- Qt 5.15
- Qt Bluetooth
- C++ Classes
- QBluetoothServiceDiscoveryAgent
- Qt 5.15.12 Reference Documentation

# QBluetoothServiceDiscoveryAgent Class

The QBluetoothServiceDiscoveryAgent class enables you to query for Bluetooth services. More...

Header: | #include <QBluetoothServiceDiscoveryAgent>
qmake: | QT += bluetooth
Since: | Qt 5.2

This class was introduced in Qt 5.2.

- List of all members, including inherited members

## Public Types

enum | DiscoveryMode { MinimalDiscovery, FullDiscovery }
enum | Error { NoError, PoweredOffError, InputOutputError, InvalidBluetoothAdapterError, UnknownError }

## Public Functions

| QBluetoothServiceDiscoveryAgent(const QBluetoothAddress &deviceAdapter, QObject *parent = nullptr)
| QBluetoothServiceDiscoveryAgent(QObject *parent = nullptr)
virtual | ~QBluetoothServiceDiscoveryAgent()
QList<QBluetoothServiceInfo> | discoveredServices() const
Error | error() const
QString | errorString() const
bool | isActive() const
QBluetoothAddress | remoteAddress() const
bool | setRemoteAddress(const QBluetoothAddress &address)
void | setUuidFilter(const QList<QBluetoothUuid> &uuids)
void | setUuidFilter(const QBluetoothUuid &uuid)
QList<QBluetoothUuid> | uuidFilter() const

## Public Slots

void | clear()
void | start(DiscoveryMode mode = MinimalDiscovery)
void | stop()

## Signals

void | canceled()
void | error(QBluetoothServiceDiscoveryAgent::Error error)
void | finished()
void | serviceDiscovered(const QBluetoothServiceInfo &info)

## Detailed Description

The discovery process relies on the Bluetooth Service Discovery Process (SDP). The following steps are required to query the services provided by all contactable Bluetooth devices:

- create an instance of QBluetoothServiceDiscoveryAgent,
- connect to either the serviceDiscovered() or finished() signals,
- and call start().

     void MyClass::startServiceDiscovery()
     {

         // Create a discovery agent and connect to its signals
         QBluetoothServiceDiscoveryAgent *discoveryAgent = new QBluetoothServiceDiscoveryAgent(this);
         connect(discoveryAgent, SIGNAL(serviceDiscovered(QBluetoothServiceInfo)),
                 this, SLOT(serviceDiscovered(QBluetoothServiceInfo)));

         // Start a discovery
         discoveryAgent->start();

         //...
     }

     // In your local slot, read information about the found devices
     void MyClass::serviceDiscovered(const QBluetoothServiceInfo &service)
     {
         qDebug() << "Found new service:" << service.serviceName()
                  << '(' << service.device().address().toString() << ')';
     }

By default a minimal service discovery is performed. In this mode, the returned QBluetoothServiceInfo objects are guaranteed to contain only device and service UUID information. Depending on platform and device capabilities, other service information may also be available. The minimal service discovery mode relies on cached SDP data of the platform. Therefore it is possible that this discovery does not find a device although it is physically available. In such cases a full discovery must be performed to force an update of the platform cache. However for most use cases a minimal discovery is adequate as it is much quicker and other classes which require up-to-date information such as QBluetoothSocket::connectToService() will perform additional discovery if required. If the full service information is required, pass FullDiscovery as the discoveryMode parameter to start().

This class may internally utilize QBluetoothDeviceDiscoveryAgent to find unknown devices.

The service discovery may find Bluetooth Low Energy services too if the target device is a combination of a classic and Low Energy device. Those devices are required to advertise their Low Energy services via SDP. If the target device only supports Bluetooth Low Energy services, it is likely to not advertise them via SDP. The QLowEnergyController class should be utilized to perform the service discovery on Low Energy devices.

On iOS, this class cannot be used because the platform does not expose an API which may permit access to QBluetoothServiceDiscoveryAgent related features.

See also QBluetoothDeviceDiscoveryAgent and QLowEnergyController.

## Member Type Documentation

### enum QBluetoothServiceDiscoveryAgent::DiscoveryMode

This enum describes the service discovery mode.

Constant | Value | Description
--- | --- | ---
`QBluetoothServiceDiscoveryAgent::MinimalDiscovery` | `0` | Performs a minimal service discovery. The QBluetoothServiceInfo objects returned may be incomplete and are only guaranteed to contain device and service UUID information. Since a minimal discovery relies on cached SDP data it may not find a physically existing device until a `FullDiscovery` is performed.
`QBluetoothServiceDiscoveryAgent::FullDiscovery` | `1` | Performs a full service discovery.

### enum QBluetoothServiceDiscoveryAgent::Error

This enum describes errors that can occur during service discovery.

Constant | Value | Description
--- | --- | ---
`QBluetoothServiceDiscoveryAgent::NoError` | `QBluetoothDeviceDiscoveryAgent::NoError` | No error has occurred.
`QBluetoothServiceDiscoveryAgent::PoweredOffError` | `QBluetoothDeviceDiscoveryAgent::PoweredOffError` | The Bluetooth adaptor is powered off, power it on before doing discovery.
`QBluetoothServiceDiscoveryAgent::InputOutputError` | `QBluetoothDeviceDiscoveryAgent::InputOutputError` | Writing or reading from the device resulted in an error.
`QBluetoothServiceDiscoveryAgent::InvalidBluetoothAdapterError` | `QBluetoothDeviceDiscoveryAgent::InvalidBluetoothAdapterError` | The passed local adapter address does not match the physical adapter address of any local Bluetooth device. This value was introduced by Qt 5.3.
`QBluetoothServiceDiscoveryAgent::UnknownError` | `QBluetoothDeviceDiscoveryAgent::UnknownError` | An unknown error has occurred.

## Member Function Documentation

### QBluetoothServiceDiscoveryAgent::QBluetoothServiceDiscoveryAgent(const QBluetoothAddress &deviceAdapter, QObject *parent = nullptr)

Constructs a new QBluetoothServiceDiscoveryAgent for deviceAdapter and with parent.

It uses deviceAdapter for the service search. If deviceAdapter is default constructed the resulting QBluetoothServiceDiscoveryAgent object will use the local default Bluetooth adapter.

If a deviceAdapter is specified that is not a local adapter error() will be set to InvalidBluetoothAdapterError. Therefore it is recommended to test the error flag immediately after using this constructor.

Note: On WinRT the passed adapter address will be ignored.

Note: On Android passing any deviceAdapter address is meaningless as Android 6.0 or later does not publish the local Bluetooth address anymore. Subsequently, the passed adapter address can never be matched against the local adapter address. Therefore the subsequent call to start() will always trigger InvalidBluetoothAdapterError.

See also error().

### QBluetoothServiceDiscoveryAgent::QBluetoothServiceDiscoveryAgent(QObject *parent = nullptr)

Constructs a new QBluetoothServiceDiscoveryAgent with parent. The search is performed via the local default Bluetooth adapter.

### `[signal]` void QBluetoothServiceDiscoveryAgent::canceled()

This signal is triggered when the service discovery was canceled via a call to stop().

### `[slot]` void QBluetoothServiceDiscoveryAgent::clear()

Clears the results of previous service discoveries and resets uuidFilter(). This function does nothing during an ongoing service discovery (see isActive()).

See also discoveredServices().

### `[signal]` void QBluetoothServiceDiscoveryAgent::error(QBluetoothServiceDiscoveryAgent::Error error)

This signal is emitted when an error occurs. The error parameter describes the error that occurred.

Note: Signal error is overloaded in this class. To connect to this signal by using the function pointer syntax, Qt provides a convenient helper for obtaining the function pointer as shown in this example:

     connect(bluetoothServiceDiscoveryAgent, QOverload<QBluetoothServiceDiscoveryAgent::Error>::of(&QBluetoothServiceDiscoveryAgent::error),
         [=](QBluetoothServiceDiscoveryAgent::Error error){ /* ... */ });

### `[signal]` void QBluetoothServiceDiscoveryAgent::finished()

This signal is emitted when the Bluetooth service discovery completes.

Unlike the QBluetoothDeviceDiscoveryAgent::finished() signal this signal will even be emitted when an error occurred during the service discovery. Therefore it is recommended to check the error() signal to evaluate the success of the service discovery discovery.

### `[signal]` void QBluetoothServiceDiscoveryAgent::serviceDiscovered(const QBluetoothServiceInfo &info)

This signal is emitted when the Bluetooth service described by info is discovered.

Note: The passed QBluetoothServiceInfo parameter may contain a Bluetooth Low Energy service if the target device advertises the service via SDP. This is required from device which support both, classic Bluetooth (BaseRate) and Low Energy services.

See also QBluetoothDeviceInfo::coreConfigurations().

### `[slot]` void QBluetoothServiceDiscoveryAgent::start(DiscoveryMode mode = MinimalDiscovery)

Starts service discovery. mode specifies the type of service discovery to perform.

On some platforms, device discovery may lead to pairing requests.

See also DiscoveryMode.

### `[slot]` void QBluetoothServiceDiscoveryAgent::stop()

Stops the service discovery process. The canceled() signal will be emitted once the search has stopped.

### `[virtual]` QBluetoothServiceDiscoveryAgent::~QBluetoothServiceDiscoveryAgent()

Destructor for QBluetoothServiceDiscoveryAgent

### QList<QBluetoothServiceInfo> QBluetoothServiceDiscoveryAgent::discoveredServices() const

Returns the list of all discovered services.

This list of services accumulates newly discovered services from multiple calls to start(). Unless clear() is called the list cannot decrease in size. This implies that if a remote Bluetooth device moves out of range in between two subsequent calls to start() the list may contain stale entries.

Note: The list of services should always be cleared before the discovery mode is changed.

See also clear().

### Error QBluetoothServiceDiscoveryAgent::error() const

Returns the type of error that last occurred. If the service discovery is done for a single remoteAddress() it will return errors that occurred while trying to discover services on that device. If the remoteAddress() is not set and devices are discovered by a scan, errors during service discovery on individual devices are not saved and no signals are emitted. In this case, errors are fairly normal as some devices may not respond to discovery or may no longer be in range. Such errors are surpressed. If no services are returned, it can be assumed no services could be discovered.

### QString QBluetoothServiceDiscoveryAgent::errorString() const

Returns a human-readable description of the last error that occurred during the service discovery.

### bool QBluetoothServiceDiscoveryAgent::isActive() const

Returns `true` if the service discovery is currently active; otherwise returns `false`. An active discovery can be stopped by calling stop().

### QBluetoothAddress QBluetoothServiceDiscoveryAgent::remoteAddress() const

Returns the remote device address. If setRemoteAddress() is not called, the function will return a default constructed QBluetoothAddress.

See also setRemoteAddress().

### bool QBluetoothServiceDiscoveryAgent::setRemoteAddress(const QBluetoothAddress &address)

Sets the remote device address to address. If address is default constructed, services will be discovered on all contactable Bluetooth devices. A new remote address can only be set while there is no service discovery in progress; otherwise this function returns false.

On some platforms the service discovery might lead to pairing requests. Therefore it is not recommended to do service discoveries on all devices. This function can be used to restrict the service discovery to a particular device.

See also remoteAddress().

### void QBluetoothServiceDiscoveryAgent::setUuidFilter(const QList<QBluetoothUuid> &uuids)

Sets the UUID filter to uuids. Only services matching the UUIDs in uuids will be returned. The matching applies to the service's ServiceId and ServiceClassIds attributes.

An empty UUID list is equivalent to a list containing only QBluetoothUuid::PublicBrowseGroup.

See also uuidFilter().

### void QBluetoothServiceDiscoveryAgent::setUuidFilter(const QBluetoothUuid &uuid)

This is an overloaded member function, provided for convenience.

Sets the UUID filter to a list containing the single element uuid. The matching applies to the service's ServiceId and ServiceClassIds attributes.

See also uuidFilter().

### QList<QBluetoothUuid> QBluetoothServiceDiscoveryAgent::uuidFilter() const

Returns the UUID filter.

See also setUuidFilter().