[Seaside] Dynamic Images without WADocumentHandler

[Ken Treis]

I've got an application that needs to draw a graph, so I'm using Cairo  
to render it. This works really well. But I didn't realize until  
recently that code like this was problematic:

(html image)
	document: ws contents
	mimeType: 'image/png'

.. since it created WADocumentHandler instances that are never  
collected. I don't want these graphs to live forever in the image --  
they don't even need to be cached on the server, and they ought to be  
specific to a particular session. It's a similar situation to when I'm  
generating a CSV export of a client's data: the document is a one-time  
throw-away.

It seems like a callback is the right way to solve this (it's what I  
used in the CSV case too), so I hacked it together like so:

renderGraphOn: html
	(html image)
		class: 'graph';
		url:	(html context actionUrl copy addParameter:
					(html callbacks registerCallback:
							[self session respond:
									[:url |
									| ws |
									ws := ByteArray new writeStream.
									self writePngOn: ws.
									Seaside.WAResponse document: ws contents mimeType: 'image/ 
png']]))

This has the desired effect of linking the graph to the current  
session, and so far is appears to be free of the memory leak I was  
creating earlier using WADocumentHandler. Is this the right approach,  
or is there a better way? And if it's the right approach, can we add  
something like a #documentCallback: method to WAImageTag to make it  
easier to code things like this?

renderGraphOn: html
	(html image)
		class: 'graph';
		documentCallback:
			[| ws |
			ws := ByteArray new writeStream.
			self writePngOn: ws.
			Seaside.WAResponse document: ws contents mimeType: 'image/png']

--
Ken Treis
Miriam Technologies, Inc.