Apache FOP XSL-FO implementation and footers
“flows” in XSL-FO can have static areas surrounding them, which are naturaly used to make headers and footers on pages. Let’s consider sales invoice like this:
Here, of course, you can position header and footer absolutely using
<fo:block-container left="xx" top="xx" position="absolute">.
However, when your invoice grows and you run out of space on the page, and you want to see something like this:
invoice how it should page correctly
In XSL-FO standard this is achived using special static sections of the page:
<fo:page-sequence-master master-name="all-pages"> <fo:repeatable-page-master-alternatives> <fo:conditional-page-master-reference page-position="any" master-reference="any-page-id"/> <fo:conditional-page-master-reference page-position="last" master-reference="last-page-id"/> </fo:repeatable-page-master-alternatives> </fo:page-sequence-master>
Here you use master-reference parameter values your page master definition , for example:
Inside that master you can define different footer. A good example is given here http://www.xmlpdf.com/ibex-examples.html
Problem is, tracking which page is “last” is very tricky in free-flow documents. Take HTML for example, the common question about how to make something 100% high is hard to answer. The content can flow differently, and, more importantly, the element that is trying to make use of that position can affect its own position. For example, now in XSL-FO, a footer on the last page can be so high that the engine will have to create another page after the page that was considered “last” just to fit it. In which case, page before last will suddenly fit all content on the last page too, because it doesn’t have footer anymore. So engine has to make a decision on how to fix that in one or another position! What I am getting to is that Apache FOP doesn’t do that yet, and in general many complex things having to do with flow. It’s working just fine with everything else it seems. This is why current version is in “maintanence” now and developers are working on redesigning for next version (which doesn’t have name yet, but either 0.30 or 1.0
So what do you do in the mid time? Well, you have 3 workarounds (none of which is perfect). First one that is normally suggested is to use fo:footnote element to emulate the footer on the last page. Don’t forget that you have to have footnote reference text, otherwise it won’t generate anything:
<fo:block id="end"> <fo:footnote><fo:inline color="white">.</fo:inline> <fo:footnote-body>...
The problem with footnote is that you can’t use absolutely positioned blocks in it, and it seems to only like tables for fine formatting. So the example that i gave above on the picture might be hard to implement.
The second workaround is to use some code inside your program (before FOP call) to tell which items will be on last page, and put them into a different page master. That may be hard for some applications, again, like in example I gave, each line in the invoice can have wrapping text and as a result starts taking more space than originally estimated.
The last solution is not perfect either - don’t make special footer.This is what I ended up doing (and it was fine in my case, since every invoice page has same footer in my clients case). So in the end it looks like this: …
You may think of switching to commercial XSL-FO generator, like RenderX or Antenna House, which seem to support flows better, and overall are high quality commercial products.
For my own software I am thinking of supporting many different renderers, based on clients ability to buy commercial renderer or to live with Apache FOP limitations (which will be fixed some day).