Overview
If you write your
XRC files
by hand (which is probably relatively rare as most people prefer to edit them
visually, but it does happen), and if you are human (which is probably not so
rare), you are bound to make mistakes in them. Some of these mistakes result
in invalid XML which give errors when loading the XRC file, some of them are
not a big deal and are just ignored by the XRC parser, but some others can
result in the layout mysteriously not working as expected, which can be
annoying. Validating XRC files, i.e. verifying that they conform to the scheme
describing all the valid XRC elements, allows to avoid all these problems at
once, so it is strongly recommended to do it -- and this post will explain
how, in details.
Setup
To validate them you need two things: a schema and a tool using it. The schema
exists in
our repository
since October 2013 and is also available at the public URL
http://www.wxwidgets.org/wxxrc.
As for the
tools, anything
supporting compact RELAX NG schema syntax can work, but in practice this seems
to limit the choices either to the online validator at
validator.nu (
Edit: this validator
can't be used for validating XRC currently as RELAX NG support
seems to be broken) or the original command-line
validator, written by James Clark, one of the persons behind RELAX NG, called
Jing, and the
offline version is, unsurprisingly, much more useful as it can be integrated
into the build process or used as part of pre-commit hook checks easily, so
this is the one we are going to look at.
The latest Jing release is available from
Google Code,
but as the entire site will be
closing soon,
it might not work any longer by the time you are reading this, so
here is
the smaller and more up-to-date version. It is completely standalone and
requires just a working Java installation, you can put the JAR file anywhere
you want and simply run
$ java -jar /full/path/to/jing/jar
(in spite of the dollar sign indicating the Unix prompt, this works under
Windows, too). The basic Jing command line syntax is just the schema file
followed by any number of files to validate, however XRC schema uses the
compact syntax which has to be explicitly selected by its
-c option,
so in the simplest case the full command line would look like
$ java -jar jing.jar -c $WXWIN/misc/schema/xrc_schema.rnc myfile.xrc
where
WXWIN environment variable is supposed to contain the full path
of your wxWidgets installation.
As an aside, it is also possible to avoid hard coding the path to the XRC
schema, which may be important to make the validation step work on different
machines. The natural way to do it would be to just use the canonical URI
instead of the path to the schema, i.e.
http://www.wxwidgets.org/wxxrc
but currently this doesn't work, seemingly because of a bug in Jing handling
HTTP-to-HTTPS redirections. The following command does work:
$ java -jar jing.jar -c https://www.wxwidgets.org/wxxrc myfile.xrc
but still has two problems: first, the URI is, formally speaking, wrong as it
should be using "http" schema. Second, and probably more importantly in
practice, it can be slow as the file needs to be downloaded from network every
time. To fix this problem, an
XML catalog mapping
the canonical URI to a local file can be used. I won't pretend to know much
about XML catalogs (because I don't), but here is a minimal one that can be
used to set up the correct redirection:
If you save the above file as
catalog.xml and adjust the value of the
uri attribute to contain the correct path to
xrc_schema.rnc
on your machine (notice that triple slashes
are needed), you should
be able to use
$ java -jar jing.jar -C catalog.xml -c http://www.wxwidgets.org/wxxrc myfile.xrc
Using (or not) Custom XRC Elements
After the setup describe above you can use the standard schema
xrc_schema.rnc to validate the contents of all the standard XRC
elements, such as
<sizeritem> or
<object
class="wxButton">. However any custom elements are simply ignored by
default because the standard schema has no knowledge of them. Of course, you
might not have any custom XRC elements at all. In this case, you should use
xrc_schema_builtin_only.rnc, located in the same
$WXWIN/misc/schema directory of your wxWidgets installation, to
forbid any of them from appearing. There is no canonical URI for it, so you
should simply pass full path to it on Jing command line.
The more interesting case is when you do define some custom XRC handlers. In
this case using
xrc_schema_builtin_only.rnc would result in errors
about invalid value of attribute "class" for all your custom elements, so you
need to define a custom schema describing just them. An example of doing it is
shown in
misc/schema/README,
but here is an even simpler custom schema:
This schema allows appearance of a custom
Frobnicator window-like
(because of
stdWindowProperties inclusion) element which can have one
specific (but optional, because of the trailing
*)
num_times
attribute, i.e. would validate the following XRC fragment:
if you issue
$ java -jar jing.jar -C catalog.xml -c frob.rnc frob.xrc
command. Of course, the same fragment would also be accepted by the standard
schema, the real benefit of using a custom one is that typos in either
"Frobnicator" or "num_times" would be detected only by the latter.
TL;DR Summary
Your easy guide to XRC validation:
- Download jing.jar
- For one-time validation of only standard XRC elements just run
$ java -jar jing.jar -c https://www.wxwidgets.org/wxxrc myfile.xrc
- For repeated use, download
XML catalog,
edit the file path in it and run
$ java -jar jing.jar -C catalog.xml -c http://www.wxwidgets.org/wxxrc myfile.xrc
- If you use custom XRC elements, consider defining a schema for them too,
it is simple to do.
