Previously, we looked at the case where calling a method initiates some asynchronous activity, and if new activity is required, the work is handed off to the existing coroutine. A different model is to have everybody do the work in parallel, but only the last one counts. Of course, this pattern assumes that the work can safely be performed in parallel, such as perform a complex calculation.<\/p>\n
Let’s assume that the object has thread affinity, so we can assume that all accesses on the UI thread are uncontended and therefore do not require a lock.<\/p>\n
std::mutex m_mutex;\r\nwinrt::hstring m_lang;\r\nint32_t m_messageId;\r\nwinrt::hstring m_message;\r\n\r\nwinrt::IAsyncAction Widget::SetMessageAsync(int32_t messageId)\r\n{\r\n auto lifetime = get_strong();\r\n {\r\n std::lock_guard guard{ m_mutex };\r\n m_messageId = messageId;\r\n }\r\n co_await RecalcAsync();\r\n}\r\n\r\nwinrt::IAsyncAction Widget::SetLanguageAsync(winrt::hstring lang)\r\n{\r\n auto lifetime = get_strong();\r\n {\r\n std::lock_guard guard{ m_mutex };\r\n m_lang = lang;\r\n }\r\n co_await RecalcAsync();\r\n}\r\n\r\nwinrt::IAsyncAction Widget::RecalcAsync()\r\n{\r\n auto lifetime = get_strong();\r\n\r\n winrt::hstring messageId;\r\n winrt::hstring lang;\r\n {\r\n std::lock_guard guard{ m_mutex };\r\n messageId = m_messageId;\r\n lang = m_lang;\r\n }\r\n\r\n auto resolved = co_await ResolveLanguageAsync(lang);\r\n auto library = co_await GetResourceLibraryAsync(resolved);\r\n auto message = co_await library.LookupResourceAsync(messageId);\r\n\r\n std::lock_guard guard{ m_mutex };\r\n if (m_messageId == messageId && m_lang == lang) {\r\n m_message = message;\r\n }\r\n}\r\n<\/pre>\nThe pattern here is that on each call to Set\u00adMessage\u00adId()<\/code> or Set\u00adLanguage()<\/code>, we update our local state variables and then call a common helper coroutine to recalculate the m_message<\/code>.<\/p>\nThe pattern in Recalc\u00adAsync<\/code> goes like this:<\/p>\n\n- Capture the member variables you need into local variables.<\/li>\n
- Do the asynchronous work, operating purely on the local variables.<\/li>\n
- When finished, compare the member variables against the local variables to see if they still match.<\/li>\n
- If so, then update the results.<\/li>\n
- If not, then somebody else changed the
m_messageId<\/code> or m_lang<\/code> in the meantime, so abandon the update.<\/li>\n<\/ul>\nIn the case where the update is abandoned, we don’t have to restart the calculation because whoever changed the message ID or the language is running their own Recalc\u00adAsync<\/code>. In fact, everyone<\/i> who changes the message ID or the language is running their own Recalc\u00adAsync<\/code>, and only the one whose calculations match the current state gets to update the result. Note that this may not be the calculation that finishes last.<\/p>\nThe “capture and compare” pattern assumes that the calculations are idempotent. If not, then you can use a counter to keep track of who is running the “real” computation.<\/p>\n
uint32_t m_counter = 0;<\/span>\r\n\r\nwinrt::IAsyncAction Widget::RecalcAsync()\r\n{\r\n auto lifetime = get_strong();\r\n\r\n uint32_t counter;<\/span>\r\n winrt::hstring messageId;\r\n winrt::hstring lang;\r\n {\r\n std::lock_guard guard{ m_mutex };\r\n counter = ++m_counter;<\/span>\r\n messageId = m_messageId;\r\n lang = m_lang;\r\n }\r\n\r\n auto resolved = co_await ResolveLanguageAsync(lang);\r\n auto library = co_await GetResourceLibraryAsync(resolved);\r\n auto message = co_await library.LookupResourceAsync(messageId);\r\n\r\n std::lock_guard guard{ m_mutex };\r\n if (m_counter == counter<\/span>) {\r\n m_message = message;\r\n }\r\n}\r\n<\/pre>\nWe use a counter to keep track of which instance of the recalculation we are managing, and when we finish our calculations, we check if the counter has changed since we started. If not, then we are the active recalculation and can update the result. If the counter doesn’t match, then somebody else triggered a recalculation while we were recalculating, and we’ll let that other recalculation set the result.<\/p>\n
\n\n\nCaller 1
\nco_await RecalcAsync();<\/code>
\n\u00a0\u00a0counter = m_counter = 1;<\/code>
\n\u00a0\u00a0co_await ResolveLanguageAsync(...);<\/code><\/td>\n <\/td>\n <\/td>\n <\/td>\n <\/td>\n<\/tr>\n \n\u00a0<\/td>\n <\/td>\n Caller 2
\nco_await RecalcAsync();<\/code>
\n\u00a0\u00a0counter = m_counter = 2;<\/code>
\n\u00a0\u00a0co_await ResolveLanguageAsync(...);<\/code><\/td>\n <\/td>\n <\/td>\n<\/tr>\n \n\u00a0\u00a0co_await GetResourceLibraryAsync(...);<\/code><\/td>\n <\/td>\n \u00a0<\/td>\n <\/td>\n <\/td>\n<\/tr>\n \n\u00a0<\/td>\n <\/td>\n \u00a0<\/td>\n <\/td>\n Caller 3
\nco_await RecalcAsync();<\/code>
\n\u00a0\u00a0counter = m_counter = 3;<\/code>
\n\u00a0\u00a0co_await ResolveLanguageAsync(...);<\/code><\/td>\n<\/tr>\n\n\u00a0<\/td>\n <\/td>\n \u00a0<\/td>\n <\/td>\n \u00a0<\/td>\n<\/tr>\n \n\u00a0<\/td>\n <\/td>\n \u00a0\u00a0co_await GetResourceLibraryAsync(...);<\/code><\/td>\n <\/td>\n \u00a0<\/td>\n<\/tr>\n \n\u00a0<\/td>\n <\/td>\n \u00a0<\/td>\n <\/td>\n \u00a0\u00a0co_await GetResourceLibraryAsync(...);<\/code><\/td>\n<\/tr>\n\n\u00a0<\/td>\n <\/td>\n \u00a0\u00a0co_await LookupResourceAsync(...);<\/code><\/td>\n <\/td>\n \u00a0<\/td>\n<\/tr>\n \n\u00a0\u00a0co_await LookupResourceAsync(...);<\/code><\/td>\n <\/td>\n \u00a0<\/td>\n <\/td>\n \u00a0<\/td>\n<\/tr>\n \n\u00a0<\/td>\n <\/td>\n \u00a0<\/td>\n <\/td>\n \u00a0\u00a0co_await LookupResourceAsync(...);<\/code><\/td>\n<\/tr>\n\n\u00a0<\/td>\n <\/td>\n \u00a0\u00a0m_counter is 3<\/code>
\n\u00a0\u00a0counter is 2<\/code>
\n\u00a0\u00a0do not update m_message<\/code>
\n\u00a0\u00a0co_return;<\/code><\/td>\n <\/td>\n \u00a0<\/td>\n<\/tr>\n \n\u00a0<\/td>\n <\/td>\n <\/td>\n <\/td>\n \u00a0\u00a0m_counter is 3<\/code>
\n\u00a0\u00a0counter is 3<\/code>
\n\u00a0\u00a0update m_message<\/code>
\n\u00a0\u00a0co_return;<\/code><\/td>\n<\/tr>\n\n\u00a0\u00a0m_counter is 3<\/code>
\n\u00a0\u00a0counter is 1<\/code>
\n\u00a0\u00a0do not update m_message<\/code>
\n\u00a0\u00a0co_return;<\/code><\/td>\n <\/td>\n <\/td>\n <\/td>\n <\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n