Android BLE characteristics getValue returns null

Question

I am trying to write text data to my BLE device. So , i am following Android Bluetooth GATT classes to do the task. But i found writing the text to the Characteristics is fine but while trying to retrieve the Characteristics value , it returns null.

MyCode :

public void writeCharacteristic(BluetoothGattCharacteristic characteristic,
                                String text) {

    String TAGS ="MyBeacon";

    if (mBluetoothAdapter == null || mBluetoothGatt == null) {
        Log.w(TAGS, "BluetoothAdapter not initialized");
        return;
    } else {
        Log.w(TAGS, "Writting ... ");
    }
    byte[] data = hexStringToByteArray(text);


    Log.w(TAGS, "Writting text = " + data);


    try {
        characteristic.setValue(URLEncoder.encode(text, "utf-8"));
    } catch (UnsupportedEncodingException e) {
        e.printStackTrace();
    }

    boolean writeValue = mBluetoothGatt.writeCharacteristic(characteristic);

    Log.w(TAGS, "Writting Status = " + writeValue);

}

// Successfully onCharacteristicWrite also gets called //

   @Override
    public void onCharacteristicWrite(BluetoothGatt gatt, BluetoothGattCharacteristic characteristic, int status) {
        super.onCharacteristicWrite(gatt, characteristic, status);

        String TAGS ="MyBeacon";

        String text = null;
        try {
            text = new String(characteristic.getValue(), "UTF-8");
        } catch (UnsupportedEncodingException e) {
            e.printStackTrace();
        }

        Log.w(TAGS, "onCharacteristicWrite = " + text+" :: "+status);

    }

but while trying to read the Characteristics it returns null.

  for (BluetoothGattCharacteristic gattCharacteristic : gattCharacteristics) {

                final byte[] data = gattCharacteristic.getValue(); // returns null

                  if (data != null && data.length > 0) {

                     Log.d("MyBeacon", " Read Data ")

                  } else {

                     Log.d("MyBeacon", " Data is null")
                  }

      }

MyBeacon

Also check the issue in other thread too.

Please help me out , suggest me some solution to write and read data successfully to my Beacon.

Answer 1

Syntax should be as follows,

mBluetoothGatt.readCharacteristic(characteristic);

Reading characteristics: You can read the characteristic using mBluetoothGatt.readCharacteristic(characteristic);

You can have to read the characteristic's descriptor as follows,

mBluetoothGatt.readDescriptor(ccc);

Once you read it, it should return data by calling the onDescriptorRead callback. Here you can set up (subscribe) to the charactersitic through either notification or indication by calling:

mBluetoothGatt.setCharacteristicNotification(characteristic, true)

once it returns true you will need to write to the descriptor again (the value of notification or indication)

BluetoothGattDescriptor clientConfig = characteristic.getDescriptor(CCC);
clientConfig.setValue(BluetoothGattDescriptor.ENABLE_NOTIFICATION_VALUE);

//clientConfig.setValue(BluetoothGattDescriptor.ENABLE_INDICATION_VALUE);
mBluetoothGatt.writeDescriptor(clientConfig);

Once this is done you will get notifications through onCharacteristicChanged callback every time the characteristic changes.

Do update me , if you have any problems while implementing,