maybe a doc thing, how do i get data from the map?

my map is drawn. I want to know the the bounding rectangle as shown. 

I want to draw new markers from my database inside that rectangle

and then if the user changes it, I want to do that again ...  new center, new markers..   for move and zoom
(I assume use watch on the ref)

and I also want to add a slider for user to zoom in/out..  