server_valuecallback.cpp

#include <iostream>
 
#include <open62541pp/node.hpp>
#include <open62541pp/server.hpp>
 
int main() {
    opcua::Server server;
 
    const opcua::NodeId currentTimeId(1, "CurrentTime");
    auto currentTimeNode =
        opcua::Node(server, opcua::ObjectId::ObjectsFolder)
            .addVariable(currentTimeId, "CurrentTime")
            .writeDisplayName({"en-US", "Current time"})
            .writeDescription({"en-US", "Current time"})
            .writeDataType<opcua::DateTime>()
            .writeValueScalar(opcua::DateTime::now());
 
    // set variable value callback to write current time before every read operation
    opcua::ValueCallback valueCallback;
    valueCallback.onBeforeRead = [&](const opcua::DataValue& value) {
        const auto timeOld = value.getValue().getScalar<opcua::DateTime>();
        const auto timeNow = opcua::DateTime::now();
        std::cout << "Time before read: " << timeOld.format("%Y-%m-%d %H:%M:%S") << std::endl;
        std::cout << "Set current time: " << timeNow.format("%Y-%m-%d %H:%M:%S") << std::endl;
        currentTimeNode.writeValueScalar(timeNow);
    };
    server.setVariableNodeValueCallback(currentTimeId, valueCallback);
 
    server.run();
}