Question

I have an ActiveX dll that loads a child dll (NPAPI plugin). My child plugin creates multiple threads and they need to communicate back to the web page. In my ActiveX (parent dll) I have connection points to fire an event for javascript to provide updates. The problem I am having is that I do not know how the child can call the parent DLL.

What I have tried:
1: Passing a function pointer to the child dll of a function that is not within the class and used a global variable that is a pointer to the ActiveX class (this), but the connection point fails when Invoke with E_UNEXPECTED

2: Passing the child dll a pointer to my class function, and get the same results.

Im still learning C++ and COM at the same time, sorry for my ignorance. Thank you.

Answer 1

COM has ways to set things up so that you can call COM methods from other threads, but the easiest way for you to do this would be to make all calls back to the activex control on the main thread. One way to do this is to create a Message Window and post a windows message to it with the LPARAM being a pointer to a data structure containing the parameters needed to make the call; you create the structure and fill it out, then post the message and free the memory after your WINPROC has made the call.

FireBreath uses this approach and it works quite well. Speaking of which, have you considered implementing your NPAPI plugin as a FireBreath plugin so that it works in both activex and npapi?

Here are some code samples that may help:

• WinMessageWindow (.cpp, .h) – this is a class FireBreath uses for creating message windows; pay particular attention to the DefaultWinProc which is used as part of an async (often cross thread) function call
• NpapiBrowserHostAsyncWin (.cpp, .h) – this is a class used on some browsers for the npapi plugin to work around NPN_PluginThreadAsyncCall not working. It utilizes WinMessageWindow to make cross thread calls in _scheduleAsyncCall

Hope that helps