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QMqttClient Class

The QMqttClient class represents the central access communicating with an MQTT broker. More...

Header: #include <QMqttClient>
CMake: find_package(Qt6 REQUIRED COMPONENTS Mqtt)
target_link_libraries(mytarget PRIVATE Qt6::Mqtt)
qmake: QT += mqtt
Inherits: QObject

Public Types

enum ClientError { NoError, InvalidProtocolVersion, IdRejected, ServerUnavailable, BadUsernameOrPassword, …, Mqtt5SpecificError }
enum ClientState { Disconnected, Connecting, Connected }
enum ProtocolVersion { MQTT_3_1, MQTT_3_1_1, MQTT_5_0 }
enum TransportType { IODevice, AbstractSocket, SecureSocket }

Properties

Public Functions

QMqttClient(QObject *parent = nullptr)
virtual ~QMqttClient() override
void authenticate(const QMqttAuthenticationProperties &prop)
bool autoKeepAlive() const
bool cleanSession() const
QString clientId() const
void connectToHost()
void connectToHostEncrypted(const QSslConfiguration &conf)
QMqttConnectionProperties connectionProperties() const
void disconnectFromHost()
QMqttClient::ClientError error() const
QString hostname() const
quint16 keepAlive() const
QMqttLastWillProperties lastWillProperties() const
QString password() const
quint16 port() const
QMqttClient::ProtocolVersion protocolVersion() const
qint32 publish(const QMqttTopicName &topic, const QByteArray &message = QByteArray(), quint8 qos = 0, bool retain = false)
qint32 publish(const QMqttTopicName &topic, const QMqttPublishProperties &properties, const QByteArray &message = QByteArray(), quint8 qos = 0, bool retain = false)
bool requestPing()
QMqttServerConnectionProperties serverConnectionProperties() const
void setConnectionProperties(const QMqttConnectionProperties &prop)
void setLastWillProperties(const QMqttLastWillProperties &prop)
void setTransport(QIODevice *device, QMqttClient::TransportType transport)
QMqttClient::ClientState state() const
QMqttSubscription * subscribe(const QMqttTopicFilter &topic, quint8 qos = 0)
QMqttSubscription * subscribe(const QMqttTopicFilter &topic, const QMqttSubscriptionProperties &properties, quint8 qos = 0)
QIODevice * transport() const
void unsubscribe(const QMqttTopicFilter &topic)
void unsubscribe(const QMqttTopicFilter &topic, const QMqttUnsubscriptionProperties &properties)
QString username() const
QByteArray willMessage() const
quint8 willQoS() const
bool willRetain() const
QString willTopic() const

Public Slots

void setAutoKeepAlive(bool autoKeepAlive)
void setCleanSession(bool cleanSession)
void setClientId(const QString &clientId)
void setError(QMqttClient::ClientError error)
void setHostname(const QString &hostname)
void setKeepAlive(quint16 keepAlive)
void setPassword(const QString &password)
void setPort(quint16 port)
void setProtocolVersion(QMqttClient::ProtocolVersion protocolVersion)
void setState(QMqttClient::ClientState state)
void setUsername(const QString &username)
void setWillMessage(const QByteArray &willMessage)
void setWillQoS(quint8 willQoS)
void setWillRetain(bool willRetain)
void setWillTopic(const QString &willTopic)

Signals

void authenticationFinished(const QMqttAuthenticationProperties &p)
void authenticationRequested(const QMqttAuthenticationProperties &p)
void autoKeepAliveChanged(bool autoKeepAlive)
void brokerSessionRestored()
void cleanSessionChanged(bool cleanSession)
void clientIdChanged(QString clientId)
void connected()
void disconnected()
void errorChanged(QMqttClient::ClientError error)
void hostnameChanged(QString hostname)
void keepAliveChanged(quint16 keepAlive)
void messageReceived(const QByteArray &message, const QMqttTopicName &topic = QMqttTopicName())
void messageSent(qint32 id)
void messageStatusChanged(qint32 id, QMqtt::MessageStatus s, const QMqttMessageStatusProperties &properties)
void passwordChanged(QString password)
void pingResponseReceived()
void portChanged(quint16 port)
void protocolVersionChanged(QMqttClient::ProtocolVersion protocolVersion)
void stateChanged(QMqttClient::ClientState state)
void usernameChanged(QString username)
void willMessageChanged(QByteArray willMessage)
void willQoSChanged(quint8 willQoS)
void willRetainChanged(bool willRetain)
void willTopicChanged(QString willTopic)

Detailed Description

An MQTT client is a program or device that uses MQTT to create a network connection to an MQTT server, also called a broker. The connection request must contain a unique client identifier. Optionally, it can contain a Will Topic, Will Message, user name, and password.

Once a connection is created, a client can send messages that other clients might be interested in receiving, subscribe to request notifications on topics, unsubscribe to remove a request for notifications, and disconnect from the broker.

Member Type Documentation

enum QMqttClient::ClientError

This enum type specifies the error state of a client.

Constant Value Description
QMqttClient::NoError 0 No error occurred.
QMqttClient::InvalidProtocolVersion 1 The broker does not accept a connection using the specified protocol version.
QMqttClient::IdRejected 2 The client ID is malformed. This might be related to its length.
QMqttClient::ServerUnavailable 3 The network connection has been established, but the service is unavailable on the broker side.
QMqttClient::BadUsernameOrPassword 4 The data in the username or password is malformed.
QMqttClient::NotAuthorized 5 The client is not authorized to connect.
QMqttClient::TransportInvalid 256 The underlying transport caused an error. For example, the connection might have been interrupted unexpectedly.
QMqttClient::ProtocolViolation 257 The client encountered a protocol violation, and therefore closed the connection.
QMqttClient::UnknownError 258 An unknown error occurred.
QMqttClient::Mqtt5SpecificError 259 The error is related to MQTT protocol level 5. A reason code might provide more details.

enum QMqttClient::ClientState

This enum type specifies the states a client can enter.

Constant Value Description
QMqttClient::Disconnected 0 The client is disconnected from the broker.
QMqttClient::Connecting 1 A connection request has been made, but the broker has not approved the connection yet.
QMqttClient::Connected 2 The client is connected to the broker.

enum QMqttClient::ProtocolVersion

This enum specifies the protocol version of the MQTT standard to use during communication with a broker.

Constant Value Description
QMqttClient::MQTT_3_1 3 MQTT Standard 3.1
QMqttClient::MQTT_3_1_1 4 MQTT Standard 3.1.1, publicly referred to as version 4
QMqttClient::MQTT_5_0 5 MQTT Standard 5.0

enum QMqttClient::TransportType

This enum type specifies the connection method to be used to instantiate a connection to a broker.

Constant Value Description
QMqttClient::IODevice 0 The transport uses a class based on a QIODevice.
QMqttClient::AbstractSocket 1 The transport uses a class based on a QAbstractSocket.
QMqttClient::SecureSocket 2 The transport uses a class based on a QSslSocket.

Property Documentation

[since 5.14] autoKeepAlive : bool

This property holds whether the client will automatically manage keep alive messages to the server.

If this property is true, then the client will automatically send a ping message to the server at the keepAlive interval.

Otherwise, a user will have to manually invoke requestPing within the specified interval of the connection. If no ping has been sent within the interval, the server will disconnect.

The default of this property is true.

This property was introduced in Qt 5.14.

Access functions:

bool autoKeepAlive() const
void setAutoKeepAlive(bool autoKeepAlive)

Notifier signal:

void autoKeepAliveChanged(bool autoKeepAlive)

See also keepAlive(), requestPing(), serverConnectionProperties(), and pingResponseReceived().

cleanSession : bool

This property holds the state after connecting to a broker.

Access functions:

bool cleanSession() const
void setCleanSession(bool cleanSession)

Notifier signal:

void cleanSessionChanged(bool cleanSession)

clientId : QString

This property holds the client's identifier value.

Each client needs to have a unique ID to be able to connect to an MQTT broker. If no client ID is specified by the user, one will be generated automatically when a connection is established.

Access functions:

QString clientId() const
void setClientId(const QString &clientId)

Notifier signal:

void clientIdChanged(QString clientId)

error : ClientError

Specifies the current error of the client.

Access functions:

QMqttClient::ClientError error() const
void setError(QMqttClient::ClientError error)

Notifier signal:

void errorChanged(QMqttClient::ClientError error)

hostname : QString

This property holds the hostname of the MQTT broker to connect to.

If no transport is specified via setTransport(), the client will instantiate a socket connection to the specified hostname itself.

Access functions:

QString hostname() const
void setHostname(const QString &hostname)

Notifier signal:

void hostnameChanged(QString hostname)

keepAlive : quint16

This property holds the interval at which regular ping messages are sent to the broker.

Once a connection to a broker is established, the client needs to send frequent updates to propagate it can still be reached. The interval between those updates is specified by this property.

The interval is specified in seconds.

If the broker does not respond within a grace period the connection will be closed.

Access functions:

quint16 keepAlive() const
void setKeepAlive(quint16 keepAlive)

Notifier signal:

void keepAliveChanged(quint16 keepAlive)

See also autoKeepAlive(), requestPing(), and pingResponseReceived().

password : QString

This property holds the password for connecting to a broker.

Access functions:

QString password() const
void setPassword(const QString &password)

Notifier signal:

void passwordChanged(QString password)

port : quint16

This property holds the port to connect to the MQTT broker.

If no transport is specified via setTransport(), the client will instantiate a socket connection to a host with this port number.

Access functions:

quint16 port() const
void setPort(quint16 port)

Notifier signal:

void portChanged(quint16 port)

protocolVersion : ProtocolVersion

This property holds the MQTT standard version to use for connections.

Specifies the version of the standard the client uses for connecting to a broker. Valid values are:

  • 3: MQTT standard version 3.1.
  • 4: MQTT standard version 3.1.1, often referred to MQTT 4.

Access functions:

QMqttClient::ProtocolVersion protocolVersion() const
void setProtocolVersion(QMqttClient::ProtocolVersion protocolVersion)

Notifier signal:

void protocolVersionChanged(QMqttClient::ProtocolVersion protocolVersion)

state : ClientState

This property holds the current state of the client.

Access functions:

QMqttClient::ClientState state() const
void setState(QMqttClient::ClientState state)

Notifier signal:

void stateChanged(QMqttClient::ClientState state)

username : QString

This property holds the user name for connecting to a broker.

Access functions:

QString username() const
void setUsername(const QString &username)

Notifier signal:

void usernameChanged(QString username)

willMessage : QByteArray

This property holds the payload of a Will Message.

Access functions:

QByteArray willMessage() const
void setWillMessage(const QByteArray &willMessage)

Notifier signal:

void willMessageChanged(QByteArray willMessage)

willQoS : quint8

This property holds the level of QoS for sending and storing the Will Message.

Access functions:

quint8 willQoS() const
void setWillQoS(quint8 willQoS)

Notifier signal:

void willQoSChanged(quint8 willQoS)

willRetain : bool

This property holds whether the Will Message should be retained on the broker for future subscribers to receive.

Access functions:

bool willRetain() const
void setWillRetain(bool willRetain)

Notifier signal:

void willRetainChanged(bool willRetain)

willTopic : QString

This property holds the Will Topic.

Access functions:

QString willTopic() const
void setWillTopic(const QString &willTopic)

Notifier signal:

void willTopicChanged(QString willTopic)

Member Function Documentation

QMqttClient::QMqttClient(QObject *parent = nullptr)

Creates a new MQTT client instance with the specified parent.

[signal, since 5.12] void QMqttClient::authenticationFinished(const QMqttAuthenticationProperties &p)

This signal is emitted after extended authentication has finished. p specifies available details on the authentication process.

After successful authentication QMqttClient::connected is emitted.

Note: Extended authentication is part of the MQTT 5.0 standard and can only be used when the client specifies MQTT_5_0 as ProtocolVersion.

This function was introduced in Qt 5.12.

See also authenticationRequested() and authenticate().

[signal, since 5.12] void QMqttClient::authenticationRequested(const QMqttAuthenticationProperties &p)

This signal is emitted after a client invoked QMqttClient::connectToHost or QMqttClient::connectToHostEncrypted and before the connection is established. In extended authentication, a broker might request additional details which need to be provided by invoking QMqttClient::authenticate. p specifies properties provided by the broker.

Note: Extended authentication is part of the MQTT 5.0 standard and can only be used when the client specifies MQTT_5_0 as ProtocolVersion.

This function was introduced in Qt 5.12.

See also authenticationFinished() and authenticate().

[signal] void QMqttClient::brokerSessionRestored()

This signal is emitted after a client has successfully connected to a broker with the cleanSession property set to false, and the broker has restored the session.

Sessions can be restored if a client has connected previously using the same clientId.

[signal] void QMqttClient::connected()

This signal is emitted when a connection has been established.

[signal] void QMqttClient::disconnected()

This signal is emitted when a connection has been closed. A connection may be closed when disconnectFromHost() is called or when the broker disconnects.

[signal] void QMqttClient::messageReceived(const QByteArray &message, const QMqttTopicName &topic = QMqttTopicName())

This signal is emitted when a new message has been received. The category of the message is specified by topic with the content being message.

[signal] void QMqttClient::messageSent(qint32 id)

Indicates that a message that was sent via the publish() function has been received by the broker. The id is the same as returned by publish() to help tracking the status of the message.

[signal, since 5.12] void QMqttClient::messageStatusChanged(qint32 id, QMqtt::MessageStatus s, const QMqttMessageStatusProperties &properties)

This signal is emitted when the status for the message identified by id changes. s specifies the new status of the message, and properties specify additional properties provided by the server.

This function was introduced in Qt 5.12.

[signal] void QMqttClient::pingResponseReceived()

This signal is emitted after the broker responds to a requestPing() call or a keepAlive() ping message, and the connection is still valid.

[override virtual] QMqttClient::~QMqttClient()

Deletes a MQTT client. If the MQTT client was not already disconnected from the MQTT broker, it will be disconnected from automatically.

[since 5.12] void QMqttClient::authenticate(const QMqttAuthenticationProperties &prop)

Sends an authentication request to the broker. prop specifies the required information to fulfill the authentication request.

This function should only be called after a QMqttClient::authenticationRequested signal has been emitted.

Note: Extended authentication is part of the MQTT 5.0 standard and can only be used when the client specifies MQTT_5_0 as ProtocolVersion.

This function was introduced in Qt 5.12.

See also authenticationRequested() and authenticationFinished().

[invokable] void QMqttClient::connectToHost()

Initiates a connection to the MQTT broker.

Note: This function can be invoked via the meta-object system and from QML. See Q_INVOKABLE.

[since 5.14] void QMqttClient::connectToHostEncrypted(const QSslConfiguration &conf)

Initiates an encrypted connection to the MQTT broker.

conf specifies the SSL configuration to be used for the connection

This function was introduced in Qt 5.14.

[since 5.12] QMqttConnectionProperties QMqttClient::connectionProperties() const

Returns the connection properties the client requests to the broker.

Note: QMqttConnectionProperties can only be used when the client specifies MQTT_5_0 as ProtocolVersion.

This function was introduced in Qt 5.12.

See also setConnectionProperties().

[invokable] void QMqttClient::disconnectFromHost()

Disconnects from the MQTT broker.

Note: This function can be invoked via the meta-object system and from QML. See Q_INVOKABLE.

[since 5.12] QMqttLastWillProperties QMqttClient::lastWillProperties() const

Returns the last will properties.

Note: QMqttLastWillProperties can only be used when the client specifies MQTT_5_0 as ProtocolVersion.

This function was introduced in Qt 5.12.

See also setLastWillProperties().

[invokable] qint32 QMqttClient::publish(const QMqttTopicName &topic, const QByteArray &message = QByteArray(), quint8 qos = 0, bool retain = false)

Publishes a message to the broker with the specified topic. qos specifies the QoS level required for transferring the message.

If retain is set to true, the message will stay on the broker for other clients to connect and receive the message.

Returns an ID that is used internally to identify the message.

Note: This function can be invoked via the meta-object system and from QML. See Q_INVOKABLE.

[invokable, since 5.12] qint32 QMqttClient::publish(const QMqttTopicName &topic, const QMqttPublishProperties &properties, const QByteArray &message = QByteArray(), quint8 qos = 0, bool retain = false)

Publishes a message to the broker with the specified properties and topic. qos specifies the QoS level required for transferring the message.

If retain is set to true, the message will stay on the broker for other clients to connect and receive the message.

Returns an ID that is used internally to identify the message.

Note: properties will only be passed to the broker when the client specifies MQTT_5_0 as ProtocolVersion.

Note: This function can be invoked via the meta-object system and from QML. See Q_INVOKABLE.

This function was introduced in Qt 5.12.

bool QMqttClient::requestPing()

Sends a ping message to the broker and expects a reply.

If the connection is active and autoKeepAlive is true, then calling this function will fail as the client is responsible for managing this process.

Using requestPing() manually requires a call every time within the keepAlive interval as long as the connection is active.

To check whether the ping is successful, connect to the pingResponseReceived() signal.

Returns true if the ping request could be sent.

See also pingResponseReceived(), autoKeepAlive(), and keepAlive().

[since 5.12] QMqttServerConnectionProperties QMqttClient::serverConnectionProperties() const

Returns the QMqttServerConnectionProperties the broker returned after a connection attempt.

This can be used to verify that client side connection properties set by QMqttClient::setConnectionProperties have been accepted by the broker. Also, in case of a failed connection attempt, it can be used for connection diagnostics.

Note: QMqttServerConnectionProperties can only be used when the client specifies MQTT_5_0 as ProtocolVersion.

This function was introduced in Qt 5.12.

See also connectionProperties().

[since 5.12] void QMqttClient::setConnectionProperties(const QMqttConnectionProperties &prop)

Sets the connection properties to prop. QMqttConnectionProperties can be used to ask the server to use a specific feature set. After a connection request the server response can be obtained by calling QMqttClient::serverConnectionProperties.

Note: The connection properties can only be set if the MQTT client is in the Disconnected state.

Note: QMqttConnectionProperties can only be used when the client specifies MQTT_5_0 as ProtocolVersion.

This function was introduced in Qt 5.12.

See also connectionProperties().

[since 5.12] void QMqttClient::setLastWillProperties(const QMqttLastWillProperties &prop)

Sets the last will properties to prop. QMqttLastWillProperties allows to set additional features for the last will message stored at the broker.

Note: The connection properties can only be set if the MQTT client is in the Disconnected state.

Note: QMqttLastWillProperties can only be used when the client specifies MQTT_5_0 as ProtocolVersion.

This function was introduced in Qt 5.12.

See also lastWillProperties().

void QMqttClient::setTransport(QIODevice *device, QMqttClient::TransportType transport)

Sets the transport to device. A transport can be either a socket type or derived from QIODevice and is specified by transport.

Note: The transport can only be exchanged if the MQTT client is in the Disconnected state.

Note: Setting a custom transport for a client does not pass over responsibility on connection management. The transport has to be opened for QIODevice based transports or connected for socket type transports before calling QMqttClient::connectToHost().

See also transport().

QMqttSubscription *QMqttClient::subscribe(const QMqttTopicFilter &topic, quint8 qos = 0)

Adds a new subscription to receive notifications on topic. The parameter qos specifies the level at which security messages are received. For more information about the available QoS levels, see Quality of Service.

This function returns a pointer to a QMqttSubscription. If the same topic is subscribed twice, the return value points to the same subscription instance. The MQTT client is the owner of the subscription.

[since 5.12] QMqttSubscription *QMqttClient::subscribe(const QMqttTopicFilter &topic, const QMqttSubscriptionProperties &properties, quint8 qos = 0)

Adds a new subscription to receive notifications on topic. The parameter properties specifies additional subscription properties to be validated by the broker. The parameter qos specifies the level at which security messages are received. For more information about the available QoS levels, see Quality of Service.

This function returns a pointer to a QMqttSubscription. If the same topic is subscribed twice, the return value points to the same subscription instance. The MQTT client is the owner of the subscription.

Note: properties will only be passed to the broker when the client specifies MQTT_5_0 as ProtocolVersion.

This function was introduced in Qt 5.12.

QIODevice *QMqttClient::transport() const

Returns the transport used for communication with the broker.

See also setTransport().

void QMqttClient::unsubscribe(const QMqttTopicFilter &topic)

Unsubscribes from topic. No notifications will be sent to any of the subscriptions made by calling subscribe().

Note: If a client disconnects from a broker without unsubscribing, the broker will store all messages and publish them on the next reconnect.

[since 5.12] void QMqttClient::unsubscribe(const QMqttTopicFilter &topic, const QMqttUnsubscriptionProperties &properties)

Unsubscribes from topic. No notifications will be sent to any of the subscriptions made by calling subscribe(). properties specifies additional user properties to be passed to the broker.

Note: If a client disconnects from a broker without unsubscribing, the broker will store all messages and publish them on the next reconnect.

Note: properties will only be passed to the broker when the client specifies MQTT_5_0 as ProtocolVersion.

This function was introduced in Qt 5.12.

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