- Qt 6.12
- Qt Bluetooth
- C++ Classes
- QBluetoothDeviceDiscoveryAgent
- Qt 6.12.0 Reference Documentation

# QBluetoothDeviceDiscoveryAgent Class

The QBluetoothDeviceDiscoveryAgent class discovers the Bluetooth devices nearby. More...

Header: | `#include <QBluetoothDeviceDiscoveryAgent>`
CMake: | `find_package(Qt6 REQUIRED COMPONENTS Bluetooth)` `target_link_libraries(mytarget PRIVATE Qt6::Bluetooth)`
qmake: | `QT += bluetooth`
Inherits: | QObject

- List of all members, including inherited members

## Public Types

enum | DiscoveryMethod { NoMethod, ClassicMethod, LowEnergyMethod }
flags | DiscoveryMethods
enum | Error { NoError, PoweredOffError, InputOutputError, InvalidBluetoothAdapterError, UnsupportedPlatformError, …, UnknownError }

## Public Functions

| QBluetoothDeviceDiscoveryAgent(QObject *parent = nullptr)
| QBluetoothDeviceDiscoveryAgent(const QBluetoothAddress &deviceAdapter, QObject *parent = nullptr)
virtual | ~QBluetoothDeviceDiscoveryAgent()
QList<QBluetoothDeviceInfo> | discoveredDevices() const
QBluetoothDeviceDiscoveryAgent::Error | error() const
QString | errorString() const
bool | isActive() const
int | lowEnergyDiscoveryTimeout() const
void | setLowEnergyDiscoveryTimeout(int timeout)

## Public Slots

void | start()
void | start(QBluetoothDeviceDiscoveryAgent::DiscoveryMethods methods)
void | stop()

## Signals

void | canceled()
void | deviceDiscovered(const QBluetoothDeviceInfo &info)
void | deviceUpdated(const QBluetoothDeviceInfo &info, QBluetoothDeviceInfo::Fields updatedFields)
`(since 6.2)` void | errorOccurred(QBluetoothDeviceDiscoveryAgent::Error error)
void | finished()

## Static Public Members

QBluetoothDeviceDiscoveryAgent::DiscoveryMethods | supportedDiscoveryMethods()

## Detailed Description

To discover the nearby Bluetooth devices:

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

     void MyClass::startDeviceDiscovery()
     {

         // Create a discovery agent and connect to its signals
         QBluetoothDeviceDiscoveryAgent *discoveryAgent = new QBluetoothDeviceDiscoveryAgent(this);
         connect(discoveryAgent, SIGNAL(deviceDiscovered(QBluetoothDeviceInfo)),
                 this, SLOT(deviceDiscovered(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() << ')';
     }

To retrieve results asynchronously, connect to the deviceDiscovered() signal. To get a list of all discovered devices, call discoveredDevices() after the finished() signal.

This class can be used to discover Classic and Low Energy Bluetooth devices. The individual device type can be determined via the QBluetoothDeviceInfo::coreConfigurations() attribute. In most cases the list returned by discoveredDevices() contains both types of devices. However not every platform can detect both types of devices. On platforms with this limitation (for example iOS only suports Low Energy discovery), the discovery process will limit the search to the type which is supported.

Note: Since Android 6.0 the ability to detect devices requires ACCESS_COARSE_LOCATION.

Note: The Win32 backend currently does not support the Received Signal Strength Indicator (RSSI), as well as the Manufacturer Specific Data, or other data updates advertised by Bluetooth LE devices after discovery.

## Member Type Documentation

### enum QBluetoothDeviceDiscoveryAgent::DiscoveryMethod
flags QBluetoothDeviceDiscoveryAgent::DiscoveryMethods

This enum descibes the type of discovery method employed by the QBluetoothDeviceDiscoveryAgent.

Constant | Value | Description
--- | --- | ---
`QBluetoothDeviceDiscoveryAgent::NoMethod` | `0x0` | The discovery is not possible. None of the available methods are supported.
`QBluetoothDeviceDiscoveryAgent::ClassicMethod` | `0x01` | The discovery process searches for Bluetooth Classic (BaseRate) devices.
`QBluetoothDeviceDiscoveryAgent::LowEnergyMethod` | `0x02` | The discovery process searches for Bluetooth Low Energy devices.

The DiscoveryMethods type is a typedef for QFlags<DiscoveryMethod>. It stores an OR combination of DiscoveryMethod values.

See also supportedDiscoveryMethods().

### enum QBluetoothDeviceDiscoveryAgent::Error

Indicates all possible error conditions found during Bluetooth device discovery.

Constant | Value | Description
--- | --- | ---
`QBluetoothDeviceDiscoveryAgent::NoError` | `0` | No error has occurred.
`QBluetoothDeviceDiscoveryAgent::PoweredOffError` | `2` | The Bluetooth adaptor is powered off, power it on before doing discovery.
`QBluetoothDeviceDiscoveryAgent::InputOutputError` | `1` | Writing or reading from the device resulted in an error.
`QBluetoothDeviceDiscoveryAgent::InvalidBluetoothAdapterError` | `3` | The passed local adapter address does not match the physical adapter address of any local Bluetooth device.
`QBluetoothDeviceDiscoveryAgent::UnsupportedPlatformError (since Qt 5.5)` | `4` | Device discovery is not possible or implemented on the current platform. The error is set in response to a call to start(). An example for such cases are iOS versions below 5.0 which do not support Bluetooth device search at all.
`QBluetoothDeviceDiscoveryAgent::UnsupportedDiscoveryMethod (since Qt 5.8)` | `5` | One of the requested discovery methods is not supported by the current platform.
`QBluetoothDeviceDiscoveryAgent::LocationServiceTurnedOffError (since Qt 6.2)` | `6` | The location service is turned off. Usage of Bluetooth APIs is not possible when location service is turned off.
`QBluetoothDeviceDiscoveryAgent::MissingPermissionsError (since Qt 6.4)` | `7` | The operating system requests permissions which were not granted by the user.
`QBluetoothDeviceDiscoveryAgent::UnknownError` | `100` | An unknown error has occurred.

## Member Function Documentation

### `[explicit]` QBluetoothDeviceDiscoveryAgent::QBluetoothDeviceDiscoveryAgent(QObject *parent = nullptr)

Constructs a new Bluetooth device discovery agent with parent parent.

### `[explicit]` QBluetoothDeviceDiscoveryAgent::QBluetoothDeviceDiscoveryAgent(const QBluetoothAddress &deviceAdapter, QObject *parent = nullptr)

Constructs a new Bluetooth device discovery agent with parent.

It uses deviceAdapter for the device search. If deviceAdapter is default constructed the resulting QBluetoothDeviceDiscoveryAgent 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.

See also error().

### `[virtual noexcept]` QBluetoothDeviceDiscoveryAgent::~QBluetoothDeviceDiscoveryAgent()

Destructor for ~QBluetoothDeviceDiscoveryAgent()

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

This signal is emitted when device discovery is aborted by a call to stop().

### `[signal]` void QBluetoothDeviceDiscoveryAgent::deviceDiscovered(const QBluetoothDeviceInfo &info)

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

The signal is emitted as soon as the most important device information has been collected. However, as long as the finished() signal has not been emitted the information collection continues even for already discovered devices. This is particularly true for signal strength information (RSSI) and manufacturer data updates. If the use case requires continuous manufacturer data or RSSI updates it is advisable to retrieve the device information via discoveredDevices() once the discovery has finished or listen to the deviceUpdated() signal.

If lowEnergyDiscoveryTimeout() is larger than 0 the signal is only ever emitted when at least one attribute of info changes. This reflects the desire to receive updates as more precise information becomes available. The exception to this behavior is the case when lowEnergyDiscoveryTimeout is set to `0`. A timeout of `0` expresses the desire to monitor the appearance and disappearance of Low Energy devices over time. Under this condition the deviceDiscovered() signal is emitted even if info has not changed since the last signal emission.

See also QBluetoothDeviceInfo::rssi() and lowEnergyDiscoveryTimeout().

### `[signal]` void QBluetoothDeviceDiscoveryAgent::deviceUpdated(const QBluetoothDeviceInfo &info, QBluetoothDeviceInfo::Fields updatedFields)

This signal is emitted when the agent receives additional information about the Bluetooth device described by info. The updatedFields flags tell which information has been updated.

During discovery, some information can change dynamically, such as signal strength and manufacturerData. This signal informs you that if your application is displaying this data, it can be updated, rather than waiting until the discovery has finished.

See also QBluetoothDeviceInfo::rssi() and lowEnergyDiscoveryTimeout().

### QList<QBluetoothDeviceInfo> QBluetoothDeviceDiscoveryAgent::discoveredDevices() const

Returns a list of all discovered Bluetooth devices.

### QBluetoothDeviceDiscoveryAgent::Error QBluetoothDeviceDiscoveryAgent::error() const

Returns the last error.

Any possible previous errors are cleared upon restarting the discovery.

### `[signal, since 6.2]` void QBluetoothDeviceDiscoveryAgent::errorOccurred(QBluetoothDeviceDiscoveryAgent::Error error)

This signal is emitted when an error occurs during Bluetooth device discovery. The error parameter describes the error that occurred.

This function was introduced in Qt 6.2.

See also error() and errorString().

### QString QBluetoothDeviceDiscoveryAgent::errorString() const

Returns a human-readable description of the last error.

See also error() and errorOccurred().

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

This signal is emitted when Bluetooth device discovery completes. The signal is not going to be emitted if the device discovery finishes with an error.

### bool QBluetoothDeviceDiscoveryAgent::isActive() const

Returns true if the agent is currently discovering Bluetooth devices, otherwise returns false.

### int QBluetoothDeviceDiscoveryAgent::lowEnergyDiscoveryTimeout() const

Returns a timeout in milliseconds that is applied to the Bluetooth Low Energy device search. A value of `-1` implies that the platform does not support this property and the timeout for the device search cannot be adjusted. A return value of `0` implies a never-ending search which must be manually stopped via stop().

See also setLowEnergyDiscoveryTimeout().

### void QBluetoothDeviceDiscoveryAgent::setLowEnergyDiscoveryTimeout(int timeout)

Sets the maximum search time for Bluetooth Low Energy device search to timeout in milliseconds. If timeout is `0` the discovery runs until stop() is called.

This reflects the fact that the discovery process for Bluetooth Low Energy devices is mostly open ended. The platform continues to look for more devices until the search is manually stopped. The timeout ensures that the search is aborted after timeout milliseconds. Of course, it is still possible to manually abort the discovery by calling stop().

The new timeout value does not take effect until the device search is restarted. In addition the timeout does not affect the classic Bluetooth device search. Depending on the platform the classic search may add more time to the total discovery process beyond timeout.

For a reliable Bluetooth Low Energy discovery, use at least 40000 milliseconds.

See also lowEnergyDiscoveryTimeout().

### `[slot]` void QBluetoothDeviceDiscoveryAgent::start()

Starts Bluetooth device discovery, if it is not already started.

The deviceDiscovered() signal is emitted as each device is discovered. The finished() signal is emitted once device discovery is complete. The discovery utilizes the maximum set of supported discovery methods on the platform.

Note: This slot is overloaded. To connect to this slot:

     // Connect using qOverload:
     connect(sender, &SenderClass::signal,
             bluetoothDeviceDiscoveryAgent, qOverload<>(&QBluetoothDeviceDiscoveryAgent::start));

     // Or using a lambda as wrapper:
     connect(sender, &SenderClass::signal,
             bluetoothDeviceDiscoveryAgent, [receiver = bluetoothDeviceDiscoveryAgent]() { receiver->start(); });

For more examples and approaches, see connecting to overloaded slots.

See also supportedDiscoveryMethods().

### `[slot]` void QBluetoothDeviceDiscoveryAgent::start(QBluetoothDeviceDiscoveryAgent::DiscoveryMethods methods)

Starts Bluetooth device discovery, if it is not already started and the provided methods are supported. The discovery methods limit the scope of the device search. For example, if the target service or device is a Bluetooth Low Energy device, this function could be used to limit the search to Bluetooth Low Energy devices and thereby reduces the discovery time significantly.

Note: methods only determines the type of discovery and does not imply the filtering of the results. For example, the search may still contain classic bluetooth devices despite methods being set to LowEnergyMethod only. This may happen due to previously cached search results which may be incorporated into the search results.

Note: Some platforms (e.g. Windows) may also provide cached results for the devices that are not currently advertising. Other platforms (like `iOS`) only provide information about currently advertising devices. You can store the received device UUID (or Bluetooth address) upon first discovery, and then use it later to establish a connection directly (see QBluetoothDeviceInfo). This way the applications can omit the later device discovery phases. Using the Bluetooth address requires that the remote device's Bluetooth address does not change.

Note: This slot is overloaded. To connect to this slot:

     // Connect using qOverload:
     connect(sender, &SenderClass::signal,
             bluetoothDeviceDiscoveryAgent, qOverload(&QBluetoothDeviceDiscoveryAgent::start));

     // Or using a lambda as wrapper:
     connect(sender, &SenderClass::signal,
             bluetoothDeviceDiscoveryAgent, [receiver = bluetoothDeviceDiscoveryAgent](QBluetoothDeviceDiscoveryAgent::DiscoveryMethods methods) { receiver->start(methods); });

For more examples and approaches, see connecting to overloaded slots.

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

Stops Bluetooth device discovery. The cancel() signal is emitted once the device discovery is canceled. start() maybe called before the cancel signal is received. Once start() has been called the cancel signal from the prior discovery will be discarded.

### `[static]` QBluetoothDeviceDiscoveryAgent::DiscoveryMethods QBluetoothDeviceDiscoveryAgent::supportedDiscoveryMethods()

This function returns the discovery methods supported by the current platform. It can be used to limit the scope of the device discovery.