Android之 BatteryService

電池的信息，電壓，溫度，充電狀態等等，都是由BatteryService來提供的。BatteryService是跑在system_process當中，在系統初始化的時候啓動，如下

     在BatteryService.java中：
          Log.i(TAG, "Starting Battery Service.");
          BatteryService battery = new BatteryService(context);
          ServiceManager.addService("battery", battery);
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1. 數據來源
        BatteryService通過JNI（com_android_server_BatteryService.cpp）讀取數據。BatteryService通過JNI註冊的不僅有函數，還有變量。如下：

    //##############在BatteryService.java中聲明的變量################
    private boolean mAcOnline;
    private boolean mUsbOnline;
    private int mBatteryStatus;
    private int mBatteryHealth;
    private boolean mBatteryPresent;
    private int mBatteryLevel;
    private int mBatteryVoltage;
    private int mBatteryTemperature;
    private String mBatteryTechnology;

//在BatteryService.java中聲明的變量，在com_android_server_BatteryService.cpp中共用，即在com_android_server_BatteryService.cpp中其實操作的也是BatteryService.java中聲明的變量

   gFieldIds.mAcOnline = env->GetFieldID(clazz, "mAcOnline", "Z");
    gFieldIds.mUsbOnline = env->GetFieldID(clazz, "mUsbOnline", "Z");
    gFieldIds.mBatteryStatus = env->GetFieldID(clazz, "mBatteryStatus", "I");
    gFieldIds.mBatteryHealth = env->GetFieldID(clazz, "mBatteryHealth", "I");
    gFieldIds.mBatteryPresent = env->GetFieldID(clazz, "mBatteryPresent", "Z");
    gFieldIds.mBatteryLevel = env->GetFieldID(clazz, "mBatteryLevel", "I");
    gFieldIds.mBatteryTechnology = env->GetFieldID(clazz, "mBatteryTechnology", "Ljava/lang/String;");
    gFieldIds.mBatteryVoltage = env->GetFieldID(clazz, "mBatteryVoltage", "I");
    gFieldIds.mBatteryTemperature = env->GetFieldID(clazz, "mBatteryTemperature", "I");

//上面這些變量的值，對應是從下面的文件中讀取的，一隻文件存儲一個數值。

    #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"

Android是運行在Linux內核上面的，/sys/class/power_supply亦是Linux內核下面的目錄。至於這些文件時怎麼生成的，則是由Platform來控制的。

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2. 數據傳送
電池的這些信息是通過何種方式，被其他應用所獲得的。可以想到的有兩種方式，第一種，應用主動從BatteryService獲得數據；第二種，BatteryService主動把數據傳送給所關心的應用程序。

BatteryService採用的是第二種方式，所有的電池的信息數據是通過Intent傳送出去的。在BatteryService.java中，Code如下：

Intent intent = new Intent(Intent.ACTION_BATTERY_CHANGED);
intent.addFlags(Intent.FLAG_RECEIVER_REGISTERED_ONLY);

        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);

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3. 數據接收
應用如果想要接收到BatteryService發送出來的電池信息，則需要註冊一個Intent爲Intent.ACTION_BATTERY_CHANGED的BroadcastReceiver。

    註冊方法如下：
              IntentFilter mIntentFilter = new IntentFilter();
              mIntentFilter.addAction(Intent.ACTION_BATTERY_CHANGED);
              registerReceiver(mIntentReceiver, mIntentFilter);

      private BroadcastReceiver mIntentReceiver = new BroadcastReceiver() {
             @Override
             public void onReceive(Context context, Intent intent) {
                   // TODO Auto-generated method stub
                     String action = intent.getAction();
                     if (action.equals(Intent.ACTION_BATTERY_CHANGED)) {

                           int nVoltage = intent.getIntExtra("voltage", 0);
                           if(nVoltage!=0){
                                   mVoltage.setText("V: " + nVoltage + "mV - Success...");
                          }
                            else{
                                   mVoltage.setText("V: " + nVoltage + "mV - fail...");
                            }
                   }
              }
      };

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4. 數據更新
電池的信息會隨着時間不停變化，自然地，就需要考慮如何實時的更新電池的數據信息。在BatteryService啓動的時候，會同時通過UEventObserver啓動一個onUEvent Thread。

        每一個Process最多只能有一個onUEvent Thread，即使這個Process中有多個UEventObserver的實例。當在一個Process中，第一次Call startObserving()方法後，這個UEvent thread就啓動了。
而一旦這個UEvent thread啓動之後，就不會停止。

     //在BatteryService.java中
     mUEventObserver.startObserving("SUBSYSTEM=power_supply");

     private UEventObserver mUEventObserver = new UEventObserver() {
        @Override
        public void onUEvent(UEventObserver.UEvent event) {
            update();
        }
     };

在UEvent thread中會不停調用 update()方法，來更新電池的信息數據。

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5. 附錄相關文件：
（1）. SystemServer.java          (frameworks/frameworks/base/services/java/com/android/server)
（2）. BatteryService.java       (frameworks/frameworks/base/services/java/com/android/server)
（3）. UEventObserver.java       (frameworks/frameworks/base/core/java/android/os)
（4）. com_android_server_BatteryService.cpp (frameworks/frameworks/base/services/jni)

參考 http://www.cnblogs.com/armlinux/archive/2010/09/15/2396916.html