how to get paragraph index in method onUpdate() ?

[treefung]

example:

content： 
`
<p>the firsr paragraph ( index 0 )</p>
<p>the second paragraph ( index 1 )</p>
<p>the third paragraph ( index 2 )</p>
<p>.... ( index n )</p>
`

if I change third paragraph <p>the third paragraph</p> to <p>the third paragraph change content</p>

how i get paragraph index in method onUpdate() ? it should be index 2

tks