Add KDMqtt client example

Adds a very basic MQTT client example.
Examlple shows how to use and integrate KDMqtt
in you KDFoundation CoreApplication.

Example connects to broker at test.mosquitto.org,
subscribes to mytopic as soon as connected to the
broker and publishes a message for mytopic as soon
as subscribing to the topic is done. The received
message is then printed using spdlog::info.

This example also outlines the steps neccessary
when deploying an application using KDMqtt on
windos (copying DLLs next to the binary).

examples/CMakeLists.txt

@@ -8,3 +8,4 @@
 #
 
 add_subdirectory(gui_window)
+add_subdirectory(mqtt_client)

examples/mqtt_client/CMakeLists.txt

@@ -0,0 +1,34 @@
+# This file is part of KDUtils.
+#
+# SPDX-FileCopyrightText: 2024 Klarälvdalens Datakonsult AB, a KDAB Group company <info@kdab.com>
+# Author: Marco Thaller <marco.thaller@kdab.com>
+#
+# SPDX-License-Identifier: MIT
+#
+# Contact KDAB at <info@kdab.com> for commercial licensing options.
+#
+
+project(mqtt_client_example LANGUAGES CXX)
+
+add_executable(
+    ${PROJECT_NAME}
+    mqtt_client.cpp
+)
+
+target_link_libraries(
+    ${PROJECT_NAME}
+    KDMqtt
+)
+
+if (WIN32)
+    # Deployment: On Windows, copy all DLLs from the mosquitto install directory next to the application binary so that they're found.
+    if (BUILD_INTEGRATION_MQTT AND MOSQUITTO_RUNTIME_DLLS)
+        foreach(MOSQUITTO_RUNTIME_DLL ${MOSQUITTO_RUNTIME_DLLS})
+            add_custom_command(TARGET ${PROJECT_NAME} POST_BUILD
+                COMMAND ${CMAKE_COMMAND} -E copy_if_different
+                "${MOSQUITTO_RUNTIME_DLL}"
+                $<TARGET_FILE_DIR:${PROJECT_NAME}>
+            )
+        endforeach()
+    endif()
+endif()

examples/mqtt_client/mqtt_client.cpp

@@ -0,0 +1,54 @@
+/*
+  This file is part of KDUtils.
+
+  SPDX-FileCopyrightText: 2024 Klarälvdalens Datakonsult AB, a KDAB Group company <info@kdab.com>
+  Author: Marco Thaller <marco.thaller@kdab.com>
+
+  SPDX-License-Identifier: MIT
+
+  Contact KDAB at <info@kdab.com> for commercial licensing options.
+*/
+#include <KDFoundation/core_application.h>
+#include <KDMqtt/mqtt.h>
+
+using namespace KDFoundation;
+using namespace KDMqtt;
+
+int main()
+{
+    const Url url("test.mosquitto.org");
+    const std::string topic = "mytopic";
+    const std::string payload = "Hello World!";
+
+    CoreApplication app;
+
+    MqttLib::instance().init();
+    MqttClient mqttClient("KDMqttClient", true, true);
+
+    auto onMqttConnectionStateChanged = [&](const MqttClient::ConnectionState &connectionState) {
+        if (connectionState == MqttClient::ConnectionState::CONNECTED) {
+            mqttClient.subscribe(topic.c_str());
+        }
+    };
+    std::ignore = mqttClient.connectionState.valueChanged().connect(onMqttConnectionStateChanged);
+
+    auto onMqttSubscriptionStateChanged = [&](const MqttClient::SubscriptionState &subscriptionState) {
+        if (subscriptionState == MqttClient::SubscriptionState::SUBSCRIBED) {
+            mqttClient.publish(nullptr, topic.c_str(), payload.length(), payload.c_str());
+        }
+    };
+    std::ignore = mqttClient.subscriptionState.valueChanged().connect(onMqttSubscriptionStateChanged);
+
+    auto onMqttMessageReceived = [&](const mosquitto_message *message) {
+        const auto timestamp = std::time(nullptr);
+        const auto timestring = std::string(std::asctime(std::localtime(&timestamp)));
+        const auto topic = std::string(message->topic);
+        const auto payload = std::string(static_cast<char *>(message->payload));
+        spdlog::info("Received MQTT message. Topic: {}. Payload: {}", topic, payload);
+    };
+    std::ignore = mqttClient.msgReceived.connect(onMqttMessageReceived);
+
+    mqttClient.connect(url);
+
+    app.exec();
+}