Hi,
I'm stumbling in a few places in GeoTools where it's hard to perform
a switch to the GeoApi filter API because of a design issue.
Basically, GeoApi does not seem to offer any way to figure out
the function arguments count and their types. Given that functions
are not standardized as filters, and that at least in GeoTools
they are pluggable, any random use can add some, this is a
major problem: the API is extensible, but not machine discoverable,
it seems that whoever designed it thought only about a human
using it, a human that discovered the functions and their
structure by some other means (supposedly documentation).
The GeoApi Function javadoc says that FilterCapabilities
should be used to provide the intel on what functions
are available, and what arguments they do provide.
Unfortunately someone had a change of mind and commented
out FilterFactory.getFilterCapabilities(), and someone
else just removed whatever function information from
the FilterCapabilities interface, leaving GeoApi
Function utterly unusable from the point of view of
code that needs to create functions automatically.
Unless I'm missing something, this has to be rectified
in order to be able and switch the source code to GeoApi
functions, but frankly, it has to be rectified also
because it's just plain wrong to have an extensible
but non discoverable API.
If filter functions are supposed to be be immutable,
something else needs to be used in order to discover their
structure, something like the factory itself providing a
list of class types for the arguments (and thus implicitly
the count of them), or like introduction a factory whose
task in life is to provide metadata about the function
it's going to instantiate, and to instantiate it, something
like:
interface FunctionFactory {
List<Class> getParameterTypes();
List<InternationalString> getParamerNames();
Function create(List<Object> parameterValues);
}
Actually for parameters descriptions we could use something
like org.geotools.data.Parameter, thought that seems a little
too much for just function arguments.
Hmmm... thoughts?
Cheers
Andrea
--
Andrea Aime
OpenGeo - http://opengeo.org
Expert service straight from the developers.
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