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我正嘗試使用IPreviewHandler接口在我的應用程序中的TPanel上顯示一個類似於Windows 7的預覽。IPreviewHandler卸載COM對象需要很長時間並凍結應用程序
當我通過調用Unload（這意味着處理COM對象）並且然後將對象作爲對象來銷燬預覽對象時，就會出現問題。應用程序將凍結（直接在析構函數之後），直到預覽主機進程退出。這可能需要幾分鐘的時間。使用Adobe預覽.pdf時會發生很多情況。
我想知道是否有辦法避免這種情況/或以不同的方式來完成文件預覽？
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.
創建預覽
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;