winrt/c++: await result from dispatched task

Question

I want to dispatch a job to the UI thread, then wait for the result and use it from another thread. Like this, but co_await does not work inside the lambda:

dispatcher.RunAsync(CoreDispatcherPriority::Normal, [&]() {
    auto res = co_await mStoreContext.RequestPurchaseAsync(L"");
});

Or even waiting for the whole RunAsync operation, if I could get my result out from it

The more idiomatic way would be co_await resume_foreground(dispatcher); auto res = co_await mStoreContext.RequestPurchaseAsync(L""); co_await resume_background(); use_result(res); — Raymond Chen
– Raymond Chen, Commented Jun 1, 2019 at 14:35

David Haim · Accepted Answer · 2018-09-20 15:54:57Z

1

That's because void can not be used as a return value from a coroutine (I can be if you use my my library).

try returning a std::future<void> instead:

dispatcher.RunAsync(CoreDispatcherPriority::Normal, [&]() -> std::future<void> {
    auto res = co_await mStoreContext.RequestPurchaseAsync(L"");
});

edited Sep 20, 2018 at 15:54

answered Sep 20, 2018 at 14:24

David Haim

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2 Comments

Rolle Over a year ago

Thanks. even better would be a way to get the result out of the complete runasync block, but this works fine

Raymond Chen Over a year ago

This is dangerous because of the reference capture. The caller cannot return until it is sure the lambda has finished running.

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