Thumbnail images that user can see in the thumbnail view mode are produced by special type of shell extensions known as ExtractImage Handlers. Taking this example you will learn how to quickly implement your own thumbnail image handler.

    The ExtractImage handler is an ActiveX library like all other Shell Extensions. The first step to be done is to create a new ActiveX Library. Create it with the following sequence of operations: open the Repository dialog using File | New | Other... menu, then select the ActiveX tab and click the ActiveX Library icon.

Fig. 1. New ActiveX Library creation.

    Save the created project.

    The next step is to add the instance of TSxDataModule to the project. Every Shell+ component should be placed on SxDataModule ¹. This special descendant of TDataModule supplies several internal methods that allow Shell+ components to safely operates in multiple Shell threads and automates their registration and initialization. Note that you can place several Shell+ components onto the same SxDataModule as well as you can add multiple TSxDataModule instances to your project. It is easy to add new SxDataModule to the project - just click corresponding icon in the Repository:

Fig. 2. How to create new SxDataModule instance.

    You can save the example now.

Add the TSxExtractImage component now. This component handles shell requests of image extraction and allows you to easy customize its behavior using a few lines of code. As usual when you place the component onto SxDataModule the CLSID property is set with a newly generated GUID. It is this GUID will be used by Shell to identify your extension. Other properties are presented in the table below:

Table 1. TSxExtractImage properties.

    Use Object Inspector to set TSxExtractImage properties as shown below.

Fig 3. Setting TSxExtractImage properties

    Every time shell needs the image to display it calls a specific interface supplied by the extension library. You have not to care about all implementation details. The only thing you will have to do is to handle the OnExtractImage event. This event is fired as a result of shell request. The event handler should return the image and it is this image will be shown by the system.

    Select the Events tab of the Object Inspector and double click the OnExtractImage handler. The body of the handler will be created by Delphi, add the following code there:

procedure TSxModule1.SxExtractImage1ExtractImage(Sender: TSxExtractImage;
  Image: TBitmap; Size: TSxSize);
var
  TempBitmap:TBitmap;
begin
  TempBitmap:=TBitmap.Create; // create temporary image
  try
    // set it's size
    TempBitmap.Width:=Size.cx; 
    TempBitmap.Height:=Size.cy;
    // prepare to draw something
    TempBitmap.Canvas.Font.Name:='Arial';
    TempBitmap.Canvas.Font.Size:=5;
    // call auxiliary drawing function, its code follows below
    RenderTextFile(Sender.FileName,TempBitmap);
    // return the result
    Image.Assign(TempBitmap);
  finally
    TempBitmap.Free;
  end;
end;

    The rendering procedure is simple enough, it draws document contents as a plain text, line by line:

procedure RenderTextFile(FileName:String;var Bitmap:TBitmap);
var
  TempFile:TextFile;
  i:Integer;
  TempString:String;
begin
  AssignFile(TempFile,FileName);
  try
    Reset(TempFile);
    i:=0;
    repeat
      if not EOF(TempFile) then
         ReadLn(TempFile,TempString)
      else
         Break;
      Bitmap.Canvas.TextOut(0,i,TempString);
      i:=i+Bitmap.Canvas.TextHeight('A');
    until(i>Bitmap.Height);
  finally
    CloseFile(TempFile);
  end;
end;

    Save the project and compile it - it is ready now! As well as any other ActiveX library the Shell+ project requires registration. You can register it in several ways:

• from command prompt using MS regsvr32 utility: regsvr32.exe Project1.dll
• from command prompt using Borland's tregsvr utility: tregsvr Project1.dll

To know more about shell extension installation please follow the link.

    Create an empty text document in any acceptable folder and switch the folder view to Thumbnails mode. If the example works correctly then thumbnail image of the test document should be empty: