Good question :-)
I will answer twice and both times be right (don't mind me I am a bit punch
drunk from writing docs):
A: The fallback value *is* needed in case you are going to encode a document -
no sense throwing out information.
A: In a related manner you may be constructing a style for use with another
system does not implement the function.
An easy example is a previous version of a GeoTools; or an environment that
does not have some of your custom functions defined.
So that scary SE quote is correct, we may not need to use the value at
execution time, however it is still of use.
And idea occurs to me - we could also use the fallback value when rendering a
style "quickly"; ie configure the renderer to not call functions for a quick
preview; and then do the full render if the user is interested. The fallback
value is always a literal so the resulting style would be "fast".
--
Jody Garnett
On Thursday, 28 April 2011 at 8:37 PM, Michael Bedward wrote:
> Hello Jody,
>
> Many thanks for this. It looks great.
>
> I have a question that is peripheral to the tutorial, and that I know
> we've gone over in the past: what is the valid use of a fallback value
> with filter functions ? The Symbology Encoding spec says:
>
> "The value of the fallbackValue attribute is used as a default value,
> if the SE implementation does not support the function. If the
> implementation supports the function, then the result value is
> determined by executing the function."
>
> In that case, a function shouldn't even have a fallback value field should it
> ?
>
> I remember struggling with this when helping to do the "env" function.
> If you look at the code for EnvFunction you'll see that a "default"
> value can be supplied as an optional extra arg to the evaluate method
> (this was Andrea's suggestion for how to have a default while not
> violating the spec). If the setFallbackValue is called an warning is
> logged.
>
> Sorry that this is banal feedback on a nice tutorial - but it's so
> good to have a teaching page about filter functions at last so I think
> it's worth checking that it's squeaky clean.
>
> Michael
>
>
> On 28 April 2011 01:08, Jody Garnett <[email protected]> wrote:
> > Available now:
> > - http://docs.geotools.org/latest/userguide/tutorial/advanced/function.html
> > Review and feedback are of course welcome.
> > --
> > Jody Garnett
> > --
> > Jody Garnett
> >
> >
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