GATT callback fails to register

I'm trying to write an application to send messages over Bluetooth Low Energy, which will then be passed on by UART in my peripheral. I've followed the steps here and the app scans for and finds the device successfully. However, connection using the BluetoothGatt = BluetoothDevice.connectGatt(context, autoconnect, callback) method fails, with logcat saying "Failed to register callback".

Call made from:

//device scan callback
private BluetoothAdapter.LeScanCallback btScanCallback = new BluetoothAdapter.LeScanCallback() 
{
    @Override
    public void onLeScan(final BluetoothDevice device, final int rssi, final byte[] scanRecord)
    {		
        some stuff
        currBtGatt = device.connectGatt(parentActivity, false, btGattCallback);
    }
};

And the Gatt callback:

//GATT callback
private BluetoothGattCallback btGattCallback = new BluetoothGattCallback()
{		
    @Override
    public void onConnectionStateChange(BluetoothGatt gatt, int status, int newState)
    {
        // if connected successfully
        if(newState == BluetoothProfile.STATE_CONNECTED)
        {
            //discover services
            updateStatus("Connected");
            gatt.discoverServices();
        }
        else if(newState == BluetoothProfile.STATE_DISCONNECTED)
        {
            updateStatus("Disconnected");
        }
    }
	
    @Override
    public void onServicesDiscovered(BluetoothGatt gatt, int status)
    {
        if(status == BluetoothGatt.GATT_SUCCESS)
        {
            //pick out the (app side) transmit channel
            currBtService = gatt.getService(uartUuids[0]);
            currBtCharacteristic = currBtService.getCharacteristic(uartUuids[1]);
        }
        else 
        {
            updateStatus("Service discovery failed");
        }
    }
};

Logcat says:

11-19 10:40:39.363: D/BluetoothAdapter(11717): stopLeScan()
11-19 10:40:39.373: D/BluetoothGatt(11717): connect() - device: DC:6D:75:0C:0F:F9, auto: false
11-19 10:40:39.373: D/BluetoothGatt(11717): registerApp()
11-19 10:40:39.373: D/BluetoothGatt(11717): registerApp() - UUID=3ba20989-5026-4715-add3-a5e31684009a
11-19 10:40:39.373: I/BluetoothGatt(11717): Client registered, waiting for callback
11-19 10:40:49.373: E/BluetoothGatt(11717): Failed to register callback
11-19 10:40:49.533: D/BluetoothGatt(11717): onClientRegistered() - status=0 clientIf=5
11-19 10:40:49.533: E/BluetoothGatt(11717): Bad connection state: 0
11-19 10:40:49.593: D/BluetoothGatt(11717): onClientConnectionState() - status=0 clientIf=5 device=DC:6D:75:0C:0F:F9
11-19 10:40:49.593: W/BluetoothGatt(11717): Unhandled exception: java.lang.NullPointerException

Interestingly, my peripheral moves to a "connected" state (I have indication LEDs) and I can connect to it from the same phone with a demonstration application or with a PC BLE dongle. Any ideas appreciated.

[EDIT] the connectGatt method returns null, which I guess is expected.

[EDIT] On inspection of API 18 source code, it appears that the "Failed to register callback" message is delivered because the method registerApp() returns false because the registerClient() method of the IBluetoothGatt "mService" throws a remote exception, probably at the line:

enforceCallingOrSelfPermission(BLUETOOTH_PERM, "Need BLUETOOTH permission");

because the log message in the very next line is never seen. So it could be a permissions thing, except that the application has bluetooth and bluetooth_admin permissions.

I finally figured this problem out. The device I am using is a Samsung Galaxy S4 and the actual problem (thanks Wibble for guidance in your answer, but you are slightly off in your conclusion) appears to be a threading issue.

In Wibble's answer, he stated that adding a button to connect fixed his issue. I started wondering why that matters, and I also can connect and disconnect fine during an entire session without a GUI button using background worker threads. As soon as I force close my application, restart it, and try to connect, I start getting the error "Failed to register callback." and nothing works any more. I almost pulled my hair out over this one :)

See my post in Samsung's forums for more detail on my exact issues.

Solution: To get around this issue, just make sure you run any BLE interaction code (device#connectGatt, connect, disconnect, etc) code in the UIThread (with a handler, local service, or Activity#runOnUiThread). Follow this rule of thumb and you will hopefully avoid this dreadful problem.

Deep in our library, I only had access to the application context. You can create a handler from a context that will post to the main thread by using new Handler(ctx.getMainLooper());

If you face other connection problems, deploy the sample app in samples\android-18\legacy\BluetoothLeGatt and see if that application works. That was kind of my baseline for realizing BLE does actually work with my peripheral, and gave me hope that if I dug enough in our library I would eventually find the answer.

EDIT: I did not see this 'Failed to register callback' issue on the Nexus 4, Nexus 5, or Nexus 7 2013 when using background threads to perform BLE operations. It may just be an issue in Samsungs 4.3 implementation.

So, My problem was running it from a recursive service. connectGatt worked fine with lollipop but older versions returned null. running on a main thread solved the problem. This is my solution:

public void connectToDevice( String deviceAddress) {
    mDeviceAddress = deviceAddress;
    final BluetoothDevice device = mBluetoothAdapter.getRemoteDevice(mDeviceAddress);

    Handler handler = new Handler(Looper.getMainLooper());
    handler.post(new Runnable() {
        @Override
        public void run() {


            if (device != null) {

                mGatt = device.connectGatt(getApplicationContext(), true, mGattCallback);
                scanLeDevice(false);// will stop after first device detection
            }
        }
    });
}

I can also confirm that Lo-Tan is the answer to check first. I've tested a lot of devices, some of them are behaving well when you run from a secondary thread. Some may block after a while, the behaviour is unpredicted.

Here is the list of things to do:

1. Maker sure you use new Handler(Looper.getMainLooper()).post(new Runnable) on any gatt operation (connect, disconnect, close) but also on the scanner operations (startScan, stopScan etc.).

2. There is race condition for direct connection on Android 6 (or maybe 5) so try to connect gatt like this:

     new Handler(getContext().get().getMainLooper()).post(() -> {
         if (CommonHelper.isNOrAbove()) {
     	    connectedGatt = connectedBLEDevice.connectGatt(context.get(), true, gattCallback, BluetoothDevice.TRANSPORT_AUTO);
             Timber.tag("HED-BT").d("Connecting BLE after N");
         } else {	
     	    try {
     		    Method connectGattMethod = connectedBLEDevice.getClass().getMethod("connectGatt", Context.class, boolean.class, BluetoothGattCallback.class, int.class);
                 connectedGatt = (BluetoothGatt) connectGattMethod.invoke(connectedBLEDevice, context.get(), false, gattCallback, BluetoothDevice.TRANSPORT_AUTO);
                 Timber.tag("HED-BT").d("Connecting BLE before N");
             } catch (Exception e) {
                 failedConnectingBLE();
             }
         }
     });
   

3. When disconnecting the gatt, call disconnect() first and close() after in the GattCallback routine.

In order to automatically connect to a Bluetooth device, ie without explicit user input as I was attempting to do, the permission BLUETOOTH_PRIVILEDGE is required. However, this is not available to third-party applications so my code failed. Adding a menu option to connect and using the same code works fine.

http://developer.android.com/reference/android/Manifest.permission.html#BLUETOOTH_PRIVILEGED