{"id":36333,"date":"2005-03-01T07:00:00","date_gmt":"2005-03-01T07:00:00","guid":{"rendered":"https:\/\/blogs.msdn.microsoft.com\/oldnewthing\/2005\/03\/01\/modality-part-7-a-timed-messagebox-the-cheap-version\/"},"modified":"2005-03-01T07:00:00","modified_gmt":"2005-03-01T07:00:00","slug":"modality-part-7-a-timed-messagebox-the-cheap-version","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/oldnewthing\/20050301-00\/?p=36333","title":{"rendered":"Modality, part 7: A timed MessageBox, the cheap version"},"content":{"rendered":"

\n\nAs we noted at the end of part 3<\/a>,\nnow that you know the conventions surrounding\n\nthe WM_QUIT<\/code> message<\/a>\n you can put them to your advantage.\n<\/p>\n

\nThe more robust you want the TimedMessageBox<\/code> function\nto be, the more work you need to do.\nHere’s the cheap version,\nbased on the\n\nsample in the Knowledge Base<\/a>,\nbut with some additional bug fixes.\n<\/p>\n

\nstatic BOOL s_fTimedOut;\nstatic HWND s_hwndMBOwnerEnable;\nvoid CALLBACK\nCheapMsgBoxTooLateProc(HWND hWnd, UINT uiMsg, UINT_PTR idEvent, DWORD dwTime)\n{\n    s_fTimedOut = TRUE;\n    if (s_hwndMBOwnerEnable) EnableWindow(s_hwndMBOwnerEnable, TRUE);\n    PostQuitMessage(42); \/\/ value not important\n}\n\/\/ Warning! Not thread-safe! See discussion.\nint CheapTimedMessageBox(HWND hwndOwner, LPCTSTR ptszText,\n    LPCTSTR ptszCaption, UINT uType, DWORD dwTimeout)\n{\n    s_fTimedOut = FALSE;\n    s_hwndMBOwnerEnable = NULL;\n    if (hwndOwner && IsWindowEnabled(hwndOwner)) {\n      s_hwndMBOwnerEnable = hwndOwner;\n    }\n    UINT idTimer = SetTimer(NULL, 0, dwTimeout, CheapMsgBoxTooLateProc);\n    int iResult = MessageBox(hwndOwner, ptszText, ptszCaption, uType);\n    if (idTimer) KillTimer(NULL, idTimer);\n    if (s_fTimedOut) {\t\t\t\/\/ We timed out\n\tMSG msg;\n\t\/\/ Eat the fake WM_QUIT message we generated\n\tPeekMessage(&msg, NULL, WM_QUIT, WM_QUIT, PM_REMOVE);\n\tiResult = -1;\n    }\n    return iResult;\n}\n<\/pre>\n
\nThis CheapTimedMessageBox<\/code> function acts just like\n\nthe MessageBox<\/code> function<\/a>,\nexcept that if the user doesn’t respond\nwithin dwTimeout<\/code> milliseconds, we return -1.\nThe limitation is that only one timed message box can be active at a time.\nIf your program is single-threaded, this is not a serious limitation,\nbut if your program is multi-threaded, this will be a problem.\n<\/p>\n
\nDo you see how it works?\n<\/p>\n
\nThe global static variable s_fTimedOut<\/code>\ntells us whether we generated\na fake WM_QUIT<\/code> message as a result of a timeout.\nWhen the MessageBox<\/code> function returns, and we indeed timed out,\nwe use\n\nthe PeekMessage<\/code> function<\/a>\nto remove the fake WM_QUIT<\/code> message from the\nqueue before returning.\n<\/p>\n
\nNote that we remove the WM_QUIT<\/code> message only if we were the ones\nwho generated it.\nIn this way, WM_QUIT<\/code> messages generated by other\nparts of the program remain in the queue for processing by the\nmain message loop.\n<\/p>\n
\nNote also that when we decide that the timeout has occurred,\nwe re-enable the original owner window before we cause\nthe message box to bail out of its message loop by posting a quit message.\nThose are the rules for\n\nthe correct order for disabling and enabling windows<\/a>.\n<\/p>\n
\nNote also that we used a thread timer rather than a window timer.\nThat’s because we don’t own the window being passed in and therefore\ndon’t know what timer IDs are safe to use.  Any timer ID we pick\nmight happen to collide with a timer ID being used by that window,\nresulting in erratic behavior.\n<\/p>\n
\nRecall that when you pass NULL<\/code> as the\nhwnd<\/code> parameter to\n\nthe SetTimer<\/code> function<\/a>\nand also pass zero as the\nnIDEvent<\/code> parameter,\nthen the SetTimer<\/code> function\ncreates a brand new timer, assigns it a unique ID, and returns the ID.\nMost people, when they read that part of the specification for\nSetTimer<\/code>,\nscratch their heads and ask themselves, “Why would\nanybody want to use this?”\n<\/p>\n
\nWell, this is one scenario where this is exactly what you want.\n<\/p>\n
\nNext comes the job of making the function a tad more robust.\nBut before we do that, we’ll need two quick sidebars.<\/p>\n","protected":false},"excerpt":{"rendered":"
As we noted at the end of part 3, now that you know the conventions surrounding the WM_QUIT message you can put them to your advantage. The more robust you want the TimedMessageBox function to be, the more work you need to do. Here’s the cheap version, based on the sample in the Knowledge Base, but […]<\/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":[25,143],"class_list":["post-36333","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-oldnewthing","tag-code","tag-modality"],"acf":[],"blog_post_summary":"
As we noted at the end of part 3, now that you know the conventions surrounding the WM_QUIT message you can put them to your advantage. The more robust you want the TimedMessageBox function to be, the more work you need to do. Here’s the cheap version, based on the sample in the Knowledge Base, but […]<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/36333","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=36333"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/36333\/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=36333"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/categories?post=36333"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/tags?post=36333"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}