{"id":35323,"date":"2005-06-14T09:02:18","date_gmt":"2005-06-14T09:02:18","guid":{"rendered":"https:\/\/blogs.msdn.microsoft.com\/oldnewthing\/2005\/06\/14\/displaying-the-dictionary-part-2-using-text-callbacks\/"},"modified":"2005-06-14T09:02:18","modified_gmt":"2005-06-14T09:02:18","slug":"displaying-the-dictionary-part-2-using-text-callbacks","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/oldnewthing\/20050614-18\/?p=35323","title":{"rendered":"Displaying the dictionary, part 2: Using text callbacks"},"content":{"rendered":"

\n\nAs we noted last time<\/a>,\nadding items to the listview takes an absurd amount of time.\nToday, we’ll make a failed attempt at improving this\nbecause it lets me illustrate a listview technique\nand it lays the groundwork for the real fix next time.\n<\/p>\n

\nInstead of creating the items in their entirety,\nlet’s set their text to LPSTR_TEXTCALLBACK<\/code>.\nThis is a placeholder value which indicates\n“I’m not going to tell you what the string is.\nIf you need it, call me back.”\n<\/p>\n

\nclass RootWindow : public Window\n{\n ...\n LRESULT OnCreate();\n LRESULT OnNotify(NMHDR* pnm);\n void OnGetDispInfo(NMLVDISPINFO* pnmv);<\/font>\n ...\n};\nLRESULT RootWindow::OnCreate()\n{\n ...\n \/\/ item.pszText = const_cast<LPWSTR>(de.m_pszTrad);<\/strike>\n item.pszText = LPSTR_TEXTCALLBACK;<\/font>\n ...\n   \/\/ item.pszText = const_cast<LPWSTR>(de.m_pszPinyin);<\/strike>\n   item.pszText = LPSTR_TEXTCALLBACK;<\/font>\n ...\n   \/\/ item.pszText = const_cast<LPWSTR>(de.m_pszEnglish);<\/strike>\n   item.pszText = LPSTR_TEXTCALLBACK;<\/font>\n ...\n}\nLRESULT RootWindow::OnNotify(NMHDR *pnm)\n{\n switch (pnm->code) {\n case LVN_GETDISPINFO:\n  OnGetDispInfo(CONTAINING_RECORD(pnm, NMLVDISPINFO, hdr));\n  break;\n }\n return 0;\n}\nvoid RootWindow::OnGetDispInfo(NMLVDISPINFO* pnmv)\n{\n if (pnmv->item.iItem < 0 || \/\/ typo fixed 11am\n     pnmv->item.iItem >= Length()) {\n  return;         \/\/ requesting invalid item\n }\n if (pnmv->item.mask & LVIF_TEXT) {\n  const DictionaryEntry& de = Item(pnmv->item.iItem);\n  LPCWSTR pszResult = L\"\";\n  switch (pnmv->item.iSubItem) {\n   case COL_TRAD:    pszResult = de.m_pszTrad;    break;\n   case COL_PINYIN:  pszResult = de.m_pszPinyin;  break;\n   case COL_ENGLISH: pszResult = de.m_pszEnglish; break;\n  }\n  pnmv->item.pszText = const_cast<LPWSTR>(pszResult);\n }\n if (pnmv->item.mask & LVIF_IMAGE) {\n  pnmv->item.iImage = -1;\n }\n if (pnmv->item.mask & LVIF_STATE) {\n     pnmv->item.state = 0;\n }\n}<\/font>\nLRESULT RootWindow::HandleMessage(\n                          UINT uMsg, WPARAM wParam, LPARAM lParam)\n{\n ...\n  case WM_NOTIFY:\n   return OnNotify(reinterpret_cast<NMHDR*>(lParam));<\/font>\n ...\n}\n<\/pre>\n
\nInstead of setting the strings when we create the listview items,\nwe set their texts to LPSTR_TEXTCALLBACK<\/code>.\nWhen the listview needs the text, it sends us a\nLVN_GETDISPINFO<\/code> notification, which we handle\nby returning the data that the listview requested.\n<\/p>\n
\nSidebar<\/strong>:\nIn our case, obtaining the missing data is very fast.\nIf it were slow, we could have optimized the function further by\nadding the line<\/p>\n
\n pnmv->item.mask |= LVIF_DI_SETITEM;\n<\/pre>\n
to the end.  This tells the listview, “Please cache these results\nand don’t ask me for them again.”\nThat way, we do the slow computation only once.\n<\/p>\n
\nAfter making these changes (though not the LVIF_DI_SETITEM<\/code>\nchange; that was just a sidebar),\nnotice that the it didn’t really help much.\nOn my machine, the startup time dropped from eleven to ten seconds,\nbut ten seconds is still way too long.\nThis optimization turns out to have been a washout.\n<\/p>\n
\n(Note also that our program is now relying heavily on the fact\nthat a vector is a fast random-access data structure.)\n<\/p>\n
\nWe’ll do better next time.<\/p>\n","protected":false},"excerpt":{"rendered":"
As we noted last time, adding items to the listview takes an absurd amount of time. Today, we’ll make a failed attempt at improving this because it lets me illustrate a listview technique and it lays the groundwork for the real fix next time. Instead of creating the items in their entirety, let’s set their […]<\/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-35323","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-oldnewthing","tag-code"],"acf":[],"blog_post_summary":"
As we noted last time, adding items to the listview takes an absurd amount of time. Today, we’ll make a failed attempt at improving this because it lets me illustrate a listview technique and it lays the groundwork for the real fix next time. Instead of creating the items in their entirety, let’s set their […]<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/35323","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=35323"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/35323\/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=35323"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/categories?post=35323"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/tags?post=35323"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}