path: root/src/android/bluetooth/src/org/qtproject/qt5/android/bluetooth/QtBluetoothLEServer.java

/****************************************************************************
 **
 ** Copyright (C) 2016 The Qt Company Ltd.
 ** Contact: https://www.qt.io/licensing/
 **
 ** This file is part of the QtBluetooth module of the Qt Toolkit.
 **
 ** $QT_BEGIN_LICENSE:LGPL$
 ** Commercial License Usage
 ** Licensees holding valid commercial Qt licenses may use this file in
 ** accordance with the commercial license agreement provided with the
 ** Software or, alternatively, in accordance with the terms contained in
 ** a written agreement between you and The Qt Company. For licensing terms
 ** and conditions see https://www.qt.io/terms-conditions. For further
 ** information use the contact form at https://www.qt.io/contact-us.
 **
 ** GNU Lesser General Public License Usage
 ** Alternatively, this file may be used under the terms of the GNU Lesser
 ** General Public License version 3 as published by the Free Software
 ** Foundation and appearing in the file LICENSE.LGPL3 included in the
 ** packaging of this file. Please review the following information to
 ** ensure the GNU Lesser General Public License version 3 requirements
 ** will be met: https://www.gnu.org/licenses/lgpl-3.0.html.
 **
 ** GNU General Public License Usage
 ** Alternatively, this file may be used under the terms of the GNU
 ** General Public License version 2.0 or (at your option) the GNU General
 ** Public license version 3 or any later version approved by the KDE Free
 ** Qt Foundation. The licenses are as published by the Free Software
 ** Foundation and appearing in the file LICENSE.GPL2 and LICENSE.GPL3
 ** included in the packaging of this file. Please review the following
 ** information to ensure the GNU General Public License requirements will
 ** be met: https://www.gnu.org/licenses/gpl-2.0.html and
 ** https://www.gnu.org/licenses/gpl-3.0.html.
 **
 ** $QT_END_LICENSE$
 **
 ****************************************************************************/

package org.qtproject.qt5.android.bluetooth;

import android.bluetooth.BluetoothDevice;
import android.bluetooth.BluetoothGattCharacteristic;
import android.bluetooth.BluetoothGattDescriptor;
import android.bluetooth.BluetoothGattService;
import android.content.Context;
import android.bluetooth.BluetoothAdapter;
import android.bluetooth.BluetoothGatt;
import android.bluetooth.BluetoothGattServer;
import android.bluetooth.BluetoothGattServerCallback;
import android.bluetooth.BluetoothManager;
import android.bluetooth.BluetoothProfile;
import android.bluetooth.le.AdvertiseCallback;
import android.bluetooth.le.AdvertiseData;
import android.bluetooth.le.AdvertiseData.Builder;
import android.bluetooth.le.AdvertiseSettings;
import android.bluetooth.le.BluetoothLeAdvertiser;
import android.os.ParcelUuid;
import android.util.Log;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Iterator;
import java.util.LinkedList;
import java.util.List;
import java.util.ListIterator;
import java.util.HashMap;
import java.util.UUID;

public class QtBluetoothLEServer {
    private static final String TAG = "QtBluetoothGattServer";

    /* Pointer to the Qt object that "owns" the Java object */
    @SuppressWarnings({"CanBeFinal", "WeakerAccess"})
    long qtObject = 0;
    @SuppressWarnings("WeakerAccess")

    private Context qtContext = null;

    // Bluetooth members
    private final BluetoothAdapter mBluetoothAdapter;
    private BluetoothGattServer mGattServer = null;
    private BluetoothLeAdvertiser mLeAdvertiser = null;

    private ArrayList<BluetoothGattService> mPendingServiceAdditions =
                      new ArrayList<BluetoothGattService>();

    private String mRemoteName = "";
    // This function is called from Qt thread
    public synchronized String remoteName() {
        return mRemoteName;
    }

    private String mRemoteAddress = "";
    // This function is called from Qt thread
    public synchronized String remoteAddress() {
        return mRemoteAddress;
    }

    /*
        As per Bluetooth specification each connected device can have individual and persistent
        Client characteristic configurations (see Bluetooth Spec 5.0 Vol 3 Part G 3.3.3.3)
        This class manages the existing configurrations.
     */
    private class ClientCharacteristicManager {
        private final HashMap<BluetoothGattCharacteristic, List<Entry>> notificationStore = new HashMap<BluetoothGattCharacteristic, List<Entry>>();

        private class Entry {
            BluetoothDevice device = null;
            byte[] value = null;
            boolean isConnected = false;
        }

        public void insertOrUpdate(BluetoothGattCharacteristic characteristic,
                              BluetoothDevice device, byte[] newValue)
        {
            if (notificationStore.containsKey(characteristic)) {

                List<Entry> entries = notificationStore.get(characteristic);
                for (int i = 0; i < entries.size(); i++) {
                    if (entries.get(i).device.equals(device)) {
                        Entry e = entries.get(i);
                        e.value = newValue;
                        entries.set(i, e);
                        return;
                    }
                }

                // not match so far -> add device to list
                Entry e = new Entry();
                e.device = device;
                e.value = newValue;
                e.isConnected = true;
                entries.add(e);
                return;
            }

            // new characteristic
            Entry e = new Entry();
            e.device = device;
            e.value = newValue;
            e.isConnected = true;
            List<Entry> list = new LinkedList<Entry>();
            list.add(e);
            notificationStore.put(characteristic, list);
        }

        /*
            Marks client characteristic configuration entries as (in)active based the associated
            devices general connectivity state.
            This function avoids that existing configurations are not acted
            upon when the associated device is not connected.
         */
        public void markDeviceConnectivity(BluetoothDevice device, boolean isConnected)
        {
            final Iterator<BluetoothGattCharacteristic> keys = notificationStore.keySet().iterator();
            while (keys.hasNext()) {
                final BluetoothGattCharacteristic characteristic = keys.next();
                final List<Entry> entries = notificationStore.get(characteristic);
                if (entries == null)
                    continue;

                ListIterator<Entry> charConfig = entries.listIterator();
                while (charConfig.hasNext()) {
                    Entry e = charConfig.next();
                    if (e.device.equals(device))
                        e.isConnected = isConnected;
                }
            }
        }

        // Returns list of all BluetoothDevices which require notification or indication.
        // No match returns an empty list.
        List<BluetoothDevice> getToBeUpdatedDevices(BluetoothGattCharacteristic characteristic)
        {
            ArrayList<BluetoothDevice> result = new ArrayList<BluetoothDevice>();
            if (!notificationStore.containsKey(characteristic))
                return result;

            final ListIterator<Entry> iter = notificationStore.get(characteristic).listIterator();
            while (iter.hasNext())
                result.add(iter.next().device);

            return result;
        }

        // Returns null if no match; otherwise the configured actual client characteristic
        // configuration value
        byte[] valueFor(BluetoothGattCharacteristic characteristic, BluetoothDevice device)
        {
            if (!notificationStore.containsKey(characteristic))
                return null;

            List<Entry> entries = notificationStore.get(characteristic);
            for (int i = 0; i < entries.size(); i++) {
                final Entry entry = entries.get(i);
                if (entry.device.equals(device) && entry.isConnected == true)
                    return entries.get(i).value;
            }

            return null;
        }
    }

    private static final UUID CLIENT_CHARACTERISTIC_CONFIGURATION_UUID = UUID
            .fromString("00002902-0000-1000-8000-00805f9b34fb");
    ClientCharacteristicManager clientCharacteristicManager = new ClientCharacteristicManager();

    public QtBluetoothLEServer(Context context)
    {
        qtContext = context;
        mBluetoothAdapter = BluetoothAdapter.getDefaultAdapter();

        if (mBluetoothAdapter == null || qtContext == null) {
            Log.w(TAG, "Missing Bluetooth adapter or Qt context. Peripheral role disabled.");
            return;
        }

        BluetoothManager manager = (BluetoothManager) qtContext.getSystemService(Context.BLUETOOTH_SERVICE);
        if (manager == null) {
            Log.w(TAG, "Bluetooth service not available.");
            return;
        }

        mLeAdvertiser = mBluetoothAdapter.getBluetoothLeAdvertiser();

        if (!mBluetoothAdapter.isMultipleAdvertisementSupported())
            Log.w(TAG, "Device does not support Bluetooth Low Energy advertisement.");
        else
            Log.w(TAG, "Let's do BTLE Peripheral.");
    }


    // The following functions are synchronized callback handlers. The callbacks
    // from Android are forwarded to these methods to synchronize member variable
    // access with other threads (the Qt thread's JNI calls in particular).
    //
    // We use a single lock object (this server) for simplicity because:
    // - Some variables may change and would thus not be suitable as locking objects but
    //   would require their own additional objects => overhead
    // - Many accesses to shared variables are infrequent and the code paths are fast and
    //   deterministic meaning that long "wait times" on a lock should not happen
    // - Typically several shared variables are accessed in a single code block.
    //   If each variable would be protected individually, the amount of (nested) locking
    //   would become quite unreasonable

    public synchronized void handleOnConnectionStateChange(BluetoothDevice device,
                                                           int status, int newState)
    {
        if (mGattServer == null) {
            Log.w(TAG, "Ignoring connection state event, server is disconnected");
            return;
        }

        int qtControllerState = 0;
        switch (newState) {
            case BluetoothProfile.STATE_DISCONNECTED:
                qtControllerState = 0; // QLowEnergyController::UnconnectedState
                clientCharacteristicManager.markDeviceConnectivity(device, false);
                mGattServer.close();
                mPendingServiceAdditions.clear();
                mGattServer = null;
                break;
            case BluetoothProfile.STATE_CONNECTED:
                clientCharacteristicManager.markDeviceConnectivity(device, true);
                qtControllerState = 2; // QLowEnergyController::ConnectedState
                break;
        }

        mRemoteName = device.getName();
        mRemoteAddress = device.getAddress();

        int qtErrorCode;
        switch (status) {
            case BluetoothGatt.GATT_SUCCESS:
                qtErrorCode = 0;
                break;
            default:
                Log.w(TAG, "Unhandled error code on peripheral connectionStateChanged: "
                           + status + " " + newState);
                qtErrorCode = status;
                break;
        }
        leServerConnectionStateChange(qtObject, qtErrorCode, qtControllerState);
    }

    public synchronized void handleOnServiceAdded(int status, BluetoothGattService service)
    {
        if (mGattServer == null) {
            Log.w(TAG, "Ignoring service addition event, server is disconnected");
            return;
        }

        Log.d(TAG, "Service " + service.getUuid().toString() + " addition result: " + status);

        // Remove the indicated service from the pending queue
        ListIterator<BluetoothGattService> iterator = mPendingServiceAdditions.listIterator();
        while (iterator.hasNext()) {
            if (iterator.next().getUuid().equals(service.getUuid())) {
                iterator.remove();
                break;
            }
        }

        // If there are more services in the queue, add the next whose add initiation succeeds
        iterator = mPendingServiceAdditions.listIterator();
        while (iterator.hasNext()) {
            BluetoothGattService nextService = iterator.next();
            if (mGattServer.addService(nextService)) {
                break;
            } else {
                Log.w(TAG, "Adding service " + nextService.getUuid().toString() + " failed");
                iterator.remove();
            }
        }
    }

    public synchronized void handleOnCharacteristicReadRequest(BluetoothDevice device,
                                                   int requestId, int offset,
                                                   BluetoothGattCharacteristic characteristic)
    {
        if (mGattServer == null) {
            Log.w(TAG, "Ignoring characteristic read, server is disconnected");
            return;
        }
        byte[] dataArray;
        try {
            dataArray = Arrays.copyOfRange(characteristic.getValue(),
                                           offset, characteristic.getValue().length);
            mGattServer.sendResponse(device, requestId, BluetoothGatt.GATT_SUCCESS,
                                     offset, dataArray);
        } catch (Exception ex) {
            Log.w(TAG, "onCharacteristicReadRequest: " + requestId + " "
                        + offset + " " + characteristic.getValue().length);
            ex.printStackTrace();
            mGattServer.sendResponse(device, requestId, BluetoothGatt.GATT_FAILURE, offset, null);
        }
    }

    public synchronized void handleOnCharacteristicWriteRequest(BluetoothDevice device,
                                                   int requestId,
                                                   BluetoothGattCharacteristic characteristic,
                                                   boolean preparedWrite, boolean responseNeeded,
                                                   int offset, byte[] value)
    {
        if (mGattServer == null) {
            Log.w(TAG, "Ignoring characteristic write, server is disconnected");
            return;
        }
        Log.w(TAG, "onCharacteristicWriteRequest");
        int resultStatus = BluetoothGatt.GATT_SUCCESS;
        boolean sendNotificationOrIndication = false;
        if (!preparedWrite) { // regular write
            if (offset == 0) {
                characteristic.setValue(value);
                leServerCharacteristicChanged(qtObject, characteristic, value);
                sendNotificationOrIndication = true;
            } else {
                // This should not really happen as per Bluetooth spec
                Log.w(TAG, "onCharacteristicWriteRequest: !preparedWrite, offset "
                            + offset + ", Not supported");
                resultStatus = BluetoothGatt.GATT_REQUEST_NOT_SUPPORTED;
            }
        } else {
            Log.w(TAG, "onCharacteristicWriteRequest: preparedWrite, offset "
                        + offset + ", Not supported");
            resultStatus = BluetoothGatt.GATT_REQUEST_NOT_SUPPORTED;

            // TODO we need to record all requests and execute them in one go once onExecuteWrite() is received
            // we use a queue to remember the pending requests
            // TODO we are ignoring the device identificator for now -> Bluetooth spec requires a queue per device
        }


        if (responseNeeded)
            mGattServer.sendResponse(device, requestId, resultStatus, offset, value);
        if (sendNotificationOrIndication)
            sendNotificationsOrIndications(characteristic);
    }

    public synchronized void handleOnDescriptorReadRequest(BluetoothDevice device, int requestId,
                                              int offset, BluetoothGattDescriptor descriptor)
    {
        if (mGattServer == null) {
            Log.w(TAG, "Ignoring descriptor read, server is disconnected");
            return;
        }

        byte[] dataArray = descriptor.getValue();
        try {
            if (descriptor.getUuid().equals(CLIENT_CHARACTERISTIC_CONFIGURATION_UUID)) {
                dataArray = clientCharacteristicManager.valueFor(
                            descriptor.getCharacteristic(), device);
                if (dataArray == null)
                    dataArray = descriptor.getValue();
            }

            dataArray = Arrays.copyOfRange(dataArray, offset, dataArray.length);
            mGattServer.sendResponse(device, requestId, BluetoothGatt.GATT_SUCCESS,
                                     offset, dataArray);
        } catch (Exception ex) {
            Log.w(TAG, "onDescriptorReadRequest: " + requestId + " "
                                                   + offset + " " + dataArray.length);
            ex.printStackTrace();
            mGattServer.sendResponse(device, requestId, BluetoothGatt.GATT_FAILURE,
                                     offset, null);
        }
    }

    public synchronized void handleOnDescriptorWriteRequest(BluetoothDevice device, int requestId,
                                     BluetoothGattDescriptor descriptor, boolean preparedWrite,
                                     boolean responseNeeded, int offset, byte[] value)
    {
        if (mGattServer == null) {
            Log.w(TAG, "Ignoring descriptor write, server is disconnected");
            return;
        }
        int resultStatus = BluetoothGatt.GATT_SUCCESS;
        if (!preparedWrite) { // regular write
            if (offset == 0) {
                descriptor.setValue(value);

                if (descriptor.getUuid().equals(CLIENT_CHARACTERISTIC_CONFIGURATION_UUID)) {
                    clientCharacteristicManager.insertOrUpdate(descriptor.getCharacteristic(),
                                                               device, value);
                }

                leServerDescriptorWritten(qtObject, descriptor, value);
            } else {
                // This should not really happen as per Bluetooth spec
                Log.w(TAG, "onDescriptorWriteRequest: !preparedWrite, offset "
                            + offset + ", Not supported");
                resultStatus = BluetoothGatt.GATT_REQUEST_NOT_SUPPORTED;
            }

        } else {
            Log.w(TAG, "onDescriptorWriteRequest: preparedWrite, offset "
                        + offset + ", Not supported");
            resultStatus = BluetoothGatt.GATT_REQUEST_NOT_SUPPORTED;
            // TODO we need to record all requests and execute them in one go once onExecuteWrite() is received
            // we use a queue to remember the pending requests
            // TODO we are ignoring the device identificator for now -> Bluetooth spec requires a queue per device
        }

        if (responseNeeded)
            mGattServer.sendResponse(device, requestId, resultStatus, offset, value);
    }

    public synchronized void handleOnExecuteWrite(BluetoothDevice device,
                                            int requestId, boolean execute)
    {
        if (mGattServer == null) {
            Log.w(TAG, "Ignoring execute write, server is disconnected");
            return;
        }
        // TODO not yet implemented -> return proper GATT error for it
        mGattServer.sendResponse(device, requestId,
                                 BluetoothGatt.GATT_REQUEST_NOT_SUPPORTED, 0, null);
    }

     /*
      * Call back handler for the Gatt Server.
      */
    private BluetoothGattServerCallback mGattServerListener = new BluetoothGattServerCallback()
    {
        @Override
        public void onConnectionStateChange(BluetoothDevice device, int status, int newState) {
            super.onConnectionStateChange(device, status, newState);
            handleOnConnectionStateChange(device, status, newState);
        }

        @Override
        public void onServiceAdded(int status, BluetoothGattService service) {
            super.onServiceAdded(status, service);
            handleOnServiceAdded(status, service);
        }

        @Override
        public void onCharacteristicReadRequest(BluetoothDevice device,
                                                int requestId, int offset,
                                                BluetoothGattCharacteristic characteristic)
        {
            super.onCharacteristicReadRequest(device, requestId, offset, characteristic);
            handleOnCharacteristicReadRequest(device, requestId, offset, characteristic);
        }

        @Override
        public void onCharacteristicWriteRequest(BluetoothDevice device, int requestId,
                                                 BluetoothGattCharacteristic characteristic,
                                                 boolean preparedWrite, boolean responseNeeded,
                                                 int offset, byte[] value)
        {
            super.onCharacteristicWriteRequest(device, requestId, characteristic,
                                               preparedWrite, responseNeeded, offset, value);
            handleOnCharacteristicWriteRequest(device, requestId, characteristic,
                                               preparedWrite, responseNeeded, offset, value);
        }

        @Override
        public void onDescriptorReadRequest(BluetoothDevice device, int requestId,
                                            int offset, BluetoothGattDescriptor descriptor)
        {
            super.onDescriptorReadRequest(device, requestId, offset, descriptor);
            handleOnDescriptorReadRequest(device, requestId, offset, descriptor);
        }

        @Override
        public void onDescriptorWriteRequest(BluetoothDevice device, int requestId,
                                             BluetoothGattDescriptor descriptor,
                                             boolean preparedWrite, boolean responseNeeded,
                                             int offset, byte[] value)
        {
            super.onDescriptorWriteRequest(device, requestId, descriptor, preparedWrite,
                                           responseNeeded, offset, value);
            handleOnDescriptorWriteRequest(device, requestId, descriptor, preparedWrite,
                                           responseNeeded, offset, value);
        }

        @Override
        public void onExecuteWrite(BluetoothDevice device, int requestId, boolean execute)
        {
            super.onExecuteWrite(device, requestId, execute);
            handleOnExecuteWrite(device, requestId, execute);
        }

        @Override
        public void onNotificationSent(BluetoothDevice device, int status) {
            super.onNotificationSent(device, status);
            Log.w(TAG, "onNotificationSent" + device + " " + status);
        }

//        MTU change disabled since it requires API level 22. Right now we only enforce lvl 21
//        @Override
//        public void onMtuChanged(BluetoothDevice device, int mtu) {
//            super.onMtuChanged(device, mtu);
//        }
    };

    // This function is called from Qt thread
    public synchronized boolean connectServer()
    {
        if (mGattServer != null)
            return true;

        BluetoothManager manager = (BluetoothManager) qtContext.getSystemService(Context.BLUETOOTH_SERVICE);
        if (manager == null) {
            Log.w(TAG, "Bluetooth service not available.");
            return false;
        }

        mGattServer = manager.openGattServer(qtContext, mGattServerListener);

        return (mGattServer != null);
    }

    // This function is called from Qt thread
    public synchronized void disconnectServer()
    {
        if (mGattServer == null)
            return;

        mPendingServiceAdditions.clear();
        mGattServer.close();
        mGattServer = null;

        mRemoteName = mRemoteAddress = "";
        leServerConnectionStateChange(qtObject, 0 /*NoError*/, 0 /*QLowEnergyController::UnconnectedState*/);
    }

    // This function is called from Qt thread
    public boolean startAdvertising(AdvertiseData advertiseData,
                                    AdvertiseData scanResponse,
                                    AdvertiseSettings settings)
    {
        if (mLeAdvertiser == null)
            return false;

        if (!connectServer()) {
            Log.w(TAG, "Server::startAdvertising: Cannot open GATT server");
            return false;
        }

        Log.w(TAG, "Starting to advertise.");
        mLeAdvertiser.startAdvertising(settings, advertiseData, scanResponse, mAdvertiseListener);

        return true;
    }

    // This function is called from Qt thread
    public void stopAdvertising()
    {
        if (mLeAdvertiser == null)
            return;

        mLeAdvertiser.stopAdvertising(mAdvertiseListener);
        Log.w(TAG, "Advertisement stopped.");
    }

    // This function is called from Qt thread
    public synchronized void addService(BluetoothGattService service)
    {
        if (!connectServer()) {
            Log.w(TAG, "Server::addService: Cannot open GATT server");
            return;
        }

        // When we add a service, we must wait for onServiceAdded callback before adding the
        // next one. If the pending service queue is empty it means that there are no ongoing
        // service additions => add the service to the server. If there are services in the
        // queue it means there is an initiated addition ongoing, and we only add to the queue.
        if (mPendingServiceAdditions.isEmpty()) {
            if (mGattServer.addService(service))
                mPendingServiceAdditions.add(service);
            else
                Log.w(TAG, "Adding service " + service.getUuid().toString() + " failed.");
        } else {
            mPendingServiceAdditions.add(service);
        }
    }

    /*
        Check the client characteristics configuration for the given characteristic
        and sends notifications or indications as per required.

        This function is called from Qt and Java threads and calls must be protected
     */
    private void sendNotificationsOrIndications(BluetoothGattCharacteristic characteristic)
    {
        final ListIterator<BluetoothDevice> iter =
                clientCharacteristicManager.getToBeUpdatedDevices(characteristic).listIterator();

        // TODO This quick loop over multiple devices should be synced with onNotificationSent().
        //      The next notifyCharacteristicChanged() call must wait until onNotificationSent()
        //      was received. At this becomes an issue when the server accepts multiple remote
        //      devices at the same time.
        while (iter.hasNext()) {
            final BluetoothDevice device = iter.next();
            final byte[] clientCharacteristicConfig = clientCharacteristicManager.valueFor(characteristic, device);
            if (clientCharacteristicConfig != null) {
                if (Arrays.equals(clientCharacteristicConfig, BluetoothGattDescriptor.ENABLE_NOTIFICATION_VALUE)) {
                    mGattServer.notifyCharacteristicChanged(device, characteristic, false);
                } else if (Arrays.equals(clientCharacteristicConfig, BluetoothGattDescriptor.ENABLE_INDICATION_VALUE)) {
                    mGattServer.notifyCharacteristicChanged(device, characteristic, true);
                }
            }
        }
    }

    /*
        Updates the local database value for the given characteristic with \a charUuid and
        \a newValue. If notifications for this task are enabled an approproiate notification will
        be send to the remote client.

        This function is called from the Qt thread.
     */
    public boolean writeCharacteristic(BluetoothGattService service, UUID charUuid, byte[] newValue)
    {
        BluetoothGattCharacteristic foundChar = null;
        List<BluetoothGattCharacteristic> charList = service.getCharacteristics();
        for (BluetoothGattCharacteristic iter: charList) {
            if (iter.getUuid().equals(charUuid) && foundChar == null) {
                foundChar = iter;
                // don't break here since we want to check next condition below on next iteration
            } else if (iter.getUuid().equals(charUuid)) {
                Log.w(TAG, "Found second char with same UUID. Wrong char may have been selected.");
                break;
            }
        }

        if (foundChar == null) {
            Log.w(TAG, "writeCharacteristic: update for unknown characteristic failed");
            return false;
        }

        synchronized (this) // a value update might be in progress
        {
            foundChar.setValue(newValue);
            // Value is updated even if server is not connected, but notifying is not possible
            if (mGattServer != null)
                sendNotificationsOrIndications(foundChar);
        }
        return true;
    }

    /*
        Updates the local database value for the given \a descUuid to \a newValue.

        This function is called from the Qt thread.
     */
    public boolean writeDescriptor(BluetoothGattService service, UUID charUuid, UUID descUuid,
                                   byte[] newValue)
    {
        BluetoothGattDescriptor foundDesc = null;
        BluetoothGattCharacteristic foundChar = null;
        final List<BluetoothGattCharacteristic> charList = service.getCharacteristics();
        for (BluetoothGattCharacteristic iter: charList) {
            if (!iter.getUuid().equals(charUuid))
                continue;

            if (foundChar == null) {
                foundChar = iter;
            } else {
                Log.w(TAG, "Found second char with same UUID. Wrong char may have been selected.");
                break;
            }
        }

        if (foundChar != null)
            foundDesc = foundChar.getDescriptor(descUuid);

        if (foundChar == null || foundDesc == null) {
            Log.w(TAG, "writeDescriptor: update for unknown char or desc failed (" + foundChar + ")");
            return false;
        }

        // we even write CLIENT_CHARACTERISTIC_CONFIGURATION_UUID this way as we choose
        // to interpret the server's call as a change of the default value.
        synchronized (this) // a value update might be in progress
        {
            foundDesc.setValue(newValue);
        }
        return true;
    }

    /*
     * Call back handler for Advertisement requests.
     */
    private AdvertiseCallback mAdvertiseListener = new AdvertiseCallback()
    {
        @Override
        public void onStartSuccess(AdvertiseSettings settingsInEffect) {
            super.onStartSuccess(settingsInEffect);
        }

        @Override
        public void onStartFailure(int errorCode) {
            Log.e(TAG, "Advertising failure: " + errorCode);
            super.onStartFailure(errorCode);

            // changing errorCode here implies changes to errorCode handling on Qt side
            int qtErrorCode = 0;
            switch (errorCode) {
                case AdvertiseCallback.ADVERTISE_FAILED_ALREADY_STARTED:
                    return; // ignore -> noop
                case AdvertiseCallback.ADVERTISE_FAILED_DATA_TOO_LARGE:
                    qtErrorCode = 1;
                    break;
                case AdvertiseCallback.ADVERTISE_FAILED_FEATURE_UNSUPPORTED:
                    qtErrorCode = 2;
                    break;
                default: // default maps to internal error
                case AdvertiseCallback.ADVERTISE_FAILED_INTERNAL_ERROR:
                    qtErrorCode = 3;
                    break;
                case AdvertiseCallback.ADVERTISE_FAILED_TOO_MANY_ADVERTISERS:
                    qtErrorCode = 4;
                    break;
            }

            if (qtErrorCode > 0)
                leServerAdvertisementError(qtObject, qtErrorCode);
        }
    };

    public native void leServerConnectionStateChange(long qtObject, int errorCode, int newState);
    public native void leServerAdvertisementError(long qtObject, int status);
    public native void leServerCharacteristicChanged(long qtObject,
                                                     BluetoothGattCharacteristic characteristic,
                                                     byte[] newValue);
    public native void leServerDescriptorWritten(long qtObject,
                                                 BluetoothGattDescriptor descriptor,
                                                 byte[] newValue);
}