{"id":97385,"date":"2017-11-10T07:00:00","date_gmt":"2017-11-10T22:00:00","guid":{"rendered":"https:\/\/blogs.msdn.microsoft.com\/oldnewthing\/?p=97385"},"modified":"2019-03-13T01:34:40","modified_gmt":"2019-03-13T08:34:40","slug":"20171110-00","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/oldnewthing\/20171110-00\/?p=97385","title":{"rendered":"Cancelling the INamespace­Walk::<\/CODE>Walk<\/CODE> operation a little faster"},"content":{"rendered":"

We saw last time<\/a> that you can stop a INamespace­Walk::<\/code>Walk<\/code> operation by returning a COM error code from the Enter­Folder<\/code> or Found­Item<\/code> callback. However, that may not be fast enough. <\/p>\n

I noted some time ago that if you’re going to enumerate the contents of a directory, you’d best do it all at once<\/a>. And that’s what INamespace­Walk::<\/code>Walk<\/code> does. After it enters a directory, it enumerates the whole thing at one shot, and then (optionally) sorts it, and then calls the Found­Item<\/code> method for each item that was found. <\/p>\n

If you happen to enter a large directory, then the “enumerate the whole thing at one shot” step can take a while. But there’s a way to sneak in during the enumeration phase and cancel the operation: Implement the IAction­Progress<\/code> interface on your INamespace­Walk­CB<\/code> object. Note that this works only if you do not<\/i> pass the NSWF_<\/code>SHOW_<\/code>PROGRESS<\/code> flag. If you pass the NSWF_<\/code>SHOW_<\/code>PROGRESS<\/code> flag, then the progress dialog’s Cancel button controls the cancellation. <\/p>\n

Assuming you don’t pass the NSWF_<\/code>SHOW_<\/code>PROGRESS<\/code> flag, the INamespace­Walk::<\/code>Walk<\/code> method will call IAction­Progress::<\/code>Begin<\/code> to get the party started, and IAction­Progress::<\/code>End<\/code> when it’s all over. In between, it will call IAction­Progress::<\/code>QueryCancel<\/code>. If your IAction­Progress::<\/code>QueryCancel<\/code> method returns *pfCancelled = TRUE<\/code>, then the INamespace­Walk::<\/code>Walk<\/code> operation will abandon the enumeration, unwind all the entered folders with Leave­Folder<\/code>, and then return HRESULT_<\/code>FROM_<\/code>WIN32(<\/code>ERROR_<\/code>CANCELLED)<\/code>. <\/p>\n

Let’s use this technique to cancel the INamespace­Walk::<\/code>Walk<\/code> operation a bit more quickly. Make the following changes to the program we had last time: <\/p>\n

\n#define STRICT\n#include <windows.h>\n#include <shlobj.h>\n#include <wrl\/client.h>\n#include <wrl\/implements.h>\n#include <stdio.h> \/\/ Horrors! Mixing stdio and C++!\n\nnamespace wrl = Microsoft::WRL;\n\nclass WalkCallback : public wrl::RuntimeClass<\n  wrl::RuntimeClassFlags<wrl::ClassicCom>,\n  INamespaceWalkCB,\n  IActionProgress<\/font>> \/\/ New interface!\n{\npublic:\n  \/\/ INamespaceWalkCB\n  IFACEMETHODIMP FoundItem(IShellFolder *,\n   PCUITEMID_CHILD) override\n   { m_itemCount++; return TimeoutStatus(); }\n\n  IFACEMETHODIMP EnterFolder(IShellFolder *,\n   PCUITEMID_CHILD) override\n   { m_folderCount++; return TimeoutStatus(); }\n\n  IFACEMETHODIMP LeaveFolder(IShellFolder *,\n   PCUITEMID_CHILD) override { return S_OK; }\n\n  IFACEMETHODIMP InitializeProgressDialog(PWSTR *ppszTitle,\n    PWSTR *ppszCancel) override\n    { *ppszTitle = nullptr; *ppszCancel = nullptr;\n      return E_NOTIMPL; }\n\n  \/\/ IActionProgress - new interface!\n  IFACEMETHODIMP Begin(SPACTION, SPBEGINF) override\n  { return S_OK; }\n\n  IFACEMETHODIMP UpdateProgress(ULONGLONG, ULONGLONG) override\n  { return S_OK; }\n\n  IFACEMETHODIMP UpdateText(SPTEXT, LPCWSTR, BOOL) override\n  { return S_OK; }\n\n  IFACEMETHODIMP QueryCancel(BOOL *pfCancelled) override\n  { *pfCancelled = IsTimedOut(); return S_OK; }\n\n  IFACEMETHODIMP ResetCancel() override { return S_OK; }\n  IFACEMETHODIMP End() override { return S_OK; }<\/font>\n\n  int ItemCount() const { return m_itemCount; }\n  int FolderCount() const { return m_folderCount; }\n\nprivate:\n  bool IsTimedOut()\n    { return GetTickCount() - m_startTime<\/a> > 1000; }\n\n  HRESULT TimeoutStatus()\n    { return IsTimedOut() ?\n      HRESULT_FROM_WIN32(ERROR_CANCELLED) : S_OK; }\n\n  DWORD m_startTime = GetTickCount();\n  int m_itemCount = 0;\n  int m_folderCount = 0;\n};\n\nint __cdecl wmain(int argc, PWSTR argv[])\n{\n  CCoInitialize<\/a> coinit;\n\n  wrl::ComPtr<INamespaceWalk> walk;\n  CoCreateInstance(CLSID_NamespaceWalker, nullptr,\n    CLSCTX_INPROC_SERVER, IID_PPV_ARGS(&walk));\n\n  wrl::ComPtr<IShellItem> root;\n  SHCreateItemFromParsingName(argv[1], nullptr,\n    IID_PPV_ARGS(&root));\n\n  auto callback = wrl::Make<WalkCallback>();\n\n  HRESULT hr = walk->Walk(root.Get(), NSWF_DEFAULT,\n    100, callback.Get());\n\n  printf(\"Walk completed with result 0x%08x\\n\", hr);\n  printf(\"Found %d items and %d folders\\n\",\n   callback->ItemCount(), callback->FolderCount());\n\n  return 0;\n}\n<\/pre>\n
All we did was add IAction­Progress<\/code> support to our callback object. When asked if we want to cancel the operation, we report whether the operation has timed out. <\/p>\n
Adding this extra support will not be noticeable when enumerating relatively small directories from relatively fast media.<\/p>\n","protected":false},"excerpt":{"rendered":"
You can use IAction­Progress<\/CODE><\/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-97385","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-oldnewthing","tag-code"],"acf":[],"blog_post_summary":"
You can use IAction­Progress<\/CODE><\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/97385","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=97385"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/97385\/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=97385"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/categories?post=97385"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/tags?post=97385"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}