QLowEnergyCharacteristic Class

The QLowEnergyCharacteristic class stores information about a Bluetooth Low Energy service characteristic. More...

Header: #include <QLowEnergyCharacteristic>
qmake: QT += bluetooth
Since: Qt 5.4

Public Types

enum PropertyType { Unknown, Broadcasting, Read, WriteNoResponse, Write, …, ExtendedProperty }
flags PropertyTypes

Public Functions

QLowEnergyCharacteristic(const QLowEnergyCharacteristic &other)
QLowEnergyCharacteristic()
QLowEnergyCharacteristic &operator=(const QLowEnergyCharacteristic &other)
~QLowEnergyCharacteristic()
QLowEnergyDescriptor clientCharacteristicConfiguration() const
QLowEnergyDescriptor descriptor(const QBluetoothUuid &uuid) const
QList<QLowEnergyDescriptor> descriptors() const
bool isValid() const
QString name() const
QLowEnergyCharacteristic::PropertyTypes properties() const
QBluetoothUuid uuid() const
QByteArray value() const

Static Public Members

const QByteArray CCCDDisable
const QByteArray CCCDEnableIndication
const QByteArray CCCDEnableNotification
bool operator!=(const QLowEnergyCharacteristic &a, const QLowEnergyCharacteristic &b)
bool operator==(const QLowEnergyCharacteristic &a, const QLowEnergyCharacteristic &b)

Detailed Description

QLowEnergyCharacteristic provides information about a Bluetooth Low Energy service characteristic's name(), uuid(), value(), properties(), and descriptors(). To obtain the characteristic's specification and information, it is necessary to connect to the device using the QLowEnergyService and QLowEnergyController classes.

The characteristic value may be written via the QLowEnergyService instance that manages the service to which this characteristic belongs. The QLowEnergyService::writeCharacteristic() function writes the new value. The QLowEnergyService::characteristicWritten() signal is emitted upon success. The value() of this object is automatically updated accordingly.

Characteristics may contain none, one or more descriptors. They can be individually retrieved using the descriptor() function. The descriptors() function returns all descriptors as a list. The general purpose of a descriptor is to add contextual information to the characteristic. For example, the descriptor might provide format or range information specifying how the characteristic's value is to be interpreted.

See also QLowEnergyService and QLowEnergyDescriptor.

Member Type Documentation

enum QLowEnergyCharacteristic::PropertyType
flags QLowEnergyCharacteristic::PropertyTypes

This enum describes the properties of a characteristic.

ConstantValueDescription
QLowEnergyCharacteristic::Unknown0x00The type is not known.
QLowEnergyCharacteristic::Broadcasting0x01Allow for the broadcasting of Generic Attributes (GATT) characteristic values.
QLowEnergyCharacteristic::Read0x02Allow the characteristic values to be read.
QLowEnergyCharacteristic::WriteNoResponse0x04Allow characteristic values without responses to be written.
QLowEnergyCharacteristic::Write0x08Allow for characteristic values to be written.
QLowEnergyCharacteristic::Notify0x10Permits notification of characteristic values.
QLowEnergyCharacteristic::Indicate0x20Permits indications of characteristic values.
QLowEnergyCharacteristic::WriteSigned0x40Permits signed writes of the GATT characteristic values.
QLowEnergyCharacteristic::ExtendedProperty0x80Additional characteristic properties are defined in the characteristic's extended properties descriptor.

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

See also properties().

Member Function Documentation

QLowEnergyCharacteristic::QLowEnergyCharacteristic(const QLowEnergyCharacteristic &other)

Construct a new QLowEnergyCharacteristic that is a copy of other.

The two copies continue to share the same underlying data which does not detach upon write.

QLowEnergyCharacteristic::QLowEnergyCharacteristic()

Construct a new QLowEnergyCharacteristic. A default-constructed instance of this class is always invalid.

See also isValid().

QLowEnergyCharacteristic &QLowEnergyCharacteristic::operator=(const QLowEnergyCharacteristic &other)

Makes a copy of other and assigns it to this QLowEnergyCharacteristic object. The two copies continue to share the same service and controller details.

QLowEnergyCharacteristic::~QLowEnergyCharacteristic()

Destroys the QLowEnergyCharacteristic object.

[since 6.2] QLowEnergyDescriptor QLowEnergyCharacteristic::clientCharacteristicConfiguration() const

Returns the Client Characteristic Configuration Descriptor or an invalid QLowEnergyDescriptor instance if no Client Characteristic Configuration Descriptor exists.

BTLE characteristics can support notifications and/or indications. In both cases, the peripheral will inform the central on each change of the characteristic's value. In the BTLE attribute protocol, notification messages are not confirmed by the central, while indications are confirmed. Notifications are considered faster, but unreliable, while indications are slower and more reliable.

If a characteristic supports notification or indication, these can be enabled by writing special bit patterns to the Client Characteristic Configuration Descriptor. For convenience, these bit patterns are provided as QLowEnergyCharacteristic::CCCDDisable, QLowEnergyCharacteristic::CCCDEnableNotification, and QLowEnergyCharacteristic::CCCDEnableIndication.

Enabling e.g. notification for a characteristic named mycharacteristic in a service called myservice could be done using the following code.

auto cccd = mycharacteristic.clientCharacteristicConfiguration();
if (!cccd.isValid()) {
    // your error handling
    return error;
}
myservice->writeDescriptor(cccd, QLowEnergyCharacteristic::CCCDEnableNotification);

Note: Calling characteristic.clientCharacteristicConfiguration() is equivalent to calling characteristic.descriptor(QBluetoothUuid::DescriptorType::ClientCharacteristicConfiguration).

This function was introduced in Qt 6.2.

See also descriptor().

QLowEnergyDescriptor QLowEnergyCharacteristic::descriptor(const QBluetoothUuid &uuid) const

Returns the descriptor for uuid or an invalid QLowEnergyDescriptor instance.

See also descriptors().

QList<QLowEnergyDescriptor> QLowEnergyCharacteristic::descriptors() const

Returns the list of descriptors belonging to this characteristic; otherwise an empty list.

See also descriptor().

bool QLowEnergyCharacteristic::isValid() const

Returns true if the QLowEnergyCharacteristic object is valid, otherwise returns false.

An invalid characteristic object is not associated with any service (default-constructed) or the associated service is no longer valid due to a disconnect from the underlying Bluetooth Low Energy device, for example. Once the object is invalid it cannot become valid anymore.

Note: If a QLowEnergyCharacteristic instance turns invalid due to a disconnect from the underlying device, the information encapsulated by the current instance remains as it was at the time of the disconnect. Therefore it can be retrieved after the disconnect event.

QString QLowEnergyCharacteristic::name() const

Returns the human-readable name of the characteristic.

The name is based on the characteristic's uuid() which must have been standardized. The complete list of characteristic types can be found under Bluetooth.org Characteristics.

The returned string is empty if the uuid() is unknown.

See also QBluetoothUuid::characteristicToString().

QLowEnergyCharacteristic::PropertyTypes QLowEnergyCharacteristic::properties() const

Returns the properties of the characteristic.

The properties define the access permissions for the characteristic.

QBluetoothUuid QLowEnergyCharacteristic::uuid() const

Returns the UUID of the characteristic if isValid() returns true; otherwise a null UUID.

QByteArray QLowEnergyCharacteristic::value() const

Returns the cached value of the characteristic.

If the characteristic's properties() permit writing of new values, the value can be updated using QLowEnergyService::writeCharacteristic().

The cache is updated during the associated service's detail discovery, a successful read/write operation or when an update notification is received.

The returned QByteArray always remains empty if the characteristic does not have the read permission. In such cases only the QLowEnergyService::characteristicChanged() or QLowEnergyService::characteristicWritten() may provice information about the value of this characteristic.

Member Variable Documentation

[since 6.2] const QByteArray QLowEnergyCharacteristic::CCCDDisable

Bit pattern to write into Client Characteristic Configuration Descriptor to disable both notification and indication.

This documentation was introduced in Qt 6.2.

See also QLowEnergyCharacteristic::clientCharacteristicConfiguration.

[since 6.2] const QByteArray QLowEnergyCharacteristic::CCCDEnableIndication

Bit pattern to write into Client Characteristic Configuration Descriptor to enable indication.

This documentation was introduced in Qt 6.2.

See also QLowEnergyCharacteristic::clientCharacteristicConfiguration.

[since 6.2] const QByteArray QLowEnergyCharacteristic::CCCDEnableNotification

Bit pattern to write into Client Characteristic Configuration Descriptor to enable notification.

This documentation was introduced in Qt 6.2.

See also QLowEnergyCharacteristic::clientCharacteristicConfiguration.

Related Non-Members

bool operator!=(const QLowEnergyCharacteristic &a, const QLowEnergyCharacteristic &b)

Returns true if a and b are not equal; otherwise false.

Two QLowEnergyCharcteristic instances are considered to be equal if they refer to the same characteristic on the same remote Bluetooth Low Energy device or both instances have been default-constructed.

bool operator==(const QLowEnergyCharacteristic &a, const QLowEnergyCharacteristic &b)

Returns true if a is equal to b, otherwise false.

Two QLowEnergyCharacteristic instances are considered to be equal if they refer to the same characteristic on the same remote Bluetooth Low Energy device or both instances have been default-constructed.

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