{"id":109953,"date":"2024-07-03T07:00:00","date_gmt":"2024-07-03T14:00:00","guid":{"rendered":"https:\/\/devblogs.microsoft.com\/oldnewthing\/?p=109953"},"modified":"2024-06-24T09:10:03","modified_gmt":"2024-06-24T16:10:03","slug":"20240703-00","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/oldnewthing\/20240703-00\/?p=109953","title":{"rendered":"How do I produce a Windows Runtime asynchronous activity from C++\/CX?"},"content":{"rendered":"

You might be working in a code base written in C++\/CX.<\/p>\n

First, I’m sorry.<\/p>\n

Second, maybe you need to produce an IAsyncAction^<\/code> or one of its relatives. How do you do that?<\/p>\n

You use the Concurrency::create_async<\/code> method<\/a>.<\/p>\n

The Concurrency::create_async<\/code> method studies its parameter and infers what kind of Windows Runtime asynchronous activity to produce based on the signature of the lambda.<\/p>\n

Given a lambda whose function call operator has the signature R(Params...)<\/code>, the create_async<\/code> function returns the following Windows Runtime interface:<\/p>\n\n\n\n\n\n\n
\u00a0<\/th>\nParams…<\/th>\n<\/tr>\n
()<\/code>
\n(cancellation_token)<\/code><\/td>\n		(progress_reporter<P>)<\/code>
\n(progress_reporter<P>, cancellation_token)<\/code><\/td>\n<\/tr>\n
R<\/code> = void<\/code>
\nR<\/code> = task<void><\/code><\/td>\n		IAsyncAction^<\/code><\/td>\nIAsyncActionWithProgress<P>^<\/code><\/td>\n<\/tr>\n
R<\/code> = T<\/code>
\nR<\/code> = task<T><\/code><\/td>\n		IAsyncOperation<T>^<\/code><\/td>\nIAsyncOperationWithProgress<T, P>^<\/code><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

Inside the lambda, you can use the progress_reporter<P><\/code> to produce progress reports, and you can use the optional cancellation_token<\/code> to detect whether the asynchronous activity has been canceled.<\/p>\n

Here’s the simplest case: An IAsyncAction^<\/code> with no cancellation.<\/p>\n

task<Widget^> GetWidgetAsync(String^);\r\ntask<void> EnableWidgetAsync(Widget^, bool);\r\n\r\n\/\/ Old school: Task chain\r\nIAsyncAction^ EnableWidgetByIdAsync(String^ id, bool enable)\r\n{\r\n    return create_async([=]()\r\n        -> task<void> {\r\n        return GetWidgetAsync(id).then(\r\n        [=](Widget^ widget) {\r\n            return EnableWidgetAsync(widget, enable);\r\n        });\r\n    });\r\n}\r\n\r\n\/\/ New hotness: co_await\r\nIAsyncAction^ EnableWidgetByIdAsync(String^ id, bool enable)\r\n{\r\n    return create_async([=]()\r\n        -> task<void> {\r\n        Widget^ widget = co_await GetWidgetAsync(id);\r\n        co_await EnableWidgetAsync(widget, enable);\r\n    });\r\n}\r\n<\/pre>\n
With cancellation but no progress:<\/p>\n
void ThrowIfCanceled(cancellation_token const& cancel)\r\n{\r\n    if (cancel.is_canceled()) cancel_current_task();\r\n}\r\n\r\n\/\/ Old school: Task chain\r\nIAsyncAction^ EnableWidgetByIdAsync(String^ id, bool enable)\r\n{\r\n    return create_async([=](cancellation_token cancel<\/span>)\r\n        -> task<void> {\r\n        return GetWidgetAsync(id).then(\r\n        [=](Widget^ widget) { \/\/ explicitly: [enable, cancel<\/span>]\r\n            ThrowIfCanceled(cancel);<\/span>\r\n            return EnableWidgetAsync(widget, enable);\r\n        });\r\n    });\r\n}\r\n\r\n\/\/ New hotness: co_await\r\nIAsyncAction^ EnableWidgetByIdAsync(String^ id, bool enable)\r\n{\r\n    return create_async([=](cancellation_token cancel<\/span>)\r\n        -> task<void> {\r\n        Widget^ widget = co_await GetWidgetAsync(id);\r\n        ThrowIfCanceled(cancel);<\/span>\r\n        co_await EnableWidgetAsync(widget, enable);\r\n    });\r\n}\r\n<\/pre>\n
With progress but no cancellation:<\/p>\n
\/\/ Old school: Task chain\r\nIAsyncActionWithProgress<int>^\r\n    EnableWidgetByIdAsync(String^ id, bool enable)\r\n{\r\n    return create_async([=](progress_reporter<int> progress<\/span>)\r\n        -> task<void> {\r\n        progress.report(0);<\/span>\r\n        return GetWidgetAsync(id).then(\r\n        [=](Widget^ widget) { \/\/ explicitly: [enable, progress<\/span>]\r\n            progress.report(1);<\/span>\r\n            return EnableWidgetAsync(widget, enable);\r\n        });\r\n    });\r\n}\r\n\r\n\/\/ New hotness: co_await\r\nIAsyncAction^ EnableWidgetByIdAsync(String^ id, bool enable)\r\n{\r\n    return create_async([=](progress_reporter<int> progress<\/span>))\r\n        -> task<void> {\r\n        progress.report(0);<\/span>\r\n        Widget^ widget = co_await GetWidgetAsync(id);\r\n        progress.report(1);<\/span>\r\n        co_await EnableWidgetAsync(widget, enable);\r\n    });\r\n}\r\n<\/pre>\n
And with both progress and cancellation:<\/p>\n
\/\/ Old school: Task chain\r\nIAsyncActionWithProgress<int>^\r\n    EnableWidgetByIdAsync(String^ id, bool enable)\r\n{\r\n    return create_async([=](progress_reporter<int> pr<\/span>ogress,<\/span>\r\n                           cancellation_token cancel<\/span>)\r\n        -> task<void> {\r\n        progress.report(0);<\/span>\r\n        return GetWidgetAsync(id).then(\r\n        [=](Widget^ widget) { \/\/ explicitly: [enable, progress, cancel<\/span>]\r\n            ThrowIfCanceled(cancel);<\/span>\r\n            progress.report(1);     <\/span>\r\n            return EnableWidgetAsync(widget, enable);\r\n        });\r\n    });\r\n}\r\n\r\n\/\/ New hotness: co_await\r\nIAsyncAction^ EnableWidgetByIdAsync(String^ id, bool enable)\r\n{\r\n    return create_async([=](progress_reporter<int> pr<\/span>ogress,<\/span>\r\n                           cancellation_token cancel<\/span>)\r\n        -> task<void> {\r\n        progress.report(0);<\/span>\r\n        Widget^ widget = co_await GetWidgetAsync(id);\r\n        ThrowIfCanceled(cancel);<\/span>\r\n        progress.report(1);     <\/span>\r\n        co_await EnableWidgetAsync(widget, enable);\r\n    });\r\n}\r\n<\/pre>\n
We can generalize into a single pattern:<\/p>\n
IAsyncSomething^\r\n    DoSomethingAsync(Arg1 arg1, Arg2, arg2, ...)\r\n{\r\n    return create_async([=](\r\n        progress_reporter<P> progress[optional]<\/sub>,\r\n        cancellation_token cancel[optional]<\/sub>)\r\n        -> task<P> {\r\n        \u27e6 async stuff which may include...\r\n            progress.report(value);[optional]<\/sub>\r\n            ThrowIfCanceled(cancel);[optional]<\/sub>\r\n        \u27e7\r\n    });\r\n}\r\n<\/pre>\n
You can also register a callback function on the cancellation_token<\/code> that will be invoked when the activity is canceled. I leave using the cancellation callback as an exercise.<\/p>\n
There are also variants for IAsync\u00adOperation<\/code>. I’ll show just one of them and let you figure out the others:<\/p>\n
\/\/ Old school: Task chain\r\nIAsyncOperationWithProgress<bool, int>^\r\n    EnableWidgetByIdAsync(String^ id, bool enable)\r\n{\r\n    return create_async([=](progress_reporter<int> progress,\r\n                           cancellation_token cancel)\r\n        -> task<bool<\/span>> {\r\n        progress.report(0);\r\n        return GetWidgetAsync(id).then(\r\n        [=](Widget^ widget) { \/\/ explicitly: [enable, progress, cancel]\r\n            ThrowIfCanceled(cancel);\r\n            progress.report(1);\r\n            if (!widget) {                                         <\/span>\r\n                return task_from_result(false); \/\/ widget not found<\/span>\r\n            }                                                      <\/span>\r\n            return EnableWidgetAsync(widget, enable).then(         <\/span>\r\n                []() { return true; });                            <\/span>\r\n        });\r\n    });\r\n}\r\n\r\n\/\/ New hotness: co_await\r\nIAsyncAction^ EnableWidgetByIdAsync(String^ id, bool enable)\r\n{\r\n    return create_async([=](progress_reporter<int> progress,\r\n                           cancellation_token cancel)\r\n        -> task<bool<\/span>> {\r\n        progress.report(0);\r\n        Widget^ widget = co_await GetWidgetAsync(id);\r\n        ThrowIfCanceled(cancel);\r\n        progress.report(1);\r\n        if (!widget) {                             <\/span>\r\n            co_return false;                       <\/span>\r\n        }                                          <\/span>\r\n        co_await EnableWidgetAsync(widget, enable);<\/span>\r\n        co_return true;                            <\/span>\r\n    });\r\n}\r\n<\/pre>\n","protected":false},"excerpt":{"rendered":"
The Parallel Patterns Library has special support for C++\/CX.<\/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-109953","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-oldnewthing","tag-code"],"acf":[],"blog_post_summary":"
The Parallel Patterns Library has special support for C++\/CX.<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/109953","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=109953"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/109953\/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=109953"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/categories?post=109953"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/tags?post=109953"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}