Hi brynn,thank you for your suggestion!
I found that the text generated by Measure Path extension is not what I want cause it generates another text component, but I would use it as an alternative if I can't figure this out eventually.
Besides, this Measure Path extension menu has a tab for user to choose different input options, it solves a huge problem for me! I was wondering how to create different input options for different user demands, and it solves this problem perfectly! Many thanks!!
For the little error window suggestion, I saw a function called "inkex.errormsg()" in the source code. However, if this errormsg window is raised, the whole software is not accessible until I close this window, and I can't return to this page again. So it is not desirable for this situation.
Again, thanks a lot for your help!
I would keep waiting for others' opinions!