mBatteryCharging = batteryCharging;
mBatteryLow = batteryLow;
mBatteryFull = batteryFull;
updateLights();
}
}
o PowerManagerService 这里主要是做两件事件，先是检查是否在充电时不允许睡眠，并采用相应的行动，其次是触发一个用户行为（会影响下一次睡眠的时间）。
private final class BatteryReceiver extends BroadcastReceiver {
@Override
public void onReceive(Context context, Intent intent) {
synchronized (mLocks) {
boolean wasPowered = mIsPowered;
mIsPowered = mBatteryService.isPowered();
if (mIsPowered != wasPowered) {
// update mStayOnWhilePluggedIn wake lock
updateWakeLockLocked();
// treat plugging and unplugging the devices as a user activity.
// users find it disconcerting when they unplug the device
// and it shuts off right away.
// temporarily set mUserActivityAllowed to true so this will work
// even when the keyguard is on.
synchronized (mLocks) {
boolean savedActivityAllowed = mUserActivityAllowed;
mUserActivityAllowed = true;
userActivity(SystemClock.uptimeMillis(), false);
mUserActivityAllowed = savedActivityAllowed;
}
}
}
}
}
o LocationManagerService 这里似乎没有什么用处，我没找到mCollector赋值的地方。
if (action.equals(Intent.ACTION_BATTERY_CHANGED)) {
log("PowerStateBroadcastReceiver: Battery changed");
synchronized (mLocationListeners) {
int scale = intent.getIntExtra(BATTERY_EXTRA_SCALE, 100);
int level = intent.getIntExtra(BATTERY_EXTRA_LEVEL, 0);
boolean plugged = intent.getIntExtra(BATTERY_EXTRA_PLUGGED, 0) != 0;
// Notify collector battery state
if (mCollector != null) {
mCollector.updateBatteryState(scale, level, plugged);
}
}
}
o WifiService 根据电源状态来决定是否需要定时唤醒(没搞得太明白，看Wifi服务时再研究)。
if (action.equals(Intent.ACTION_BATTERY_CHANGED)) {
/*
* Set a timer to put Wi-Fi to sleep, but only if the screen is off
* AND we are transitioning from a state in which the device was supposed
* to stay awake to a state in which it is not supposed to stay awake.
* If "stay awake" state is not changing, we do nothing, to avoid resetting
* the already-set timer.
*/
int pluggedType = intent.getIntExtra("plugged", 0);
if (mScreenOff && shouldWifiStayAwake(stayAwakeConditions, mPluggedType) &&
!shouldWifiStayAwake(stayAwakeConditions, pluggedType)) {
long triggerTime = System.currentTimeMillis() + idleMillis;
mAlarmManager.set(AlarmManager.RTC_WAKEUP, triggerTime, mIdleIntent);
mPluggedType = pluggedType;
return;
}
mPluggedType = pluggedType;
}
o StatusBarPolicy用来更新状态栏上的充电图标。
if (action.equals(Intent.ACTION_BATTERY_CHANGED)) {
updateBattery(intent);
}