Is there a Preview Handler VCL for Windows 7?

Question

This article

http://msdn.microsoft.com/en-gb/library/bb776867.aspx

describes preview handlers in Windows as

Preview handlers are called when an item is selected to show a lightweight, rich, read-only preview of the file's contents in the view's reading pane. This is done without launching the file's associated application.

and ...

A preview handler is a hosted application. Hosts include the Microsoft Windows Explorer in Windows Vista or Microsoft Outlook 2007.

Is there some Delphi VCL code which can be used as a startingpoint for such a handler?

Answer 1

@Mjn, right know I'm writing an article for my blog to implement Preview Handlers from Delphi, but due to lack of time, I do not know when this is complete, as others users mention by the moment no exist a VCL component in Delphi to implement preview handlers, in the past I implemented a couple of preview handlers for a customer but using Delphi-Prism and C#.

As starting point here I leave some tips.

• You must use the IPreviewHandler, InitializeWithFile, InitializeWithStream, IPreviewHandlerFrame, IPreviewHandlerVisuals interfaces.

This is the Delphi translation of the headers of these interfaces

uses
  Windows, ActiveX, AxCtrls, ShlObj, ComObj;

type


  IIPreviewHandler = interface(IUnknown)
    ['{8895b1c6-b41f-4c1c-a562-0d564250836f}']
    function SetWindow(hwnd: HWND; var RectangleRef: TRect): HRESULT; stdcall;
    function SetRect(var RectangleRef: TRect): HRESULT; stdcall;
    function DoPreview(): HRESULT; stdcall;
    function Unload(): HRESULT; stdcall;
    function SetFocus(): HRESULT; stdcall;
    function QueryFocus(phwnd: HWND): HRESULT; stdcall;
    function TranslateAccelerator(PointerToWindowMessage: MSG): HRESULT; stdcall;
  end;

  IInitializeWithFile = interface(IUnknown)
    ['{b7d14566-0509-4cce-a71f-0a554233bd9b}']
    function Initialize(pszFilePath: LPWSTR; grfMode: DWORD):HRESULT;stdcall;
  end;

  IInitializeWithStream = interface(IUnknown)
    ['{b824b49d-22ac-4161-ac8a-9916e8fa3f7f}']
    function Initialize(pstream: IStream; grfMode: DWORD): HRESULT; stdcall;
  end;

  IIPreviewHandlerFrame = interface(IUnknown)
    ['{fec87aaf-35f9-447a-adb7-20234491401a}']
    function GetWindowContext(pinfo: HWND): HRESULT; stdcall;
    function TranslateAccelerator(PointerToWindowMessage: MSG): HRESULT; stdcall;
  end;

  IIPreviewHandlerVisuals = interface(IUnknown)
    ['{8327b13c-b63f-4b24-9b8a-d010dcc3f599}']
        function SetBackgroundColor(color: COLORREF ): HRESULT; stdcall;
        function SetFont(plf:LOGFONTW): HRESULT; stdcall;  
        function SetTextColor(color: COLORREF): HRESULT; stdcall;
  end;

• You must create a com dll with a class which descend from these interfaces IIPreviewHandler, IIPreviewHandlerVisuals, IOleWindow, IObjectWithSite to manage the visualization and a second class to load the files to show. this class must descend from IPreviewHandler, IInitializeWithStream.

something like this

  TMyPreviewHandler = class(IIPreviewHandler, IIPreviewHandlerVisuals, IOleWindow, IObjectWithSite)

  TMyStream = class(IIPreviewHandler, IInitializeWithStream, IStream)

• Now you must create your own implementation of the methods for the parent interfaces. this is the list of the methods which you need implement.

  IPreviewHandler -> DoPreview, SetWindow, SetRect, Unload, SetFocus, TranslateAccelerator, QueryFocus.

  IObjectWithSite -> GetSite, SetSite.

  IOleWindow -> GetWindow

  IPreviewHandlerVisuals - > SetBackgroundColor, SetFont, SetColor

  InitializeWithStream -> Initialize

• finally you must register your COM in the system as well as the file extensions which will use you PrevieHandler class.

• Check this project as a starting point Windows Preview Handler Pack (is written in C#) and this article View Data Your Way With Our Managed Preview Handler Framework