{"id":107742,"date":"2023-01-23T07:00:00","date_gmt":"2023-01-23T15:00:00","guid":{"rendered":"https:\/\/devblogs.microsoft.com\/oldnewthing\/?p=107742"},"modified":"2023-01-22T17:27:00","modified_gmt":"2023-01-23T01:27:00","slug":"20230123-00","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/oldnewthing\/20230123-00\/?p=107742","title":{"rendered":"Inside C++\/WinRT: Coroutine completions: The oversimplified version"},"content":{"rendered":"

C++\/WinRT coroutines use the Completed<\/code> delegate property to be notified when an asynchronous operation is complete. There are multiple parts of the completion handler. Today we’ll look at an oversimplified version, and then we will gradually build it up.<\/p>\n

template <typename Async>\r\nstruct await_adapter\r\n{\r\n    await_adapter(Async const& async) : async(async) { }\r\n\r\n    Async const& async;\r\n\r\n    bool await_ready() const noexcept\r\n    {\r\n        return false;\r\n    }\r\n\r\n    void await_suspend(std::experimental::coroutine_handle<> handle) const\r\n    {\r\n        async.Completed([handle](auto&& ...)\r\n        {\r\n            handle.resume();\r\n        });\r\n    }\r\n\r\n    auto await_resume() const\r\n    {\r\n        return async.GetResults();\r\n    }\r\n};\r\n<\/pre>\n
To co_await<\/code> an IAsyncAction<\/code> or IAsyncOperation<\/code>, we register a completion callback that resumes the awaiting coroutine. When the coroutine resumes, it will call await_resume()<\/code> to obtain the result of the co_await<\/code>, and we propagate the result of the GetResults()<\/code> method.<\/p>\n
We don’t particularly care about the parameters passed to the completion delegate: Those tell us whether the asynchronous work completed successfully, was cancelled, or failed outright, but we don’t need to remember that information because GetResults()<\/code> will report the information again: If the asynchronous work did not complete successfully, then GetResults()<\/code> will thrown an exception describing why it was not successful.<\/p>\n
As things go, this is a fairly standard implementation of a coroutine awaiter, although there is a race condition we need to fix:<\/p>\n
    void await_suspend(std::experimental::coroutine_handle<> handle) const\r\n    {\r\n        auto extend_lifetime = async;<\/span>\r\n        async.Completed([handle](auto&& ...)\r\n        {\r\n            handle.resume();\r\n        });\r\n    }\r\n<\/pre>\n
If the completion handler runs before Completed()<\/code> returns, then we end up destroying the async<\/code> while there is still an active call on it. This is a problem I called out  at the start of my coroutine series<\/a>. To fix this, we make a local copy of the async<\/code> to extend its lifetime to the end of the await_suspend<\/code> function.<\/p>\n
This is just the starting point for C++\/WinRT coroutine completion handlers. Next time, we’ll add apartment-preserving behavior.<\/p>\n","protected":false},"excerpt":{"rendered":"
Resuming the coroutine when the asynchronous work completes.<\/p>\n","protected":false},"author":1069,"featured_media":111744,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[1],"tags":[25],"class_list":["post-107742","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-oldnewthing","tag-code"],"acf":[],"blog_post_summary":"
Resuming the coroutine when the asynchronous work completes.<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/107742","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/users\/1069"}],"replies":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/comments?post=107742"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/107742\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/media\/111744"}],"wp:attachment":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/media?parent=107742"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/categories?post=107742"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/tags?post=107742"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}