Using BLE - Read GATT characteristics

Question

I'm trying to read GATT characteristic values from a Bluetooth LE device (a Heart Rate bracelet). Its specs are:

Services

Characteristics

I have not yet figured out how to "read" the specifications and "translate" them into code.

I need to show on my app the heartbeats detected by the device. What is the way to read the GATT values? A code example would be much appreciated :)

Follow my actual source code.

---

SETUP THE BLUETOOT CONNECTION

    private BluetoothAdapter mBluetoothAdapter;
    private BluetoothGatt mBluetoothGatt;
    private Handler mHandler;

    private static final int REQUEST_ENABLE_BT = 1;
    private static final long SCAN_PERIOD = 10000;

    // ...

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_bluetooth);
        mHandler = new Handler();

        // BLE is supported?
        if (!getPackageManager().hasSystemFeature(PackageManager.FEATURE_BLUETOOTH_LE)) {
            Toast.makeText(this, "Bluetooth Low Energy non supportato", Toast.LENGTH_SHORT).show();
            finish();
        }

        final BluetoothManager bluetoothManager = (BluetoothManager) getSystemService(Context.BLUETOOTH_SERVICE);
        mBluetoothAdapter = bluetoothManager.getAdapter();

        // Bluetooth is supported?
        if (mBluetoothAdapter == null) {
            Toast.makeText(this, "Bluetooth non supportato", Toast.LENGTH_SHORT).show();
            finish();
        }
    }

    @Override
    protected void onResume() {
        super.onResume();

        // Bluetooth is enabled?
        if (!mBluetoothAdapter.isEnabled()) {
            Intent enableBtIntent = new Intent(BluetoothAdapter.ACTION_REQUEST_ENABLE);
            startActivityForResult(enableBtIntent, REQUEST_ENABLE_BT);
        }

        scanLeDevice(true);
    }

    @Override
    protected void onPause() {
        super.onPause();
        if (mBluetoothAdapter != null && mBluetoothAdapter.isEnabled()) {
            scanLeDevice(false);
        }
    }

---

DISCOVER BLE DEVICES AND CONNECT WITH HEART RATE MONITOR

    // Device scan callback.
    private BluetoothAdapter.LeScanCallback mLeScanCallback =
            new BluetoothAdapter.LeScanCallback() {
                @Override
                public void onLeScan(final BluetoothDevice device, int rssi, byte[] scanRecord) {
                    runOnUiThread(new Runnable() {
                        @Override
                        public void run() {
                            Log.i(TAG, "Name: " + device.getName() + " (" + device.getAddress() + ")");
                            String deviceAddress = device.getAddress();
                            if (deviceAddress.equals("C0:19:37:54:9F:30")) {
                                connectToDevice(device);
                            }
                        }
                    });
                }
            };

    public void connectToDevice(BluetoothDevice device) {
        if (mBluetoothGatt == null) {
            Log.i(TAG, "Attempting to connect to device " + device.getName() + " (" + device.getAddress() + ")");
            mBluetoothGatt = device.connectGatt(this, true, gattCallback);
            scanLeDevice(false);// will stop after first device detection
        }
    }

    private final BluetoothGattCallback gattCallback = new BluetoothGattCallback() {
        @Override
        public void onConnectionStateChange(BluetoothGatt gatt, int status, int newState) {
            Log.i(TAG, "Status: " + status);
            switch (newState) {
                case BluetoothProfile.STATE_CONNECTED:
                    Log.i(TAG, "STATE_CONNECTED");
                    //BluetoothDevice device = gatt.getDevice(); // Get device
                    gatt.discoverServices();
                    break;
                case BluetoothProfile.STATE_DISCONNECTED:
                    Log.e(TAG, "STATE_DISCONNECTED");
                    break;
                default:
                    Log.e(TAG, "STATE_OTHER");
            }
        }

        @Override
        public void onServicesDiscovered(BluetoothGatt gatt, int status) {
            List<BluetoothGattService> services = gatt.getServices();
            Log.i(TAG, "Services: " + services.toString());

            BluetoothGattCharacteristic bpm = services.get(2).getCharacteristics().get(0);
            gatt.readCharacteristic(services.get(0).getCharacteristics().get(0));
        }

        @Override
        public void onCharacteristicRead(BluetoothGatt gatt, BluetoothGattCharacteristic characteristic, int status) {
            // my attempt to read and print characteristics
            byte[] charValue = characteristic.getValue();
            byte flag = charValue[0];
            Log.i(TAG, "Characteristic: " + flag);
            //gatt.disconnect();
        }
    };

Answer 1

try with this inside gattCallback:

        @Override
        public synchronized void onCharacteristicChanged(BluetoothGatt gatt,
                                            BluetoothGattCharacteristic characteristic) {


            final byte[] dataInput = characteristic.getValue();

}

EDIT

And I think you are going to receive a byte with the hear rate data, use this function to get the int value:

public int unsignedByteToInt(byte b) {
    return b & 0xFF;
}

And call it inside onCharacteristicChanged():

final byte[] dataInput = characteristic.getValue();
int hearRate = unsignedByteToInt(dataInput);

EDIT 2

Create a notification listener for the heart rate:

public void setHeartRateNotification(boolean enable){

    String uuidHRCharacteristic = "YOUR CHARACTERISTIC";

    BluetoothGattService mBluetoothLeService = null;
    BluetoothGattCharacteristic mBluetoothGattCharacteristic = null;

    for (BluetoothGattService service : mBluetoothGatt.getServices()) {
        if ((service == null) || (service.getUuid() == null)) {

            continue;
        }
        if (uuidAccelService.equalsIgnoreCase(service.getUuid().toString())) {

            mBluetoothLeService = service;
        }
    }

    if(mBluetoothLeService!=null) {
        mBluetoothGattCharacteristic =
                mBluetoothLeService.getCharacteristic(UUID.fromString(uuidHRCharacteristic));
    }
    else{
        Log.i("Test","mBluetoothLeService is null");
    }

    if(mBluetoothGattCharacteristic!=null) {

        setCharacteristicNotification(mBluetoothGattCharacteristic, enable);

        Log.i("Test","setCharacteristicNotification:"+true);
    }
    else{
        Log.i("Test","mBluetoothGattCharacteristic is null");
    }


}

And set it onServiceDiscover inside gattCallback:

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

    Log.i("Test", "onServicesDiscovered received: " + status);

    setHeartRateNotification(true);

}