app.R

library(shiny)
library(shinyjs)

appCSS <- "
#loading-content {
  position: absolute;
  background: #000000;
  opacity: 0.9;
  z-index: 100;
  left: 0;
  right: 0;
  height: 100%;
  text-align: center;
  color: #FFFFFF;
}
"

ui <- fluidPage(
  useShinyjs(),
  inlineCSS(appCSS),
  
  # Loading message
  div(
    id = "loading-content",
    h2("Loading...")
  ),
  
  # The main app code goes here
  hidden(
    div(
      id = "app-content",
      p("This is a simple example of a Shiny app with a loading screen.")
    )
  )
)

server <- function(input, output) {
  # Simulate work being done for 1 second
  Sys.sleep(1)
  
  # Hide the loading message when the rest of the server function has executed
  hide(id = "loading-content", anim = TRUE, animType = "fade")    
  show("app-content")
}

shinyApp(ui, server)