delphi – IPreviewHandler卸载COM对象需要很长时间并冻结应用程序

我正在尝试使用IPreviewHandler接口在我的应用程序中的TPanel上显示类似预览的 Windows 7.

当我通过调用Unload(用于处理COM对象)然后弄脏对象来销毁预览对象时,会出现问题.应用程序将冻结(在析构函数后直接),直到预览主机进程退出.这可能需要几分钟.用adobe预览.pdfs时会发生很多事情.

我想知道是否有办法避免这种/或以不同的方式来完成文件预览？

unit uHostPreview;

interface

uses
  Winapi.ShlObj, Winapi.Messages, Winapi.ShLwApi, Winapi.Windows,
  System.Classes,
  Vcl.Controls, Vcl.Dialogs;

type
  THostPreviewHandler = class(TCustomControl)
  private
    m_fileStream        : TFileStream;
    m_previewGUIDStr    : string;
    m_name              : string;
    m_memStream         : TMemoryStream;
    m_previewUnloading  : Boolean;
    m_loadFromMemStream : Boolean;
    m_hwnd              : HWND;
    m_previewHandler    : IPreviewHandler;
    m_msg               : string;
    procedure WMSize(var Message: TWMSize); message WM_SIZE;
    function  CreateFileFromStream(const in_Stream : TMemoryStream) : string;
  protected
    procedure Paint; override;

  public

    procedure   LoadPreviewHandler;
    constructor Create(AOwner: TWinControl; in_FileName : String) overload; reintroduce;
    constructor Create(AOwner: TWinControl; in_Stream : TMemoryStream;
      in_name : string) overload; reintroduce;
    destructor  Destroy; override;
  end;

implementation

uses
 SysUtils, Graphics, ComObj, ActiveX,
 Registry, PropSys, ObBase, System.IOUtils;

constructor THostPreviewHandler.Create(AOwner: TWinControl; in_fileName : String) overload;
begin
  inherited Create(AOwner);

  m_hwnd              := AOwner.handle;
  m_previewHandler    := nil;
  m_previewGUIDStr    := '';
  m_fileStream        := nil;
  m_name              := in_fileName;
  m_loadFromMemStream := False;
  m_msg               := 'No Preview Available.';
end;

constructor THostPreviewHandler.Create(AOwner: TWinControl; in_stream : TMemoryStream;
in_name : string) overload;
begin
  inherited Create(AOwner);

 m_hwnd               := AOwner.handle;
  m_previewHandler    := nil;
  m_previewGUIDStr    := '';
  m_fileStream        := nil;
  m_memStream         := in_stream;
  m_name              := in_name;
  m_loadFromMemStream := True;
  m_msg               := 'No Preview Available.';
end;

//As Soon as the destructor finishes the application freezes until Preview Host processes end!!!
destructor THostPreviewHandler.Destroy;
begin
  if (m_previewHandler<>nil) then
  begin
    m_previewHandler.Unload;
    m_previewHandler := nil;
   end;

  if m_fileStream<>nil then
    FreeAndNil(m_fileStream);
  m_memStream := nil;

  inherited;
end;


procedure THostPreviewHandler.Paint;
var
  lpRect: TRect;
begin
//Now Done in the load preview. Means previews don't stall when rapidly switching between different files.
{ if (m_previewGUIDStr<>'') and (m_previewHandler<>nil) and not m_previewLoaded then
 begin
   m_previewLoaded := true;
   m_previewHandler.DoPreview;
   m_previewHandler.SetFocus;
 end
 else }
 if m_previewGUIDStr='' then
 begin
   lpRect:=Rect(0, 0, Self.Width, Self.Height);
   Canvas.Brush.Style :=bsClear;
   Canvas.Font.Color  :=clWindowText;
   DrawText(Canvas.Handle, PChar(m_msg) ,Length(m_msg), lpRect, DT_VCENTER or DT_CENTER or DT_SINGLELINE);
 end;
end;

function GetPreviewHandlerCLSID(const AFileName: string): string;
const
  SID_IPreviewHandler = '{8895B1C6-B41F-4C1C-A562-0D564250836F}';
var
  Buffer               : array [0..1024] of Char;
  BufSize              : DWord;
  RegQueryRes          : HResult;
  fileExtension        : string;
  LRegistry            : TRegistry;
  LExt, LFileClass     : string;
  LPerceivedType, LKey : string;

begin
  Result := '';

  fileExtension := ExtractFileExt(AFileName);

  // Searches the registry for the preview handler for the current file extension
  BufSize := Length(Buffer);
  RegQueryRes := AssocQueryString(
    ASSOCF_INIT_DEFAULTTOSTAR,
    ASSOCSTR_SHELLEXTENSION,
    PChar(fileExtension),
    SID_IPreviewHandler,
    Buffer,
    @BufSize
  );
  If RegQueryRes = S_OK then
  begin
    Result := String(Buffer)
  end
end;

procedure THostPreviewHandler.LoadPreviewHandler;
const
  GUID_ISHELLITEM = '{43826d1e-e718-42ee-bc55-a1e261c37bfe}';
var
  prc                   : TRect;
  LPreviewGUID          : TGUID;
  LInitializeWithFile   : IInitializeWithFile;
  LInitializeWithStream : IInitializeWithStream;
  LInitializeWithItem   : IInitializeWithItem;
  LIStream              : IStream;
  LShellItem            : IShellItem;
  fname                 : string;
begin
  HandleNeeded;

  m_previewGUIDStr:=GetPreviewHandlerCLSID(m_name);

  //If no matching preview handler is found. Exit.
  if m_previewGUIDStr='' then
  begin
    exit;
  end;

  if m_fileStream<>nil then
    FreeAndNil(m_fileStream);

  LPreviewGUID:= StringToGUID(m_previewGUIDStr);

  //Create a COM object to do the preview handling
  m_previewHandler := CreateComObject(LPreviewGUID) As IPreviewHandler;
  if (m_previewHandler = nil) then
  begin
    exit;
  end;

  if m_previewHandler.QueryInterface(IInitializeWithStream, LInitializeWithStream) = S_OK then
  begin
    if m_loadFromMemStream then
    begin
      LIStream := TStreamAdapter.Create(m_memStream, soReference) as IStream;
    end
    else
    begin
      m_fileStream := TFileStream.Create(m_name, fmOpenRead or fmShareDenyNone);
      LIStream := TStreamAdapter.Create(m_fileStream, soReference) as IStream;
    end;
    LInitializeWithStream.Initialize(LIStream, STGM_READ);
  end
  else if (m_previewHandler.QueryInterface(IInitializeWithFile, LInitializeWithFile) = S_OK) then
  begin
    if not m_loadFromMemStream then
    begin
      LInitializeWithFile.Initialize(StringToOleStr(m_name), STGM_READ);
    end
    else
    begin
      fname := CreateFileFromStream(m_memStream);
      LInitializeWithFile.Initialize(StringToOleStr(fname), STGM_READ);
    end;
  end
  else if ((m_previewHandler.QueryInterface(IInitializeWithItem, LInitializeWithItem) = S_OK) and (not m_loadFromMemStream)) then
  begin
    if not m_loadFromMemStream then
    begin
      SHCreateItemFromParsingName(PChar(m_name), nil, StringToGUID(GUID_ISHELLITEM), LShellItem);
      LInitializeWithItem.Initialize(LShellItem, 0);
    end
    else
    begin
      fname := CreateFileFromStream(m_memStream);
      SHCreateItemFromParsingName(PChar(fname), nil, StringToGUID(GUID_ISHELLITEM), LShellItem);
      LInitializeWithItem.Initialize(LShellItem, 0);
    end;
  end
  else
  begin
    m_msg := 'Preview Could Not be Intialized.';
  end;

  prc := ClientRect;
  m_previewHandler.SetWindow(m_hwnd, prc);
  m_previewHandler.DoPreview;
end;

function THostPreviewHandler.CreateFileFromStream(const in_Stream : TMemoryStream) : string;
var
tempPath : string;
begin
  tempPath := TPath.GetTempPath;
  tempPath := tempPath + m_name;
  in_Stream.SaveToFile(tempPath);
  result := tempPath;
end;

procedure THostPreviewHandler.WMSize(var Message: TWMSize);
var
  prc  : TRect;
begin
  inherited;
  if m_previewHandler<>nil then
  begin
    prc := ClientRect;
    m_previewHandler.SetRect(prc);
  end;
end;

end.

创建预览

if m_attachPreview<>nil then
  begin
    FreeAndNil(m_attachPreview);
  end;

  memStream := TMemoryStream.Create;
  memStream.LoadFromFile('C:\Test');

  if loadFromStream then
  begin
  //Preview can be loaded from a stream or a file   
  m_attachPreview := THostPreviewHandler.Create(pnlPreview, TMemoryStream, name);
  end
  else
  begin
    m_attachPreview := THostPreviewHandler.Create(pnlPreview, filePath);
  end;

  m_attachPreview.Top := 0;
  m_attachPreview.Left := 0;
  m_attachPreview.Width := pnlPreview.ClientWidth;
  m_attachPreview.Height := pnlPreview.ClientHeight;
  m_attachPreview.Parent := pnlPreview;
  m_attachPreview.Align  := alClient;
  m_attachPreview.LoadPreviewHandler;

我们也注意到这种烦人的行为,不好的是你无法控制预览处理程序加载和卸载所需的时间.我们最终使用了一个带有workerthread的线程池,用于每个预览文件,在那些我们现在进行加载和卸载的线程中,这样工作正常且没有延迟.这是作为ShellBrowser组件的一部分的读取使用控件： https://www.jam-software.de/shellbrowser_delphi/file-preview.shtml