BluetoothGattServer is always disconnecting after 30 seconds

Question

I've been trying to figure out how to get these phones to talk to each other via Bluetooth. I have the android phone set up as a peripheral, and I have my iPhone running the nrf connect app. I am able to advertise and connect to the Android phone from the iPhone, and I am able to subscribe to notifications and see the characteristic updated. The problem is that if I don't send a characteristic notification, after about 7-10 seconds then the connection is lost. I get a callback on the connectionStateChanged callback handler, and I can't figure out what's causing this. I don't think it's the nrf app, because I've connected to other things and it just stays connected forever. Is there something I'm missing?

Here's some code:

private BluetoothManager bluetoothManager;
private BluetoothGattServer gattServer;
private BluetoothGattCharacteristic test_characteristic;
private BluetoothDevice connected_device;


private BluetoothGattServerCallback gattServerCallback = new BluetoothGattServerCallback() {
    @Override
    public void onConnectionStateChange(BluetoothDevice device, int status, int newState) {
        super.onConnectionStateChange(device, status, newState);
        if (newState == 2){ //CONNECTED
            connected_device = device;
        }
        Log.i("chase", "Connection state changed: "+newState);
    }
};

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    BluetoothRegulator.sharedInstance().appContext = getApplicationContext();
    bluetoothManager = (android.bluetooth.BluetoothManager) this.getSystemService(Context.BLUETOOTH_SERVICE);
    BluetoothRegulator.sharedInstance().initialize(bluetoothManager);

    if( ContextCompat.checkSelfPermission(this, android.Manifest.permission.ACCESS_COARSE_LOCATION) != PackageManager.PERMISSION_GRANTED) {
        ActivityCompat.requestPermissions(this, new String[]{android.Manifest.permission.ACCESS_COARSE_LOCATION}, 1);
    }


    gattServer = bluetoothManager.openGattServer(this, gattServerCallback);
    String uuid_string = "92D9D153-9BE6-43FF-9672-3E2904628B9D";
    BluetoothGattService service = new BluetoothGattService(UUID.fromString(uuid_string), BluetoothGattService.SERVICE_TYPE_PRIMARY);

    uuid_string = "43FF0001-9BE6-43FF-9672-3E2904628B9D";
    test_characteristic = new BluetoothGattCharacteristic(
            UUID.fromString(uuid_string),
            BluetoothGattCharacteristic.PROPERTY_BROADCAST | BluetoothGattCharacteristic.PROPERTY_NOTIFY | BluetoothGattCharacteristic.PROPERTY_READ | BluetoothGattCharacteristic.PROPERTY_WRITE,
            BluetoothGattCharacteristic.PERMISSION_READ | BluetoothGattCharacteristic.PERMISSION_WRITE);
    test_characteristic.setValue( ""+System.currentTimeMillis() );
    service.addCharacteristic(test_characteristic);
    gattServer.addService(service);

    findViewById(R.id.send_data_btn).setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            shot_detected_characteristic.setValue( ""+System.currentTimeMillis() );
            gattServer.notifyCharacteristicChanged(connected_device, shot_detected_characteristic, false);
        }
    });

    findViewById(R.id.advertise).setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            advertise();
        }
    });

}
private void advertise() {
    BluetoothAdapter adapter = BluetoothAdapter.getDefaultAdapter();
    BluetoothLeAdvertiser advertiser = adapter.getBluetoothLeAdvertiser();
    AdvertiseSettings settings = new AdvertiseSettings.Builder()
            .setAdvertiseMode( AdvertiseSettings.ADVERTISE_MODE_LOW_LATENCY )
            .setTxPowerLevel( AdvertiseSettings.ADVERTISE_TX_POWER_HIGH )
            .setConnectable( true )
            .build();
    String uuid_string = "92C9D164-9BE6-43FF-9672-3E2804618B9C";
    ParcelUuid pUuid = new ParcelUuid( UUID.fromString( uuid_string ) );

    AdvertiseData data = new AdvertiseData.Builder()
            .setIncludeDeviceName( true )
            .addServiceData( pUuid, "TEST".getBytes( Charset.forName( "UTF-8" ) ) )
            .build();

    AdvertiseCallback advertisingCallback = new AdvertiseCallback() {
        @Override
        public void onStartSuccess(AdvertiseSettings settingsInEffect) {
            Log.e( "BLE", "Advertising Started");
            super.onStartSuccess(settingsInEffect);
        }

        @Override
        public void onStartFailure(int errorCode) {
            Log.e( "BLE", "Advertising onStartFailure: " + errorCode );
            super.onStartFailure(errorCode);
        }
    };
    advertiser.startAdvertising( settings, data, advertisingCallback );
}

Edit: Actually, I'm noticing that it has nothing to do with being idle. It's actually disconnecting pretty much at the 30 second mark every time.

EDIT: So I added onCharacteristicRead & onCharacteristicWrite callbacks. But that doesn't seem to change anything, and that makes sense to me, because I am not doing anything in them except calling super.onCharacteristicRead() anyway. Also, those aren't going to be called if I am not making reads and writes. And it seems like the Bluetooth should stay connected more than 30 seconds even if there is not a read or write request.

private BluetoothGattServerCallback gattServerCallback = new BluetoothGattServerCallback() {
    @Override
    public void onConnectionStateChange(BluetoothDevice device, int status, int newState) {
        super.onConnectionStateChange(device, status, newState);
        if (newState == 2){
            connected_device = device;
            advertiser.stopAdvertising(new AdvertiseCallback() {});
        }
    }

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

};

SOLUTION

So the problem lies in the onCharacteristicReadRequest callback. According to the docs:

An application must call BluetoothGattServer#sendResponse to complete the request.

So once I updated my callback to look like this:

@Override
public void onCharacteristicReadRequest(BluetoothDevice device, int requestId, int offset, BluetoothGattCharacteristic characteristic) {
    super.onCharacteristicReadRequest(device, requestId, offset, characteristic);
    gattServer.sendResponse(device, requestId, 1, offset, characteristic.getValue());
}

and then made a read every few seconds, then it wouldn't time out and disconnect.

Answer 1

Per the ble core specification, for ATT

A transaction not completed within 30 seconds shall time out.

In practice, this usually manifests as a disconnect by one side of the connection.

Some types of GATT requests require responses (i.e Indications, Read Requests, Write Requests). Doesn't look like you are implementing callbacks such as onCharacteristicReadRequest & onCharacteristicWriteRequest in your Gatt server implementation which could lead to this problem.