Use {later} to watch for sf objects and automatically mapview them

mapview_watcher.R

# This will watch your global environment for sf objects and mapview them automagically.
library(later)
library(mapview)

last_value = NULL

dir <- tempfile()
dir.create(dir)
htmlFile <- file.path(dir, "index.html")

get_sf_data = function() {
  dat = ls(envir = .GlobalEnv)
  cls = lapply(lapply(dat, get), class)
  spatdat = dat[grep("sf", cls)]
  lapply(spatdat, get)
}

mv_watch = function() {
  spatdat_lst = get_sf_data()  
  if (length(spatdat_lst) > 0 && !identical(spatdat_lst, last_value)) {
    m = mapview(spatdat_lst)
    mapshot(m, htmlFile)
    
    # (code to write some content to the file)
    rstudioapi::viewer(htmlFile)
    last_value <<- spatdat_lst
  }
  later::later(mv_watch, 0.25)
}

## initiate the watcher
mv_watch()

library(sf)

nc = st_read(system.file("shape/nc.shp", package="sf"))
nc = nc[, 1]