BluetoothGattCallback only calls onConnectionStateChange

Question

My device.connectGatt() only triggers onConnectionStateChage. The status is 0, and the connection is established. I tested it on a 4.4 and 5.1 system - with the same result. This is my code:

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

        //BluetoothGattCharacteristic tempChar = null;
        //tempChar.setValue("test");

        if(status == 0) {
            gatt.connect();
            //gatt.disconnect();


        }
    }

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

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

    @Override
    public void onCharacteristicWrite(BluetoothGatt gatt, BluetoothGattCharacteristic characteristic, int status) {
        super.onCharacteristicWrite(gatt, characteristic, status);
        // Try to send some data to the device
        characteristic.setValue("test");
        gatt.writeCharacteristic(characteristic);
    }

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


    }

    @Override
    public void onDescriptorRead(BluetoothGatt gatt, BluetoothGattDescriptor descriptor, int status) {
        super.onDescriptorRead(gatt, descriptor, status);
    }

    @Override
    public void onDescriptorWrite(BluetoothGatt gatt, BluetoothGattDescriptor descriptor, int status) {
        super.onDescriptorWrite(gatt, descriptor, status);

    }

    @Override
    public void onReliableWriteCompleted(BluetoothGatt gatt, int status) {
        super.onReliableWriteCompleted(gatt, status);
    }

    @Override
    public void onReadRemoteRssi(BluetoothGatt gatt, int rssi, int status) {
        super.onReadRemoteRssi(gatt, rssi, status);
    }

    @Override
    public void onMtuChanged(BluetoothGatt gatt, int mtu, int status) {
        super.onMtuChanged(gatt, mtu, status);
    }

    @Override
    protected Object clone() throws CloneNotSupportedException {
        return super.clone();
    }

    @Override
    public boolean equals(Object o) {
        return super.equals(o);
    }

    @Override
    protected void finalize() throws Throwable {
        super.finalize();
    }

    @Override
    public int hashCode() {
        return super.hashCode();
    }

    @Override
    public String toString() {
        return super.toString();
    }
};

I added a break-point to all the calls and the only one that gets hit is the onConnectionStateChange. Is this supposed to work like this? Where do I get to write either a characteristic or a description and push it to the BLE module? Also, the BLE module (HC-10 controlled by an Arduino) is sending data out (tested it with a different app) ... so I would expect it to hit the onCharacteristicOnChage method as well. What am I missing ?

Answer 1

Upon connection, Android is not aware of any services or characteristics on the device.

Hence, you must discoverServices() once connected:

@Override
public void onConnectionStateChange(BluetoothGatt gatt, int status, int newState) {
    if (newState == BluetoothProfile.STATE_CONNECTED) {
        gatt.discoverServices();
    }
}

Once you get a callback, you can do a write:

@Override
public void onServicesDiscovered(BluetoothGatt gatt, int status) {
    if (status == BluetoothGatt.GATT_SUCCESS) {
        BluetoothGattCharacteristic writeChar = mBluetoothGatt.getService(myServiceUUID)
                    .getCharacteristic(myWriteCharUUID);
        byte[] data = new byte[10];
        writeChar.setValue(data);
        gatt.writeCharacteristic(writeChar);
    }
}