ToElement methods do not export data from custom xml namespaces

[scpeters]

Environment

• OS Version: any
• Source or binary build? any

Description

• Expected behavior: if you have an SDFormat file with data in custom XML namespaces (like custom_elems_attrs.sdf), if you load the file into an sdf::Root and then write it out again with Root::ToElement(), the resulting Element-tree should also have the custom data fields.
• Actual behavior: it does not include the custom data fields.

Steps to reproduce

1. TODO: write code to reproduce this

Output

[azeey]

I think Root::Element() would work though and maybe ToElement should check if the DOM object was loaded from XML (i.e, has a valid Element()) and return that.

[scpeters]

I think Root::Element() would work though and maybe ToElement should check if the DOM object was loaded from XML (i.e, has a valid Element()) and return that.

but if you make changes via the DOM API, will those be reflected in the Element() output?

[azeey]

but if you make changes via the DOM API, will those be reflected in the Element() output?

They would not. I guess if we want that we'd have to store the custom elements and attributes somehow and incorporate them into the output of ToElement. It wouldn't be trivial though since each element of the DOM object might have a custom attribute or custom element.

[scpeters]

but if you make changes via the DOM API, will those be reflected in the Element() output?

They would not. I guess if we want that we'd have to store the custom elements and attributes somehow and incorporate them into the output of ToElement. It wouldn't be trivial though since each element of the DOM object might have a custom attribute or custom element.

One approach would be to add CustomData class to the SDFormat c++ classes that stores a Param_V of custom attributes and an ElementPtr_V of custom child elements and populates it during parsing. Drawbacks:

• This would require touching every single c++ class.
• The data hierarchy is more complicated for classes that have child elements without their own c++ class, such as //joint/axis/dynamics/* and //joint/axis/limit that are stored within sdf::JointAxis

[scpeters]

Based on this portion of Plugin_TEST.cc, I believe that data from custom XML namespaces in the Plugin::Contents (child elements of <plugin/>) will be correctly exported by Plugin::ToElement. Exporting custom attributes of <plugin/> is not currently supported. In order to export custom attributes from a <plugin/> element for use in gz-sim, in addition to modifying the sdf::Plugin constructor in sdf/Plugin.hh, the corresponding gz/msgs/plugin.proto message file would need to be updated as well.