Android BLE: Successfully write ENABLE_NOTIFICATION_VALUE value to BluetoothGattDescriptor but onCharacteristicChanged never fires

Question

I have a BLE peripheral that works fine with a given Service / Characteristic UUID on iOS and Android 7.0. On Android 6.0 Marshmallow the onCharacteristicChanged method will not fire despite setting the ENABLE_NOTIFICATION_VALUE on the descriptor. Please help me figure out how to get the onCharacteristicChanged to fire for an Android device running Android OS 6.0 Marshmallow. Here is the code I am using to try to get notifications to work:

    boolean success = mBluetoothGatt.setCharacteristicNotification(characteristic, true);
    Log.e(TAG, "set char notification = " + (success ? "It worked :)" : "It did not work :("));

    if (UUID_DATA_PACKET.equals(characteristic.getUuid())) {
        BluetoothGattDescriptor descriptor = characteristic.getDescriptor(
                UUID.fromString(EkoCoreGattAttributes.CLIENT_CHARACTERISTIC_CONFIG));

        descriptor.setValue(BluetoothGattDescriptor.ENABLE_NOTIFICATION_VALUE);

        success = mBluetoothGatt.writeDescriptor(descriptor);
        Log.e(TAG, "set descriptor = " + descriptor.getCharacteristic().getWriteType() + ", success = " + (success ? "It worked :)" : "It did not work :("));
    }

In the code above both the setCharacteristicNotification and writeDescriptor calls return success (1 / true / etc). Additionally the onDescriptorWrite callback returns GATT_SUCCESS. However, when we read the descriptor at a later point in the code it is found that the notification value is still set to disabled. We have tried many solutions such as putting a delay between the setCharacteristicNotification and writeDescriptor calls but haven't found a solution to this issue. Pairing with the peripheral in question is no problem, but getting the notifications seems somehow impossible. Any tips would be appreciated.

Answer 1

Adding the following before writing the descriptor fixed my issue:

characteristic.setWriteType(BluetoothGattCharacteristic.WRIT‌​E_TYPE_DEFAULT);

Clearly there is some bug in Android 6. I hope this answer helps someone else. It was quite the headache to find.