Qt Quick 订阅

使用Qt Quick Controls 创建一个能够订阅MQTT主题的应用程序。

带有主机名、端口、主题字段以及“连接”和“订阅”按钮的 MQTT 订阅界面

Qt Quick “订阅”示例演示了如何将QMqttClient 注册为QML类型，并在Qt Quick 应用程序中使用它。

Qt MQTT 当前版本中未提供 QML API。不过，您可以将该模块的 C++ 类提供给 QML 使用。

创建客户端

连接到QMqttSubscription::messageReceived( ) 以接收发送到消息代理的所有消息：

QmlMqttSubscription::QmlMqttSubscription(QMqttSubscription *s, QmlMqttClient *c)
    : sub(s)
    , client(c)
{
    connect(sub, &QMqttSubscription::messageReceived, this, &QmlMqttSubscription::handleMessage);
}

创建一个以QMqttClient 类为基类的QmlMqttClient 类：

QmlMqttClient::QmlMqttClient(QObject *parent)
    : QObject(parent)
{
    connect(&m_client, &QMqttClient::hostnameChanged, this, &QmlMqttClient::hostnameChanged);
    connect(&m_client, &QMqttClient::portChanged, this, &QmlMqttClient::portChanged);
    connect(&m_client, &QMqttClient::stateChanged, this, &QmlMqttClient::stateChanged);
}

使用subscribe() 函数创建订阅对象：

QmlMqttSubscription* QmlMqttClient::subscribe(const QString &topic)
{
    auto sub = m_client.subscribe(topic, 0);
    auto result = new QmlMqttSubscription(sub, this);
    return result;
}

使用QMqttMessage 对象存储接收到的消息的有效载荷：

void QmlMqttSubscription::handleMessage(const QMqttMessage &qmsg)
{
    emit messageReceived(qmsg.payload());
}

在 QML 中注册类

在main.cpp 文件中，从 subscription 模块加载 QML 类型 Main：

    QGuiApplication app(argc, argv);
    QQmlApplicationEngine engine;

    QObject::connect(
            &engine, &QQmlApplicationEngine::objectCreationFailed, &app,
            []() { QCoreApplication::exit(EXIT_FAILURE); }, Qt::QueuedConnection);

    engine.loadFromModule(u"subscription"_s, u"Main"_s);

现在，在Main.qml 文件中使用MqttClient类型创建一个MQTT客户端：

    MqttClient {
        id: client
        hostname: hostnameField.text
        port: portField.text
    }

文件：

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