windows - 为什么 ReadDirectoryChangesW 会忽略事件？

Question

我使用 ReadDirectoryChangesW 监视指定目录并在检测到更改时更新索引结构。我使用以下代码(大致)

var
  InfoPointer : PFileNotifyInformation;
  NextOffset : DWORD;
...
while (not Terminated) do begin
  if ReadDirectoryChangesW (FDirHandle, FBuffer, FBufferLength, True,
                            FFilter, @BytesRead, @FOverlap, nil) then
    begin
    WaitResult := WaitForMultipleObjects (2, @FEventArray, False, INFINITE);
    if (WaitResult = waitFileChange) then
      begin 
      InfoPointer := FBuffer;
      repeat
        NextOffset := InfoPointer.NextEntryOffset;
        ...
        PByte (InfoPointer) := PByte (InfoPointer) + NextOffset;
      until NextOffset = 0;
      end;
    end;
end;  

过滤器是

FFilter :=  FILE_NOTIFY_CHANGE_FILE_NAME or
            FILE_NOTIFY_CHANGE_DIR_NAME or
            FILE_NOTIFY_CHANGE_SIZE or
            FILE_NOTIFY_CHANGE_LAST_WRITE;

目录句柄是这样获取的:

FDirHandle := CreateFile (PChar (FDirectoryWatch.WatchedDirectory),
                          FILE_LIST_DIRECTORY or GENERIC_READ,
                          FILE_SHARE_READ or FILE_SHARE_WRITE or FILE_SHARE_DELETE,
                          nil, OPEN_EXISTING, FILE_FLAG_BACKUP_SEMANTICS or   
                          FILE_FLAG_OVERLAPPED, 0);         

当我删除多个文件时，我只收到一个事件，NextOffset 为 0!当我删除一个目录时，我只得到该目录的一个事件。如果我希望目录中的每个文件都有一个事件怎么办？

如有任何帮助，我们将不胜感激。

Answer 1

在我看来，您正在混合使用 ReadDirectoryChangesW() 的各种方式，您在打开目录时同时指定了 FILE_FLAG_OVERLAPPED 标志并提供指向lpOverlapped 参数，表示您要等待结构中的事件并处理异步 I/O；同时，您在工作线程的循环中调用 ReadDirectoryChangesW()。我会先将 lpOverlapped 设置为 nil 再试一次，因为您有一个专用线程并且可以使用同步模式。

在 ReadDirectoryChangesW() 的文档中API函数描述了不同的使用方法。请注意，缓冲区也有可能溢出，因此更改事件无论如何都可能丢失。也许您应该重新考虑仅依赖此功能的策略，比较目录内容的快照也可以。

编辑:

您编辑的代码看起来更好。然而，在我的测试中，ReadDirectoryChangesW() 确实像宣传的那样工作，返回的缓冲区中有多个数据条目，或者有多个缓冲区要处理。这取决于时间安排，在 Delphi 中遇到断点后，我在一个缓冲区中获得了多个条目。

为了完整起见，我附上了使用 Delphi 5 实现的测试代码:

type
  TWatcherThread = class(TThread)
  private
    fChangeHandle: THandle;
    fDirHandle: THandle;
    fShutdownHandle: THandle;
  protected
    procedure Execute; override;
  public
    constructor Create(ADirectoryToWatch: string);
    destructor Destroy; override;

    procedure Shutdown;
  end;

constructor TWatcherThread.Create(ADirectoryToWatch: string);
const
  FILE_LIST_DIRECTORY = 1;
begin
  inherited Create(TRUE);
  fChangeHandle := CreateEvent(nil, FALSE, FALSE, nil);
  fDirHandle := CreateFile(PChar(ADirectoryToWatch),
    FILE_LIST_DIRECTORY or GENERIC_READ,
    FILE_SHARE_READ or FILE_SHARE_WRITE or FILE_SHARE_DELETE,
    nil, OPEN_EXISTING, FILE_FLAG_BACKUP_SEMANTICS or FILE_FLAG_OVERLAPPED, 0);
  fShutdownHandle := CreateEvent(nil, FALSE, FALSE, nil);
  Resume;
end;

destructor TWatcherThread.Destroy;
begin
  if fDirHandle <> INVALID_HANDLE_VALUE then
    CloseHandle(fDirHandle);
  if fChangeHandle <> 0 then
    CloseHandle(fChangeHandle);
  if fShutdownHandle <> 0 then
    CloseHandle(fShutdownHandle);
  inherited Destroy;
end;

procedure TWatcherThread.Execute;
type
  PFileNotifyInformation = ^TFileNotifyInformation;
  TFileNotifyInformation = record
    NextEntryOffset: DWORD;
    Action: DWORD;
    FileNameLength: DWORD;
    FileName: WideChar;
  end;
const
  BufferLength = 65536;
var
  Filter, BytesRead: DWORD;
  InfoPointer: PFileNotifyInformation;
  Offset, NextOffset: DWORD;
  Buffer: array[0..BufferLength - 1] of byte;
  Overlap: TOverlapped;
  Events: array[0..1] of THandle;
  WaitResult: DWORD;
  FileName, s: string;
begin
  if fDirHandle <> INVALID_HANDLE_VALUE then begin
    Filter := FILE_NOTIFY_CHANGE_FILE_NAME or FILE_NOTIFY_CHANGE_DIR_NAME
      or FILE_NOTIFY_CHANGE_SIZE or FILE_NOTIFY_CHANGE_LAST_WRITE;

    FillChar(Overlap, SizeOf(TOverlapped), 0);
    Overlap.hEvent := fChangeHandle;

    Events[0] := fChangeHandle;
    Events[1] := fShutdownHandle;

    while not Terminated do begin
      if ReadDirectoryChangesW (fDirHandle, @Buffer[0], BufferLength, TRUE,
        Filter, @BytesRead, @Overlap, nil)
      then begin
        WaitResult := WaitForMultipleObjects(2, @Events[0], FALSE, INFINITE);
        if WaitResult = WAIT_OBJECT_0 then begin
          InfoPointer := @Buffer[0];
          Offset := 0;
          repeat
            NextOffset := InfoPointer.NextEntryOffset;
            FileName := WideCharLenToString(@InfoPointer.FileName,
              InfoPointer.FileNameLength);
            SetLength(FileName, StrLen(PChar(FileName)));
            s := Format('[%d] Action: %.8xh, File: "%s"',
               [Offset, InfoPointer.Action, FileName]);
            OutputDebugString(PChar(s));
            PByte(InfoPointer) := PByte(DWORD(InfoPointer) + NextOffset);
            Offset := Offset + NextOffset;
          until NextOffset = 0;
        end;
      end;
    end;
  end;
end;

procedure TWatcherThread.Shutdown;
begin
  Terminate;
  if fShutdownHandle <> 0 then
    SetEvent(fShutdownHandle);
end;

////////////////////////////////////////////////////////////////////////////////

procedure TForm1.FormCreate(Sender: TObject);
begin
  fThread := TWatcherThread.Create('D:\Temp');
end;

procedure TForm1.FormDestroy(Sender: TObject);
begin
  if fThread <> nil then begin
    TWatcherThread(fThread).Shutdown;
    fThread.Free;
  end;
end;

删除一个目录确实只会返回一个改变，对其中包含的文件没有任何影响。但它确实有意义，因为您只查看父目录的句柄。如果您需要子目录的通知，您可能还需要监视它们。