Two simple examples of iterative printing in Shiny

shiny-simple-iterative-printing.R

# By default Shiny will always wait for any reactive dependencies to finish (re)calculcating 
# before displaying the output that depends on them. Without async reactivity in Shiny (which 
# is not currently available, but it's in the works), it's actually not really possible to 
# make a reactive dependency be available before it finishes computing. But you can redefine 
# your reactive so that its output is broken though.
#
# You can achieve this using reactive values, an observer and invalidateLater(). If this is 
# new for you, I'd recommend, you read the documentation about invalidateLater().
# 
# I'll show two simple examples here. For a more elaborate example, see this gist.
#
# The first and probably best approach is to do something like:

library(shiny)

ui <- fluidPage(
  verbatimTextOutput("loc")
)

server <- function(input, output, session) {
  rv <- reactiveValues(lines = character(0), n = 0)
  
  observe({
    # Re-execute this reactive expression immediately after 
    # it finishes (though you can also specify a longer time:
    # e.g. 1000 means re-execute after 1000 milliseconds have
    # passed since it was last executed)
    invalidateLater(0, session)
    isolate({
      if (rv$n >= 100) {
        return()
      } else {
        rv$n <- rv$n + 1
        rv$lines <- c(rv$lines, paste("Loop var =", rv$n))
      }
    })
  })
  
  output$loc <- renderPrint({
    rv$lines
  }) 
}

shinyApp(ui, server)


# This will re-execute the observer repeatedly until you've reached rv$n = 100 (actually, 
# it will keep re-executing forever and it always stops because rv$n will never be 
# decremented once it reaches 100). So in the first iteration, rv$lines starts as nothing 
# and then becomes the string "Loop var = 1". It returns that and the renderPrint() call
# will print that to the screen. However, immediately after the observer finished, it will
# re-start and the rv$lines object will now have two strings:  "Loop var = 1" and 
# "Loop var = 2". It returns that and the renderPrint() call will print that to the screen.
# And so on... This happens very fast in the example above, but will be very clear if you 
# change the line invalidateLater(0, session) to something like a 1/4 of a second: 
# invalidateLater(250, session).
#
# This first approach is IMO intuitive and it stays neatly within the reactive framework 
# that Shiny strongly encourages. However, it does have one drawback: the renderPrint() 
# call will always print all the strings, each time that rv$lines changes, even though, 
# you're just adding a line at the end (so ideally you wouldn't have to re-print everything).
# This isn't an issue for 100 iterations of printing text, but for larger number or 
# heavier objects (images, for example), this can become noticeable.
#
# But there is another option that is fairly new to Shiny, which is to use insertUI(). 
# This option follows imperative, not reactive, logic, so I'd only recommend it if you feel 
# like you understand reactivity AND it makes sense for your use case. In particular, this 
# approach will let you print things one after the other (instead of always printing one 
# big block), but it loses track of them. If these are values that could change later, or 
# that you'd like to read in some other place, you shouldn't use this approach. (There are
# ways to use insertUI in more complicated ways, but that's another discussion, because I 
# don't think that's what you need in this post.) This has the advantage of being faster 
# and not keeping a possible very large set of strings/objects in memory. If this is new 
# for you, I'd recommend, you read the documentation about insertUI().

library(shiny)

ui <- fluidPage(
  div(id = "placeholder")
)

server <- function(input, output, session) {
  rv <- reactiveValues(n = 0)
  
  observe({
    invalidateLater(0, session)
    isolate({
      if (rv$n >= 100) {
        return()
      } else {
        rv$n <- rv$n + 1
        insertUI("#placeholder", ui = p("Loop var = ", rv$n))
      }
    })
  })
}

shinyApp(ui, server)

Mostly copied from my reply here: https://groups.google.com/forum/#!topic/shiny-discuss/0ExyzzaaXTU