BaseNetworkObsever 定義要 Monitor Interface的狀態時的 動作：

StateChange
Add
Remove

叫 Network Manager 在出現 interface .. StateChange, Add, Remove 時分別 call 這三個 function.
所以..
我們寫下我們自己 implement 的 class

    private class InterfaceObserver extends BaseNetworkObserver {
        @Override
        public void interfaceLinkStateChanged(String iface, boolean up) {
            updateInterfaceState(iface, up);
        }

        @Override
        public void interfaceAdded(String iface) {
            maybeTrackInterface(iface);
        }

        @Override
        public void interfaceRemoved(String iface) {
            stopTrackingInterface(iface);
        }
    }

然後在系統啟動的時候，向 Network Management Service 註冊：

        // Start tracking interface change events.
        mInterfaceObserver = new InterfaceObserver();
        try {
            mNMService.registerObserver(mInterfaceObserver);
        } catch (RemoteException e) {
            Log.e(TAG, "Could not register InterfaceObserver " + e);
        }