\nThis question was inspired by an actual customer question,\nbut I changed a lot of it around to make for a more interesting story.\n(Trust me, the original story was even more boring<\/i>.)\n<\/p>\n
\nA customer’s background thread posted a message to the main UI thread\nto signal something (details not important).\nThey found that the posted message was never received if the user was\nin the process of dragging the main window around at the time the message\nwas posted.\nWhy would dragging a window cause posted messages to be lost?\n“We used to post a thread message, but then we saw that\n\nthread messages are eaten by modal loops<\/i><\/a>,\nso we switched to posting a message, as you recommended.\nBut that didn’t help.”\n<\/p>\n \nDragging a window doesn’t cause messages to be lost.\nThe modal loop\n\ncreated by the window dragging code<\/a>\ncalls\n \n“Oh, we don’t handle the message in the window procedure.\nWe process it here:\n<\/p>\n \nCould that be the problem?”\n<\/p>\n \nYes, that’s the problem.\n<\/p>\n \nThe customer saw the recommendation to use \nIf you read\n\nthe original recommendation<\/a>,\nyou’ll see that the problem is that when a modal loop runs,\nyour message loop is no longer in control,\nand therefore any customizations you’ve made to your message loop\nwill not be in effect.\nThis is normally a good thing.\nFor example, if a dialog box calls \nIf there is some sort of message processing you want to happen\nregardless of which message loop is control,\nthen you can’t put it in your custom message loop because\n(tautologically)\nyour custom message loop is not in control\nwhen it is not running.\nBut message loops will call\n This question was inspired by an actual customer question, but I changed a lot of it around to make for a more interesting story. (Trust me, the original story was even more boring.) A customer’s background thread posted a message to the main UI thread to signal something (details not important). They found that the […]<\/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-3313","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-oldnewthing","tag-code"],"acf":[],"blog_post_summary":" This question was inspired by an actual customer question, but I changed a lot of it around to make for a more interesting story. (Trust me, the original story was even more boring.) A customer’s background thread posted a message to the main UI thread to signal something (details not important). They found that the […]<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/3313","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=3313"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/3313\/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=3313"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/categories?post=3313"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/tags?post=3313"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}DispatchMessage<\/code>\nto deliver the posted message to its target window’s window procedure.\n<\/p>\n\nBOOL MyApp::PreTranslateMessage(MSG *pmsg)\n{\n if (pmsg->message == OUR_SPECIAL_MESSAGE) {\n ... special code here ...\n return TRUE; \/\/ handled\n }\n return FALSE; \/\/ not handled\n}\n<\/pre>\nPostMessage<\/code>\ninstead of PostThreadMessage<\/code> but simply\nblindly followed the advice rather than\n\nunderstanding its rationale<\/a>\nso they could apply the advice correctly.\n<\/p>\nMessageBox<\/code>,\nthe dialog keyboard shortcuts shouldn’t be active while the message box\nis displayed.\nIt would be very strange if hitting Enter<\/kbd> caused the\ndialog box to invoke its default button\nwhile the modal message box is still on the screen<\/i>.\nThe result would most likely be\n\na dialog box without underlying support<\/a>,\nwhich leads to unhappiness.\n<\/p>\nDispatchMessage<\/code>,\nand that will deliver the message to your window procedure.\n(Of course, the converse also applies:\nIf you want<\/i> the behavior to be suspended when a modal operation\nis in progress,\nyou can put it in your message loop.)<\/p>\n","protected":false},"excerpt":{"rendered":"