public void start() {
......
  //注册intent, 主要关注的是ACTION_BATTERY_CHANGED消息。
  // 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);
  mContext.registerReceiver(mIntentReceiver, filter, null, mHandler);
}

private BroadcastReceiver mIntentReceiver = new BroadcastReceiver() {
  @Override
  public void onReceive(Context context, Intent intent) {
    String action = intent.getAction();
    if (action.equals(Intent.ACTION_BATTERY_CHANGED)) {
      ......
      mBatteryStatus = intent.getIntExtra(BatteryManager.EXTRA_STATUS,
          BatteryManager.BATTERY_STATUS_UNKNOWN);
      ......
      if (!plugged
          && (bucket < oldBucket || oldPlugged)
          && mBatteryStatus != BatteryManager.BATTERY_STATUS_UNKNOWN
          && bucket < 0) {
        showLowBatteryWarning();
      ......
    }
  }
};