How to wait for an activity to be "ready" on android

I click on an element,
then I want to wait for the activity to be “ready” (ie has finish to launch)
to click to a button on this new activity

find_elements(:xpath, xpath_first_3dmodels)
exists { name "ar_view" }

But i do not understand why the exist is not blocking, and the back is immediately triggered,
what am i doing wrong ?

Take a look at this thread. I think you want to do a wait until current_activity produces the activity you desire before continuing.