@Override
    public void onServicesDiscovered(final BluetoothGatt gatt, int status) {
        ................
    }

    @Override
    public void onConnectionStateChange(final BluetoothGatt gatt, int status, int newState) {
        Log.d(TAG, String.format("BluetoothGattCallback: onConnectionStateChange: status:%d newState:%d", status, newState));
        if (newState == BluetoothProfile.STATE_DISCONNECTED) {
            Log.d(TAG, "Disconnected from GATT server. Flag = " + connectionStateFlag);
            mainThreadHandler.postDelayed(new Runnable() {
                @Override
                public void run() {
                    gatt.getDevice().connectGatt(applicationContext, true, callback);
                }
            }, 2000);
            return;
        }

        if (newState == BluetoothProfile.STATE_CONNECTED) {
            Log.d(TAG, "Connected to GATT server. Flag = " + connectionStateFlag);
                Log.i(TAG, "Connected to GATT server.");
                   ...
        }

    }

@Override
    public void onDescriptorWrite(final BluetoothGatt gatt, BluetoothGattDescriptor descriptor, int status) {
        Log.d(TAG, "onDescriptorWrite()");
    }
};