Busy or notSource:
Displays a spinner indicating if there are any calculations running on the server side.
character or list. If character given, then it selects from one of a few available loaders (look Details for more). By using list you can create your custom loder. List needs to have structure
loader <- list(css<-"CSS CODE", html <- div("CODE TO DISPLAY")).
character with position of the parameter. Default "top right".
Currently available loaders: "dots", "spinner".
Solution inspired by: https://colinfay.me/watch-r-shiny/.