yaml vs xacro

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I would like to have a configuration file that I can set some properties like in box use-case here. I have 3 questions regarding this:

1. Which one better suits this purpose, using yaml or xacro?

2. Let say I want to have one property (boxOK, only to illustrate as below) or some of them to be parsed with a delay (I mean it fires after some time, say 20ms after loading the launch file), how do I achieve this?

   <id="width" value="1"/>
   <id="length" value="3"/>
   <id="height" value="3"/>
   <id="boxOK" value="true"/>
   
   <id="width2" value="3"/>
   <id="length2" value="3"/>
   <id="height2" value="4"/>
   
   <id="width3" value="2"/>
   <id="length3" value="3"/>
   <id="height3" value="3"/>
   

3. In any case, if I would set in either yaml or xacro, where in launch file normally I use something like below:

<param name="boxes_num" value="4" />

then, in the node I could invoke and assign like:

ros::param::get("boxes_num", boxes_num_);

My question is how would I do this using yaml or xacro.

Thanks in advance.