[xep-support] Marker values in expressions

[G. Ken Holman]

At 2004-01-22 17:51 +1100, Ben Reid wrote:
>Would I be right in saying that you cannot use <fo:markers> and 
><fo:retrieve-marker>
>in calculations?

Correct, though you can use markers to store the results of calculations 
made at transformation time to be retrieved at rendering time.

>That is if I define a value using a marker, it cannot be accessed through 
>an expression
>in an <xsl:value-of select=""/>.

Correct, it cannot.  <xsl:value-of/> is an expression in XSLT and the 
*result* of XSLT is passed on to an XSL-FO engine for rendering without any 
feedback loop.

My first comment is that you can repetitively calculate intermediate totals 
from a stream of data, seeding the result in a marker at each stage with 
the intermediate total at that stage, and then when the FO engine lays out 
a page it can retrieve into static content the first or the last of the 
markers from the page to allow you to render that particular intermediate 
result (of the very many that you calculate) on the page in which the 
result appears.

Note that XSL-FO *does* have an expression language, but only for property 
values and not for displayed values.  If you have a complex property value 
calculation you can simplify your work by producing an expression in the 
property that gets calculated by the FO engine, rather than making 
contortions in your XSLT to figure that value out.

I hope this helps.

................ Ken

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