as the title says, I have a Core singleton that inherits from QObject that is constructed on the main thread. I also Have a QTcpServer that is creating a new QThread for each new connection, that is where I serve Http Requests. sometimes I need to invoke a signal called newMessage(...) that belongs to the Core singleton from an Http request handler thread, the purpose is to notify a QWebSocket server living on the main thread to propagate the event to the connected websockets.

so it's like

HttpResponse HttpController::callback(HttpRequest *request){
//a callback of an object living inside a handler thread
//some code
emit Core::instance()->newMessage(...) //this object is living on the main thread
//do some work
//return HttpResponse(...)
}