app.R

library(shiny)
library(shinyjs)

jscode <- "
// Define a JavaScript function to navigate the page to where the file is served
shinyjs.navigate = function(url) {
  window.location.href = window.location.origin + window.location.pathname + url;
}
"

ui <- fluidPage(
  useShinyjs(),
  extendShinyjs(text = jscode, functions = c("navigate")),
  
  # Portion of the app that lets the user cutomize a plot and then see
  # a URL to serve that plot
  hidden(div(id = "app",
    h3("Serve an image of a plot from 1 to n"),
    sliderInput("num", "n", 1, 50, 10),
    selectInput("col", "Colour", 
                choices = c("black", "brown", "red", "blue", "green")),
    uiOutput("link")
  )),
  
  # Portion of the app shown when a user is on a page that serves a file
  hidden(div(id = "creating", h3("Creating the file...")))
)

server <- function(input, output, session) {
  
  # Show the user a friendly URL for accessing the plot 
  output$link <- renderUI({
    queryBuild <- sprintf("?num=%s&col=%s", input$num, input$col)
    externalUrl <- sprintf(
      "%s%s%s",
      session$clientData$url_hostname,
      session$clientData$url_pathname,
      queryBuild
    )
    
    tags$a(externalUrl, href = queryBuild, target = "_blank")
  })
  
  # Logic for associating the URL with the plot
  observe({
    query <- parseQueryString(session$clientData$url_search)
    
    # if we see a 'num' GET param, it means this is a request for a plot image
    if (!is.null(query$num)) {
      
      show("creating")
      
      # create a URL endpoint for the image
      myurl <- session$registerDataObj(
        name   = 'getplot',
        data   = query,
        filter = function(data, req) {
          
          # create the image (or it could be a text file or any other file)
          # the file needs to be saved so that it's possible to read it
          image <- tempfile()
          col <- data$col
          tryCatch({
            png(image, width = 600, height = 600)
            plot(seq(1, as.numeric(data$num)), main = date(), col = col)
          }, finally = dev.off())
          
          # submit an OK response, sending the file (image) as the content
          # if you're serving a different type of file, remember to change
          # the "image/png" to the correct MIME type
          shiny:::httpResponse(
            200, 'image/png', readBin(image, 'raw', file.info(image)[, 'size'])
          )
        }
      )
      
      # After creating the requested file and associating it with a URL, go
      # to that webpage using JavaScript
      js$navigate(myurl)
    } else {
      # if this is not a request for a file, show the Shiny app
      show("app")
    }
  })
}

shinyApp(ui, server)