Hi,
so following up from last week discussion, it
seems there is currently no supported way to know
what the arguments of a function are, you are
supposed to know before hand (Jody mentioned
the FunctionFinder, but that does not provide
any argument lookup method).
So it seems that to get unstuck I will have to
make a change proposal affecting both GeoTools
and GeoApi.
In particular, I'm thinking to propose something
along these lines:
- register a factory for the functions, not the
functions themselves
- a factory can create multiple functions and
provide metadata about them (the number of
arguments and the return type, -1 if the
number of arguments cannot be determined
beforehand)
- for most of the existing functions implementing
FunctionExpression, we can just create a factory
that looks up the functions, and grabs the
metadata using the FunctionExpression interface
(which we could un-deprecate if we make it stop
extending org.geotools.filter.Expression)
- for the few others (if any) a new factory
will be created
This would solve the lookup issue at the GeoTools
level. At the GeoApi level things are a bit more
complicated because the function lookup support
has been mutilated quite a bit.
We should at least:
- add back FilterCapabilities to FilterFactory
- add getFunctions to FilterCapabilities,
which in turn provides the getFunctionNames
and getFunctionName methods
- decide if FunctionName interface is good for
us, or not. At the moment we do have the following:
public interface FunctionName extends Operator {
/**
* Number of arguments the function accepts.
*/
int getArgumentCount();
/**
* Argument names for documentation purposes if known.
List<String> getArgumentNames();
}
Hum, no types? Oh well, I could go for that. Going into
all the function implementations and adding the name
metadata can be somewhat painful, but as a stop gap
measure we could just name them "arg1", "arg2", ...
in FunctionExpressionImpl.
Opinions?
Cheers
Andrea
--
Andrea Aime
OpenGeo - http://opengeo.org
Expert service straight from the developers.
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