MQTT 订阅

创建与 MQTT 代理通信的应用程序。

MQTT 订阅说明了如何创建与 MQTT 代理通信的应用程序。每个订阅都会打开一个新对话框，您可以在其中查看订阅主题上的消息。

创建客户端

我们使用QMqttClient 类创建 MQTT 客户端，并设置用于连接的代理主机名和端口：

    m_client = new QMqttClient(this);
    m_client->setHostname(ui->lineEditHost->text());
    m_client->setPort(static_cast<quint16>(ui->spinBoxPort->value()));

订阅主题

当用户在客户端订阅主题时，会创建一个新的订阅对象：

void MainWindow::on_buttonSubscribe_clicked()
{
    auto subscription = m_client->subscribe(ui->lineEditTopic->text(),
                                            static_cast<quint8>(ui->spinQoS->text().toUInt()));
    if (!subscription) {
        QMessageBox::critical(this, u"Error"_s,
                              u"Could not subscribe. Is there a valid connection?"_s);
        return;
    }
    auto subWindow = new SubscriptionWindow(subscription);
    subWindow->setWindowTitle(subscription->topic().filter());
    subWindow->show();
}

我们使用QMqttSubscription 类来存储订阅的主题、状态和 QoS 级别：

SubscriptionWindow::SubscriptionWindow(QMqttSubscription *sub, QWidget *parent) :
    QWidget(parent),
    ui(new Ui::SubscriptionWindow),
    m_sub(sub)
{
    ui->setupUi(this);

    ui->labelSub->setText(m_sub->topic().filter());
    ui->labelQoS->setText(QString::number(m_sub->qos()));
    updateStatus(m_sub->state());
    connect(m_sub, &QMqttSubscription::messageReceived, this, &SubscriptionWindow::updateMessage);
    connect(m_sub, &QMqttSubscription::stateChanged, this, &SubscriptionWindow::updateStatus);
    connect(m_sub, &QMqttSubscription::qosChanged, [this](quint8 qos) {
        ui->labelQoS->setText(QString::number(qos));
    });
    connect(ui->pushButton, &QAbstractButton::clicked, m_sub, &QMqttSubscription::unsubscribe);
}

QoS 级别可分别为消息和订阅设置。为订阅设置的 QoS 级别决定了最低 QoS 级别。如果发送的消息具有更高的 QoS 级别，代理就会将该消息的 QoS 提高到更高的级别。例如，如果客户 A 以 QoS 1 订阅了主题，而客户 B 以 QoS 0 发布了主题上的消息，那么代理将自动把消息的 QoS 提高到 1。

接收信息

客户端接收信息时，会使用QMqttMessage 类来存储实际的信息有效载荷：

void SubscriptionWindow::updateMessage(const QMqttMessage &msg)
{
    ui->listWidget->addItem(msg.payload());
}

文件：

© 2025 The Qt Company Ltd. Documentation contributions included herein are the copyrights of their respective owners. The documentation provided herein is licensed under the terms of the GNU Free Documentation License version 1.3 as published by the Free Software Foundation. Qt and respective logos are trademarks of The Qt Company Ltd. in Finland and/or other countries worldwide. All other trademarks are property of their respective owners.