Android BLE onCharacteristicRead and onCharacteristicChanged never called

Question

I am trying to connect to a Bluetooth LE thermometer. Connecting to the device is working good. The only part hanging me up is the gattCallBack and it's onCharacteristicChanged / Read. the 'setNotification' and descriptor 'setValue' and 'writeDescriptor' all return true. The onCharacteristicChanged is never called to return a value.

I used a pretty handy little program from the Play Store called BLE Scanner to help me to give me more information about the device and it's services and characteristics.

This is why I simply hard coded service 2, characteristic 0. I just can't seem to figure out why after I writeDescriptor, I never see anything come back. The interesting thing is, I can use some of the other Characteristics (one being Temperature Interval) and I do receive a response (although the data is garbled.)

Also, out of curiosity, why are there 2 descriptors on this characteristic?

This code is contained in my MainActivity method. Not sure if that would make a difference here. I have looked at and tried several methods posted on here with no luck.

private final BluetoothGattCallback gattCallback = new BluetoothGattCallback()
{
    @Override
    public void onConnectionStateChange(BluetoothGatt gatt, int status, int newState)
    { ... }

    @Override
    public void onServicesDiscovered(BluetoothGatt gatt, int status)
    {
        mGatt = gatt;

        List<BluetoothGattService> services = mGatt.getServices();
        Log.i("onServicesDiscovered", services.toString());
        
        BluetoothGattCharacteristic characteristic = services.get(2).getCharacteristics().get(0);

        mGatt.setCharacteristicNotification(characteristic, true);
        
        BluetoothGattDescriptor descriptor = characteristic.getDescriptor(UUID.fromString(CLIENT_CHARACTERISTIC_CONFIG));
        descriptor.setValue(BluetoothGattDescriptor.ENABLE_INDICATION_VALUE);
        
        mGatt.writeDescriptor(descriptor);
        
    }            

    @Override
    public void onCharacteristicRead(BluetoothGatt gatt, BluetoothGattCharacteristic characteristic, int status)
    { ... }

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

UPDATE: I decided to check the the onDescriptorWrite method and Log some information.

@Override            
public void onDescriptorWrite(BluetoothGatt gatt,  BluetoothGattDescriptor descriptor, int status)
    {
        Log.i("descriptorWRITE", Integer.toString(status));
    }

Interesting thing here is that status is returning 13 which is 'A write operation exceeds the maximum length of the attribute'.

I will be looking into this further.

Answer 1

I found the problem here. I was assuming that the Thermometer was using the standard BLE service and characteristic setup. It is not. They created their own Custom Characteristic. Once I switched to that Characteristic, the 'changed' method started firing.