Published: Wednesday, February 21, 2001
Charting with Office Web Components (OWC), Part 3
By Bret Hern
Read Part 1
Read Part 2
In Part 2 we looked, in great detail, at a sample
ASP application that displayed database information in a nice-looking line graph. This graph
was saved as a GIF image on the Web server - over time these can accumulate and clutter up your
Web server's hard drive space. In this final part of this article we'll examine how to clean
up unused graphs!
Cleaning Up After Yourself
Using the absolute and relative paths we defined at the top, our ExportChartToGIF
function executes the chartspace ExportPicture method to export the generated
chart to an image format:
Function ExportChartToGIF(objCSpace, strAbsFilePath, strRelFilePath)
'Get a random filename
Dim strFileName
Randomize
strFileName = Timer & Rnd & ".gif"
'Save the image as a GIF
objCSpace.ExportPicture strAbsFilePath & "\" & strFileName, _
"gif", 600, 350
'Return the path to the GIF image of the graph
ExportChartToGIF = strRelFilePath & "/" & strFileName
End Function
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The lines immediately prior to the ExportPicture method being called are to
define a safely unique filename as the target for the exported image file. There are
alternative methods of defining temporary files for this sort of activity, but the
combination of the timer function and a random number will be sufficient for our needs --
and less resource intensive than using the temp file generation method of the FileSystemObject
object, for example (see this FAQ).
When coupled with the cleanup process described below, this export
method is safe and efficient. Also note the mild tap dance with relative paths vs. absolute
paths -- the ExportPicture method requires an absolute path, while for
safety's sake, a relative path is used to refer to the image file defined.
The parameters for the ExportPicture method are reasonably obvious -- the
final two parameters are the horizontal and vertical size of the image to be created (in
pixels).
While a simple process, exporting the chart to a graphic file does present a lingering
challenge: what to do with the file after it is displayed? The graphic file cannot be
deleted until after the script is complete and the page rendered. Therefore, without some
form of cleanup process, the number of GIF files will grow unchecked as users visit the
site. Several examples of OWC solutions in the available literature suggest either
streaming the image file as binary data to the browser, which does allow for immediate
deletion of the file within the script, or the use of sessions to permit files to be
deleted at session end. These methods are certainly reasonable and acceptable, but I prefer
an alternative approach that, when paired with the export function described above, yields
excellent results while retaining developmental simplicity:
Sub CleanUpGIF(GIFpath)
Dim objFS
Dim objFolder
Dim gif
set objFS = Server.CreateObject("Scripting.FileSystemObject")
set objFolder = objFS.GetFolder(GIFpath)
'Loop through each file in the GIFpath folder
for each gif in objFolder.Files
'Delete GIF files older than 10 minutes
if instr(gif.Name, ".gif") > 0 and _
DateDiff("n", gif.DateLastModified, now) > 10 then
objFS.DeleteFile GIFpath & "\" & gif.Name, True
end if
next
set objFolder = nothing
set objFS = nothing
End Sub
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The CleanUpGIF function simply reviews the folder passed to it for GIF files
of a certain age -- I've set it at 10 minutes -- and deletes them if older than that
threshold. This way, files are continuously being safely cleaned up after they've been
rendered and viewed. This method also works perfectly well in any sort of load balanced
situation, something which can give session-based methods a bit of heartburn.
Summary
The primary benefit of this approach to image file management is the directness of the
approach. It relies only on the Office Web Components themselves, and can be deployed
without sessions or additional componentry. By simplifying the delivery approach, we lower
the barrier to entry for this valuable toolset. It won't make up for the sketchy
documentation, but it's a start.
Happy Programming!
By Bret Hern
Attachments and Related Articles:
Download the source code in text format
Visit the Graphing with ASP Article Index
Read Using Office Web Components
Read Making Charts in ASP (using the Office Web Components (OWCs))
