windows - 为什么ReadDirectoryChangesW忽略事件？

每当检测到更改时，我都使用ReadDirectoryChangesW监视指定的目录并更新索引结构。我大致使用以下代码

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;

筛选器为

FFilter :=  FILE_NOTIFY_CHANGE_FILE_NAME or
            FILE_NOTIFY_CHANGE_DIR_NAME or
            FILE_NOTIFY_CHANGE_SIZE or
            FILE_NOTIFY_CHANGE_LAST_WRITE;

和目录句柄是这样获得的:

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，我仅收到一个事件，而NextOffset为0!当我删除目录时，该目录仅收到一个事件。如果我希望目录中的每个文件有一个事件怎么办？

任何帮助，将不胜感激。

最佳答案

在我看来，您正在混合使用ReadDirectoryChangesW()的各种方法，在打开目录时都指定了FILE_FLAG_OVERLAPPED标志，并提供了指向lpOverlapped参数的指针，这意味着您要等待结构中的事件并处理异步I/O；同时您在工作线程中的循环中调用ReadDirectoryChangesW()。我首先尝试将lpOverlapped设置为nil，因为您有专用线程并且可以使用同步模式。

在ReadDirectoryChangesW() API函数的文档中，描述了使用它的不同方法。请注意，缓冲区也有可能溢出，因此无论如何更改事件都可能丢失。也许您应该重新考虑仅依靠此功能的策略，比较目录内容的快照也可以工作。

编辑:

您编辑的代码看起来更好。但是，在我的测试中，ReadDirectoryChangesW()确实如广告所示那样工作，返回的缓冲区中有多个数据条目，或者有多个要处理的缓冲区。这取决于时间，在Delphi中达到断点后，我会在一个缓冲区中获得多个条目。

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;

删除目录确实只会返回一个更改，而不包含其中的文件。但这确实是有道理的，因为您仅在观看父目录的句柄。如果您需要有关子目录的通知，则可能还需要注意它们。