writeCharacteristic BLE API work only once

Question

I am trying to write a characteristic value on a BLE peripheral. I am using the code mentioned below(writeCharacteristics method). I am able to write the characteristics on the peripheral on a button click, but this operation is successful only once. Subsequent write are not sent to the device as I can verify this from checking BLE logs over the air. Also the callback onCharacteristicWrite is not fired when I use the write characteristic API.

Here since I set the property in the peripheral, I am not performing any check.

Code:

public class BluetoothLeService extends Service {
private final static String TAG = BluetoothLeService.class.getSimpleName();

private BluetoothManager mBluetoothManager;
private BluetoothAdapter mBluetoothAdapter;
private String mBluetoothDeviceAddress;
private BluetoothGatt mBluetoothGatt;
private int mConnectionState = STATE_DISCONNECTED;

private static final int STATE_DISCONNECTED = 0;
private static final int STATE_CONNECTING = 1;
private static final int STATE_CONNECTED = 2;

public final static String ACTION_GATT_CONNECTED =
        "com.example.bluetooth.le.ACTION_GATT_CONNECTED";
public final static String ACTION_GATT_DISCONNECTED =
        "com.example.bluetooth.le.ACTION_GATT_DISCONNECTED";
public final static String ACTION_GATT_SERVICES_DISCOVERED =
        "com.example.bluetooth.le.ACTION_GATT_SERVICES_DISCOVERED";
public final static String ACTION_DATA_AVAILABLE =
        "com.example.bluetooth.le.ACTION_DATA_AVAILABLE";
public final static String EXTRA_DATA =
        "com.example.bluetooth.le.EXTRA_DATA";

public final static UUID UUID_HEART_RATE_MEASUREMENT =
        UUID.fromString(SampleGattAttributes.HEART_RATE_MEASUREMENT);

// Implements callback methods for GATT events that the app cares about.  For example,
// connection change and services discovered.
private final BluetoothGattCallback mGattCallback = new BluetoothGattCallback() {
    @Override
    public void onConnectionStateChange(BluetoothGatt gatt, int status, int newState) {
        String intentAction;
        if (newState == BluetoothProfile.STATE_CONNECTED) {
            intentAction = ACTION_GATT_CONNECTED;
            mConnectionState = STATE_CONNECTED;
            broadcastUpdate(intentAction);
            Log.i(TAG, "Connected to GATT server.");
            // Attempts to discover services after successful connection.
            Log.i(TAG, "Attempting to start service discovery:" +
                    mBluetoothGatt.discoverServices());

        } else if (newState == BluetoothProfile.STATE_DISCONNECTED) {
            intentAction = ACTION_GATT_DISCONNECTED;
            mConnectionState = STATE_DISCONNECTED;
            Log.i(TAG, "Disconnected from GATT server.");
            broadcastUpdate(intentAction);
        }
    }

    @Override
    public void onCharacteristicWrite(BluetoothGatt gatt, BluetoothGattCharacteristic characteristic, int status) {
        super.onCharacteristicWrite(gatt, characteristic, status);
        Log.v("Callback", "Entered the callabck onCharacteristics");
    }

    @Override
    public void onServicesDiscovered(BluetoothGatt gatt, int status) {
        if (status == BluetoothGatt.GATT_SUCCESS) {
            broadcastUpdate(ACTION_GATT_SERVICES_DISCOVERED);
        } else {
            Log.w(TAG, "onServicesDiscovered received: " + status);
        }
    }


    @Override
    public void onCharacteristicRead(BluetoothGatt gatt,
                                     BluetoothGattCharacteristic characteristic,
                                     int status) {
        if (status == BluetoothGatt.GATT_SUCCESS) {
            broadcastUpdate(ACTION_DATA_AVAILABLE, characteristic);
        }
    }

    @Override
    public void onCharacteristicChanged(BluetoothGatt gatt,
                                        BluetoothGattCharacteristic characteristic) {
        broadcastUpdate(ACTION_DATA_AVAILABLE, characteristic);
    }
};

private void broadcastUpdate(final String action) {
    final Intent intent = new Intent(action);
    sendBroadcast(intent);
}
public static byte[] hexStringToByteArray(String s) {
    int len = s.length();
    byte[] data = new byte[len / 2];
    for (int i = 0; i < len; i += 2) {
        data[i / 2] = (byte) ((Character.digit(s.charAt(i), 16) << 4)
                + Character.digit(s.charAt(i+1), 16));
    }
    return data;
}
public void writeCharacteristics(byte[] data_write) {
    UUID uuid_service = UUID.fromString(SampleGattAttributes.MY_SERVICE_UUID);
    UUID uuid_char = UUID.fromString(SampleGattAttributes.MY_CHAR_UUID);

    BluetoothGattService mSVC = mBluetoothGatt.getService(uuid_service);
    BluetoothGattCharacteristic mCH = mSVC.getCharacteristic(uuid_char);
    mCH.setValue(data_write);
    mCH.setWriteType(BluetoothGattCharacteristic.WRITE_TYPE_DEFAULT);
    mBluetoothGatt.writeCharacteristic(mCH);
}

private void broadcastUpdate(final String action,
                             final BluetoothGattCharacteristic characteristic) {
    final Intent intent = new Intent(action);

    // This is special handling for the Heart Rate Measurement profile.  Data parsing is
    // carried out as per profile specifications:
    // http://developer.bluetooth.org/gatt/characteristics/Pages/CharacteristicViewer.aspx?u=org.bluetooth.characteristic.heart_rate_measurement.xml
    if (UUID_HEART_RATE_MEASUREMENT.equals(characteristic.getUuid())) {
        int flag = characteristic.getProperties();
        int format = -1;
        if ((flag & 0x01) != 0) {
            format = BluetoothGattCharacteristic.FORMAT_UINT16;
            Log.d(TAG, "Heart rate format UINT16.");
        } else {
            format = BluetoothGattCharacteristic.FORMAT_UINT8;
            Log.d(TAG, "Heart rate format UINT8.");
        }
        final int heartRate = characteristic.getIntValue(format, 1);
        Log.d(TAG, String.format("Received heart rate: %d", heartRate));
        intent.putExtra(EXTRA_DATA, String.valueOf(heartRate));
    } else {
        // For all other profiles, writes the data formatted in HEX.
        final byte[] data = characteristic.getValue();
        if (data != null && data.length > 0) {
            final StringBuilder stringBuilder = new StringBuilder(data.length);
            for(byte byteChar : data)
                stringBuilder.append(String.format("%02X ", byteChar));
            intent.putExtra(EXTRA_DATA, new String(data) + "\n" + stringBuilder.toString());
        }
    }
    sendBroadcast(intent);
}



public class LocalBinder extends Binder {
    BluetoothLeService getService() {
        return BluetoothLeService.this;
    }
}

@Override
public IBinder onBind(Intent intent) {
    return mBinder;
}

@Override
public boolean onUnbind(Intent intent) {
    // After using a given device, you should make sure that BluetoothGatt.close() is called
    // such that resources are cleaned up properly.  In this particular example, close() is
    // invoked when the UI is disconnected from the Service.
    close();
    return super.onUnbind(intent);
}

private final IBinder mBinder = new LocalBinder();

/**
 * Initializes a reference to the local Bluetooth adapter.
 *
 * @return Return true if the initialization is successful.
 */
public boolean initialize() {
    // For API level 18 and above, get a reference to BluetoothAdapter through
    // BluetoothManager.
    if (mBluetoothManager == null) {
        mBluetoothManager = (BluetoothManager) getSystemService(Context.BLUETOOTH_SERVICE);
        if (mBluetoothManager == null) {
            Log.e(TAG, "Unable to initialize BluetoothManager.");
            return false;
        }
    }

    mBluetoothAdapter = mBluetoothManager.getAdapter();
    if (mBluetoothAdapter == null) {
        Log.e(TAG, "Unable to obtain a BluetoothAdapter.");
        return false;
    }

    return true;
}

/**
 * Connects to the GATT server hosted on the Bluetooth LE device.
 *
 * @param address The device address of the destination device.
 *
 * @return Return true if the connection is initiated successfully. The connection result
 *         is reported asynchronously through the
 *         {@code BluetoothGattCallback#onConnectionStateChange(android.bluetooth.BluetoothGatt, int, int)}
 *         callback.
 */
public boolean connect(final String address) {
    if (mBluetoothAdapter == null || address == null) {
        Log.w(TAG, "BluetoothAdapter not initialized or unspecified address.");
        return false;
    }

    // Previously connected device.  Try to reconnect.
    if (mBluetoothDeviceAddress != null && address.equals(mBluetoothDeviceAddress)
            && mBluetoothGatt != null) {
        Log.d(TAG, "Trying to use an existing mBluetoothGatt for connection.");
        if (mBluetoothGatt.connect()) {
            mConnectionState = STATE_CONNECTING;
            return true;
        } else {
            return false;
        }
    }

    final BluetoothDevice device = mBluetoothAdapter.getRemoteDevice(address);
    if (device == null) {
        Log.w(TAG, "Device not found.  Unable to connect.");
        return false;
    }
    // We want to directly connect to the device, so we are setting the autoConnect
    // parameter to false.
    mBluetoothGatt = device.connectGatt(this, false, mGattCallback);
    Log.d(TAG, "Trying to create a new connection.");
    mBluetoothDeviceAddress = address;
    mConnectionState = STATE_CONNECTING;
    return true;
}

/**
 * Disconnects an existing connection or cancel a pending connection. The disconnection result
 * is reported asynchronously through the
 * {@code BluetoothGattCallback#onConnectionStateChange(android.bluetooth.BluetoothGatt, int, int)}
 * callback.
 */
public void disconnect() {
    if (mBluetoothAdapter == null || mBluetoothGatt == null) {
        Log.w(TAG, "BluetoothAdapter not initialized");
        return;
    }
    mBluetoothGatt.disconnect();
}

/**
 * After using a given BLE device, the app must call this method to ensure resources are
 * released properly.
 */
public void close() {
    if (mBluetoothGatt == null) {
        return;
    }
    mBluetoothGatt.close();
    mBluetoothGatt = null;
}

/**
 * Request a read on a given {@code BluetoothGattCharacteristic}. The read result is reported
 * asynchronously through the {@code BluetoothGattCallback#onCharacteristicRead(android.bluetooth.BluetoothGatt, android.bluetooth.BluetoothGattCharacteristic, int)}
 * callback.
 *
 * @param characteristic The characteristic to read from.
 */
public void readCharacteristic(BluetoothGattCharacteristic characteristic) {
    if (mBluetoothAdapter == null || mBluetoothGatt == null) {
        Log.w(TAG, "BluetoothAdapter not initialized");
        return;
    }
    mBluetoothGatt.readCharacteristic(characteristic);
}

/**
 * Enables or disables notification on a give characteristic.
 *
 * @param characteristic Characteristic to act on.
 * @param enabled If true, enable notification.  False otherwise.
 */
public void setCharacteristicNotification(BluetoothGattCharacteristic characteristic,
                                          boolean enabled) {
    if (mBluetoothAdapter == null || mBluetoothGatt == null) {
        Log.w(TAG, "BluetoothAdapter not initialized");
        return;
    }
    mBluetoothGatt.setCharacteristicNotification(characteristic, enabled);

    // This is specific to Heart Rate Measurement.
    if (UUID_HEART_RATE_MEASUREMENT.equals(characteristic.getUuid())) {
        BluetoothGattDescriptor descriptor = characteristic.getDescriptor(
                UUID.fromString(SampleGattAttributes.CLIENT_CHARACTERISTIC_CONFIG));
        descriptor.setValue(BluetoothGattDescriptor.ENABLE_NOTIFICATION_VALUE);
        mBluetoothGatt.writeDescriptor(descriptor);
    }
}

/**
 * Retrieves a list of supported GATT services on the connected device. This should be
 * invoked only after {@code BluetoothGatt#discoverServices()} completes successfully.
 *
 * @return A {@code List} of supported services.
 */
public List<BluetoothGattService> getSupportedGattServices() {
    if (mBluetoothGatt == null) return null;

    return mBluetoothGatt.getServices();
}

}

I am using Android N and One Plus 3T hardware. Any comments on this is greatly appreciated.

Answer 1

What is the result of method call BluetoothGatt::writeCharacteristic(BluetoothGattCharacteristic characteristic)

According to the doc

it returns boolean true, if the write operation was initiated successfully

It will return false if you try to do more than one write or read in a row. You have to wait for the result of a previous read or write before doing the next one.

This is probably why it works only the first time.