I have a WinJS app that uses WinRT component written in C++/CX. Component spawns a background thread that encapsulates work with a huge legacy thread-unsafe C++ library, which requires all calls to be made from the same thread.
I need to implement a producer/consumer pattern, where the background thread in the component is a producer, and UI thread in JavaScript app is a consumer.
The call part is easy, as JavaScript can call component methods (in UI thread), and C++ code will post a job into a queue for background thread.
The question is the callback: I need to post the data calculated by the C++ background thread into UI thread. I certainly can return an IAsyncOperation back to JavaScript, but I don't want UI thread to be blocked while this operation is waiting for an event from a background thread.
What are my options?
Solved. Look at the section: "To add an asynchronous method that fires events..." of https://learn.microsoft.com/en-us/windows/uwp/winrt-components/walkthrough-creating-a-basic-windows-runtime-component-in-cpp-and-calling-it-from-javascript-or-csharp.
The idea is that component class declares an event handler (delegate), which can be set in JavaScript. C++ background thread can fire an event in the context of UI thread, so JavaScript event handler is called right way.
Of course, this can easily be wrapped into WinJS.Promise, so JavaScript application code won't know that there are components, events etc.