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Cooking with ASP.NET, Part 2
Pages: 1, 2

Recipe 18.6: Saving and Reusing HTML Output

Problem

To improve the performance of pages that rarely change, you want to capture the output of those pages and save it for reuse when the page is requested.

Solution

Create the page that contains the desired content just as you would any other page, including the server controls you need. At the end of the Page_Load method, use the RenderControl method of the Page control to generate the HTML and then save the HTML to a file.



In the code-behind class for the page, use the .NET language of your choice to:

  1. Create an HtmlTextWriter to use for rendering the page.

  2. Use the RenderControl method of the Page control to render the output of the page to the HtmlTextWriter.

  3. Save the rendered output to a file and redirect to another page.

Example 18-16 and Example 18-17 show the VB and C# code-behind files for our application that demonstrates this solution.

Discussion

Occasionally, it's beneficial to save the HTML output from a generated page. This is commonly done when using the saved HTML can significantly improve web site performance. If the content of a page is static, for example, there is no point in dynamically generating HTML each time the page is requested. Until the advent of ASP.NET, the only way to save the HTML was to use the "Save as Complete Web Page" feature of Internet Explorer or another browser. Although this method does save the HTML, it also copies all of the page images to the local machine and changes the image references to point to the local copies. If you are trying to improve performance by capturing a static copy of the page to use on your web server, this technique does not work very well.

With ASP.NET you can easily capture the HTML exactly as it would be sent to the browser. For our example that illustrates this solution, we have used the page from Recipe 18.3 and added code to the Page_Load method to save the rendered output.

The RenderControl method of the Page control provides the ability to render the output of the page to the HtmlTextWriter passed to the method. Unfortunately, the HtmlTextWriter does not provide any methods for reading the contents, so a little more work is required to access the rendered HTML.

By first creating a StringBuilder and then using it to create a StringWriter, which is then used to create the required HtmlTextWriter, the contents of the HtmlTextWriter are available by way of the original StringBuilder. This works because the underlying storage mechanism for the StringWriter is a StringBuilder; and because the StringWriter (a stream) is used to create the HtmlTextWriter, the RenderControl method is actually writing the rendered output to the StringBuilder. Our example code to accomplish this is shown here:


VB
  renderedOutput = New StringBuilder( )
  strWriter = New StringWriter(renderedOutput)
  tWriter = New HtmlTextWriter(strWriter)

C#
  renderedOutput = new StringBuilder( );
  strWriter = new StringWriter(renderedOutput);
  tWriter = new HtmlTextWriter(strWriter);

After creating the HtmlTextWriter, the RenderControl method of the Page is called to render the HTML for the page:


VB
  Page.RenderControl(tWriter)

C#
  Page.RenderControl(tWriter);

Now that the rendered HTML is available, it needs to be saved to a file on the server. This can be accomplished by creating the file with a FileStream and using a StreamWriter to write the rendered output in the StringBuilder to the file, as shown here:


VB
  filename = Server.MapPath(".") & "\" & OUTPUT_FILENAME
  outputStream = New FileStream(filename, _
                                FileMode.Create)
  sWriter = New StreamWriter(outputStream)
  sWriter.Write(renderedOutput.ToString( ))
  sWriter.Flush( )

C#
  filename = Server.MapPath(".") + "\\" + OUTPUT_FILENAME;
  outputStream = new FileStream(filename,
                                FileMode.Create);
  sWriter = new StreamWriter(outputStream);
  sWriter.Write(renderedOutput.ToString( ));
  sWriter.Flush( );

The last step is to redirect to another page. This is necessary because allowing the page to be displayed would result in an additional rendering and an exception being thrown indicating the page has more than one server-side form element. If you need the page to be displayable anyway, a parameter can be passed in the querystring and checked in the code to determine if the output should be rendered and written to a file or handled normally.


VB
  Response.Redirect([next page])

C#
  Response.Redirect([next page]);

This technique can be used for individual controls in the same manner as for the entire page. For example, if you have a page that contains a DataGrid and you want the rendered HTML for just the DataGrid, you can call the RenderControl method of the DataGrid and then save the output as described earlier.

See Also

Recipe 18.3

Example 18-16. Capturing rendered output (.vb)

  Private Sub Page_Load(ByVal sender As System.Object, _
                        ByVal e As System.EventArgs) Handles MyBase.Load

    Const OUTPUT_FILENAME As String = "CH18CaptureRenderedOutputVB.html"

    Dim renderedOutput As StringBuilder
    Dim strWriter As StringWriter
    Dim tWriter As HtmlTextWriter
    Dim outputStream As FileStream
    Dim sWriter As StreamWriter
    Dim filename As String
    Dim nextPage As String

    Try
      'set the names of the XML and XSLT documents used in the 
      'transformation
      xmlTransform.DocumentSource = "xml/books.xml"
      xmlTransform.TransformSource = "xml/books.xslt"

      'create a HtmlTextWriter to use for rendering the page
      renderedOutput = New StringBuilder
      strWriter = New StringWriter(renderedOutput)
      tWriter = New HtmlTextWriter(strWriter)
      'render the page output
      Page.RenderControl(tWriter)
      'save the rendered output to a file
      filename = Server.MapPath(".") & "\" & OUTPUT_FILENAME
      outputStream = New FileStream(filename, _
                                    FileMode.Create)
      sWriter = New StreamWriter(outputStream)
      sWriter.Write(renderedOutput.ToString( ))
      sWriter.Flush( )
      'redirect to another page
      'NOTE: Continuing with the display of this page will result in the 
      '      page being rendered a second time which will cause an exception 
      '      to be thrown
      nextPage = DisplayMessage.PAGE_NAME & "?" & _
                 DisplayMessage.QS_PAGE_HEADER & "=Information" & "&" & _
                 DisplayMessage.QS_MESSAGE_LINE1 & "=HTML Output Saved To " & _
                 OUTPUT_FILENAME
      Response.Redirect(nextPage)

    Finally
      'clean up
      If (Not IsNothing(outputStream)) Then
        outputStream.Close( )
      End If

      If (Not IsNothing(tWriter)) Then
        tWriter.Close( )
      End If

      If (Not IsNothing(strWriter)) Then
        strWriter.Close( )
      End If
    End Try
  End Sub  'Page_Load

Example 18-17. Capturing rendered output (.cs)

private void Page_Load(object sender, System.EventArgs e)
{
  const string OUTPUT_FILENAME = "CaptureRenderedOutput_VB.html";

  StringBuilder renderedOutput = null;
  StringWriter strWriter = null;
  HtmlTextWriter tWriter = null;
  FileStream outputStream = null;
  StreamWriter sWriter = null;
  String filename = null;
  String nextPage = null;

  try
  {
    // set the names of the XML and XSLT documents used in the 
    // transformation
    xmlTransform.DocumentSource = "xml//books.xml";
    xmlTransform.TransformSource = "xml//books.xslt";

    // create a HtmlTextWriter to use for rendering the page
    renderedOutput = new StringBuilder( );
    strWriter = new StringWriter(renderedOutput);
    tWriter = new HtmlTextWriter(strWriter);
    // render the page output
    Page.RenderControl(tWriter);
    // save the rendered output to a file
    filename = Server.MapPath(".") + "\\" + OUTPUT_FILENAME;
    outputStream = new FileStream(filename,
                                  FileMode.Create);
    sWriter = new StreamWriter(outputStream);
    sWriter.Write(renderedOutput.ToString( ));
    sWriter.Flush( );
    // redirect to another page
    // NOTE: Continuing with the display of this page will result in the 
    //       page being rendered a second time which will cause an exception 
    //       to be thrown
    nextPage = DisplayMessage.PAGE_NAME + "?" +
               DisplayMessage.QS_PAGE_HEADER + "=Information" + "&" +
               DisplayMessage.QS_MESSAGE_LINE1 + "=HTML Output Saved To " +
               OUTPUT_FILENAME;
    Response.Redirect(nextPage);
  }

  finally
  {
    // clean up
    if (outputStream != null)
    {
      outputStream.Close( );
    }

    if (tWriter != null)
    {
      tWriter.Close( );
    }

    if (strWriter != null) 
    {
      strWriter.Close( );
    }
  }
}  // Page_Load

Mikkel Aaland is a professional photographer whose pioneering work in digital photography dates back to 1981. He is the author of nine books including the bestselling Photoshop Elements Solutions and O'Reilly's acclaimed Photoshop Lightroom Adventure. Visit his website at http://www.shooting-digital.com.

Geoffrey T. LeBlond


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