Vuetify — watch data you want when you toggle

There is a toggle action when it’s open and close that i wanted to watch.
i’ve spent like a week on this then finally i figured out.
just a simple way to do.
“v-model”
shit.
i can’t believe i spent a week on this.

who knows that i actually need to use v-model=”active” ?

#link

https://vuetifyjs.com/en/components/lists/#nested-lists