GATT callback fails to register

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.