Qt Bluetooth Overview
With the Qt Bluetooth API typical use cases are:
- Retrieve information about the local Bluetooth device.
- Scan for other Bluetooth devices in range and retrieve information about them.
- Push files to remote devices using the OBEX Object Push Profile (OPP).
- Connect to remote devices through a RFCOMM channel using the Serial Port Profile (SPP).
- Create a RFCOMM server that allows incoming connections using SPP.
The following sections describe how to use the Qt Bluetooth C++ API classes for the above use cases.
Retrieving Local Device Information
The Qt Bluetooth API has three main purposes. The first one is to obtain local and remote device information. The first steps in retrieving device information is to check if Bluetooth is available on the device and read the local device address and name. The QBluetoothLocalDevice is the one to provide all of this information. Additionally you can use it to turn Bluetooth on and off and set the visibility on the device.
QBluetoothLocalDevice localDevice; QString localDeviceName; // Check if Bluetooth is available on this device if (localDevice.isValid()) { // Turn Bluetooth on localDevice.powerOn(); // Read local device name localDeviceName = localDevice.name(); // Make it visible to others localDevice.setHostMode(QBluetoothLocalDevice::HostDiscoverable) }
Scanning for Bluetooth Devices
Similar to the QBluetoothLocalDevice, the API offers QBluetoothDeviceInfo which provides that information for remote devices. Although you can just create QBluetoothDeviceInfo objects on your own and fill them with data, the easier way is to use the QBluetoothDeviceDiscoveryAgent to start an automated search for visible Bluetooth devices within the connectable range.
// Create a discovery agent and connect to its signals QBluetoothDiscoveryAgent *discoveryAgent = new QBluetoothDiscoveryAgent(this); connect(discoveryAgent, SIGNAL(deviceDiscovered(const QBluetoothDeviceInfo&)), this, SLOT(deviceDiscovered(const QBluetoothDeviceInfo&))); // Start a discovery discoveryAgent->start(); ... // In your local slot, read information about the found devices void MyClass::deviceDiscovered(const QBluetoothDeviceInfo &device) { qDebug() << "Found new device:" << device.name() << '(' << device.address().toString() << ')'; }
Pushing Files to Remote Devices
Once the desired device is found, there are two main use cases provided by Qt Bluetooth. The simpler is to send files via the Obex Object Push Profile (OPP). As the name describes, this profile can only push files from one device to another, but not pull files or browse the remote file system. Because of this limitation, this profile does not require the two devices to be paired before exchanging data. To push files to remote devices, create a QBluetoothTransferRequest and ask the QBluetoothTransferManager to push the file contained in the request by calling the put() function.
// Create a transfer manager QBluetoothTransferManager *transferManager = new QBluetoothTransferManager(this); // Create the transfer request and file to be sent QBluetoothTransferRequest request(device.address()); QFile *file = new QFile("testfile.txt"); // Ask the transfer manager to send it QBluetoothTransferReply *reply = transferManager->put(request, file); // Connect to the reply's signals to be informed about the status and do cleanups when done connect(reply, SIGNAL(finished(QBluetoothTransferReply*)), this, SLOT(transferFinished(QBluetoothTransferReply*)));
Exchanging Data Between Devices
The more flexible approach for communication between two Bluetooth enabled devices, is to create a virtual serial port connection and freely exchange data over that connection. This can be done by the Serial Port Profile (SPP). The Serial Port Profile emulates a serial connection over the Bluetooth transport protocol RFCOMM.
To be able to create SPP connections, you need to register a Server one one device by using QRfcommServer.
rfcommServer = new QBluetoothServer(QBluetoothServiceInfo::RfcommProtocol, this); connect(rfcommServer, SIGNAL(newConnection()), this, SLOT(clientConnected())); bool result = rfcommServer->listen(localAdapter); if (!result) { qWarning() << "Cannot bind chat server to" << localAdapter.toString(); return; }
Connect to this server from another device playing the client role by using a QBluetoothSocket.
void ChatClient::startClient(const QBluetoothServiceInfo &remoteService) { if (socket) return; // Connect to service socket = new QBluetoothSocket(QBluetoothServiceInfo::RfcommProtocol); qDebug() << "Create socket"; socket->connectToService(remoteService); qDebug() << "ConnectToService done"; connect(socket, SIGNAL(readyRead()), this, SLOT(readSocket())); connect(socket, SIGNAL(connected()), this, SLOT(connected())); connect(socket, SIGNAL(disconnected()), this, SIGNAL(disconnected())); }
Using such a connection allows to exchange any form of data in both directions. It is perfectly suited for gaming or for syncing the state between two instances of an application on two devices. For more detailed descriptions on how to configure the server and client, please refer to the detailed description sections in the QRfcommServer and QBluetoothSocket classes. A good example to start with SPP is the Bluetooth Chat example.