实现执行异步操作的自定义Windows Workflow活动

I'm having some conceptual trouble on figuring out how to best implement this... I want to create a custom Activity class for Windows Workflow. The activity has to call out to a third party library method that itself runs another process asynchronously and may take anywhere from seconds to many hours to complete. This library offers the ability for me to either poll for the method result or to subscribe to an event that indicates its completion. In my non-workflow apps, I typically just subscribe to that event, but that doesn't seem to be reasonable in the workflow case. I'm also not sure exactly how to best implement a polling scheme. Can someone recommend some pointers to similar problems?I'm having some conceptual trouble on figuring