How do we add an event in C++/WinRt project?

Question

I am currently work on a C++/WinRt which connects(Sender) to one of the available devices(Client) around a specific region using WifiDirect. When the device wants to connect, it sends a connection request to the sender. The sender needs to detect the connection request sent by the client and connect to the client. For this I need to add an event - (On Connection requested). As soon as I add it should execute the code of OnConnectionRequested.

#include "pch.h"
#include <winrt/Windows.Foundation.Collections.h>
#include "winrt/Windows.Devices.WiFiDirect.h"
#pragma once
using namespace winrt;
using namespace Windows::Foundation;
using namespace Windows::Foundation::Collections;
using namespace Windows::Devices::WiFiDirect;
using namespace Windows::Storage::Streams;
using namespace winrt::Windows::Devices::WiFiDirect;
using namespace winrt::Windows::Devices::Enumeration;

enum class NotifyType
{
    StatusMessage,
    ErrorMessage
};

enum class CallbackContext
{
    Any,
    Same
};

class st
{
public:
    void OnConnectionRequested(WiFiDirectConnectionListener sender,
                               WiFiDirectConnectionRequestedEventArgsconnection EventArgs)
    {
        WiFiDirectConnectionRequest connectionRequest = connectionEventArgs.GetConnectionRequest();
        printf("Connection request received from ", connectionRequest.DeviceInformation().Name(), "Connection Request");
        printf("Connecting to ", connectionRequest.DeviceInformation().Name(), NotifyType::StatusMessage);
    }
    
    void start()
    {
        Windows::Devices::WiFiDirect::WiFiDirectAdvertisementPublisher _publisher;
        Windows::Devices::WiFiDirect::WiFiDirectConnectionListener _listener;
        winrt::event_token _connectionRequestedToken;
    
        try
        {
            _connectionRequestedToken = _listener.ConnectionRequested({this, &st::OnConnectionRequested});
            _publisher.Start();
            printf("Advertisement started, waiting for StatusChangedcallback...", NotifyType::StatusMessage);
        }
        catch (...)
        {
            printf("Error starting Advertisement: ", NotifyType::ErrorMessage);
        }

        getchar();
    }
};

int main()
{
    st s;
    s.start();
}

Is this the right way to add an event in C++/WinRt :

_connectionRequestedToken = _listener.ConnectionRequested({this, &st::OnConnectionRequested});

The errors are :

LNK1120 1 unresolved externals - error in Winrt.exe file

LNK2019 unresolved external symbol "public: struct winrt::hstring __thiscall winrt::impl::consume_Windows_Devices_Enumeration_IDeviceInformation::Name(void)const " (?Name@?$consume_Windows_Devices_Enumeration_IDeviceInformation@UIDeviceInformation@Enumeration@Devices@Windows@winrt@@@impl@winrt@@QBE?AUhstring@3@XZ) referenced in function "public: void __thiscall st::OnConnectionRequested(struct winrt::Windows::Devices::WiFiDirect::WiFiDirectConnectionListener,struct winrt::Windows::Devices::WiFiDirect::WiFiDirectConnectionRequestedEventArgs)" (?OnConnectionRequested@st@@QAEXUWiFiDirectConnectionListener@WiFiDirect@Devices@Windows@winrt@@UWiFiDirectConnectionRequestedEventArgs@3456@@Z) - error in Program.obj 1

What changes should I make at this line to clear the error? What does the error actually mean ? Or is there any other way to add an event in C++/WinRt project?

Answer 1

You are missing an #include directive:

#include <winrt/Windows.Devices.Enumeration.h>

See Why is the linker giving me a "LNK2019: Unresolved external symbol" error?:

If the unresolved symbol is an API from the Windows namespace headers for the C++/WinRT projection (in the winrt namespace), then the API is forward-declared in a header that you've included, but its definition is in a header that you haven't yet included. Include the header named for the API's namespace, and rebuild. For more info, see C++/WinRT projection headers.

For general instructions on how to handle events see Handle events by using delegates in C++/WinRT.

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Capturing this (raw pointer) into the delegate might be dangerous, too. This breaks the link between object lifetime and visibility, and puts the burden of lifetime management on you. See Safely accessing the this pointer with an event-handling delegate for safer, more manageable alternatives.