Creating a non-agile delegate in C++/WinRT, part 2: The synchronous coroutine

Last time, we saw that you could use an ICallback­Context to run code synchronously in another apartment from your delegate, which is important if the code that is calling your delegate is relying on the timing of your return.

We can also express this in the form of a coroutine that operates synchronously.

If we make the await_suspend invoke the handle synchronously, then the continuation of the coroutine runs synchronously with the code that called co_await.

auto resume_synchronous(ICallbackContext* context)
{
  struct awaiter : std::experimental::suspend_always
  {
    ICallbackContext* context;
    bool await_suspend(
        std::experimental::coroutine_handle<> handle)
    {
      InvokeInContext(context, handle);
      return true;
    }
  };
  return awaiter{ context };
}

This simplifies the delegate by letting you use co_await to do the dirty work.

deviceWatcher.Added(
    [=, context = CaptureCurrentApartmentContext()]
    (auto&& sender, auto&& info) -> winrt::fire_and_forget
    {
        co_await resume_synchronous(context.Get());
        viewModel.Append(winrt::make<DeviceItem>(info));
    });

Even though there is a co_await, execution continues synchronously because await_suspend runs the continuous synchronously.

Whether co_await resumes synchronously or not¹ is determined by the awaiter. If you co_await something whose awaiter resumes asynchronously, then the co_await will resume asynchronously.

deviceWatcher.Added(
    [=, context = CaptureCurrentApartmentContext()]
    (auto&& sender, auto&& info) -> winrt::fire_and_forget
    {
        auto original_context = CaptureCurrentApartmentContext();
        co_await resume_synchronous(context.Get());
        viewModel.Append(make<DeviceItem>(info));
        co_await resume_synchronous(original_context.Get());
        more_stuff();
        auto result = co_await GetMoreDataAsync();
        process_result(result);
    });

In the above example, the first two co_awaits are synchronous, but the third one (co_await GetMoreDataAsync()) is presumably asynchronous. This means that the delegate will return at the point of the third co_await, and reference parameters (sender and info) are probably not going to be valid when the coroutine resumes.

¹ Or at all. The built-in awaiter suspend_always suspends and never wakes up.