add choice element to NXDL

[prjemian]

As part of adding new geometry definitions to NXdetector, it is proposed to extend the NXDL to allow for specification of multiple choices for a named group or field.

The proposed use seems clear, to create a group with a defined name that is one specific NXDL class from a list of possibilities. The immediate children of the <choice> should all be of the same type (either all <field> or all <group> elements) and all should have the same @name attribute.

[prjemian]

See the two examples of <choice> highlighted in commit a55aa9c .

[prjemian]

created branch "choice_element_619" for this work

[zjttoefs]

This has code (in a branch), so should probably go into the next milestone, if possible.

[prjemian]

@matthew-d-jones , @zjttoefs , @mkoennecke

The example code shows:

<choice>
    <group name="pixel_shape" type="NXoff_geometry">...</group>
    <group name="pixel_shape" type="NXcylindrical_geometry">...</group>
</choice>

To ensure that the @name attribute is defined once, I'm going to change that syntax to:

<choice name="pixel_shape">
    <group type="NXoff_geometry">...</group>
    <group type="NXcylindrical_geometry">...</group>
</choice>

Now, the @name attribute is defined once and we know that any of the choices must have only that name.

[prjemian]

test the revised schema using this command:

xmllint --noout --schema nxdl.xsd contributed_definitions/NXpete.nxdl.xml

[prjemian]

before we close this ticket, delete the test file and update the documentation

[prjemian]

• end of defs_intro.rst
• autogenerated content in nxdl_desc.rst

[zjttoefs]

There is no need for choice for fields now. I can easily see this for the future, but I am happy to wait for the code until we cross that bridge.

The changed syntax to me isn't better or worse for this current use case. I've not looked into it in detail, but it does not less flexible. Can you only have a choice between things that are supposed to have the same name now? I can come up with a good example, but for transitioning to newer versions of the definitions you could have a choice between old_scheme and new_scheme (NXgeometry vs NXtransformation maybe), where the names don't have to be the same.
I'm mainly checking I understand it correctly, because all we want to have right now appears to work.

[prjemian]

Your example of a this-or-that choice with different names is already covered by the existing NXDL Schema, isn't it? One defines the two components and makes them both optional. The existing NXDL Schema does not cover the case where we must have one or the other.