feat(args): Allow specifying dynamic arg

[peci1]

This PR allows a macro to specify args whose names are defined by the macro params, e.g.

<xacro:macro name="test" params="name">
  <xacro:arg name="${name}_arg" default="test" />
  <link name="$(arg ${name}_arg)" />
</xacro:macro>
<xacro:test name="a" />

This Xacro snippet defines argument a_arg which can be either defaulted or read from CLI args.

Without this PR, the above specification works only when the arg is passed from CLI and it doesn't work when the default should have been used.

I know this is a bit of double evaluation, but this one seems pretty useful to me. The use-case I'd like to get is a general macro for calibrated joints like this one:

<xacro:macro name="calibrated_fixed_joint" params="name parent child prefix:='' lump:='1'">
    <xacro:if value="${prefix == ''}">
        <xacro:property name="pref" value="${child}" />
    </xacro:if>
    <xacro:if value="${prefix != ''}">
        <xacro:property name="pref" value="${prefix}" />
    </xacro:if>
    <xacro:arg name="${pref}_pos_x" default="0.0" />
    <xacro:arg name="${pref}_pos_y" default="0.0" />
    <xacro:arg name="${pref}_pos_" default="0.0" />
    <xacro:arg name="${pref}_rpy_r" default="0.0" />
    <xacro:arg name="${pref}_rpy_p" default="0.0" />
    <xacro:arg name="${pref}_rpy_y" default="0.0" />
    <xacro:fixed_joint name="${name}" parent="$(arg prefix)${parent}" child="$(arg prefix)${child}"
                       xyz="$(arg ${pref}_pos_x) $(arg ${pref}_pos_y) $(arg ${pref}_pos_z)"
                       rpy="$(arg ${pref}_rpy_r) $(arg ${pref}_rpy_p) $(arg ${pref}_rpy_y)"
                       lump="${lump}"/>
</xacro:macro>

[rhaschke]

Thanks for this improvement. Merged via da4b384 / 5dccc59.

[peci1]

Thanks! I see you're still keping noetic-devel alive, that's nice. I guess this is for ROS-O?

[rhaschke]

Thanks! I see you're still keping noetic-devel alive, that's nice. I guess this is for ROS-O?

Yes, indeed.