shiny app : disable downloadbutton

Question

My shiny app produces some files that user can download. I have put downloadbutton in the ui for this purpose. However, when the page launches and before any calculation is

Answer 1

Based on your comment:

yes data processing depends on the user input. USer will upload some files and click anAction button to start the processing. The download button is in a tab set.

Let's say the action button is named input$start_proc.

In server.R:

shinyServer(function(input, output, session) {
   #... other code
   observe({
       if (input$start_proc > 0) {
           # crunch data...
           # when data is ready:
           session$sendCustomMessage("download_ready", list(...))
           # you can put extra information you want to send to the client 
           # in the ... part.
       } 
   })
   #... other code
})

Then in ui.R, you can write some javascript to handler the custom message event.

---

A full example is:

server.R

library(shiny)

fakeDataProcessing <- function(duration) {
  # does nothing but sleep for "duration" seconds while
  # pretending some background task is going on...
  Sys.sleep(duration)
}

shinyServer(function(input, output, session) {

  observe({
    if (input$start_proc > 0) {
      fakeDataProcessing(5)
      # notify the browser that the data is ready to download
      session$sendCustomMessage("download_ready", list(fileSize=floor(runif(1) * 10000)))
    }
  })

  output$data_file <- downloadHandler(
       filename = function() {
         paste('data-', Sys.Date(), '.csv', sep='')
       },
       content = function(file) {
         write.csv(data.frame(x=runif(5), y=rnorm(5)), file)
       }
  )
})

ui.R

library(shiny)

shinyUI(fluidPage(
  singleton(tags$head(HTML(
'
  
'
))),
  tabsetPanel(
    tabPanel('Data download example',
      actionButton("start_proc", h5("Click to start processing data")),
      hr(),

      downloadButton("data_file"),
      helpText("Download will be available once the processing is completed.")
    )
  )
))

In the example the data processing is faked by waiting for 5 seconds. Then the download button will be ready. I also added some "fake" fileSize information in the message to demonstrate that how you can send extra information to the user.

Note that because Shiny implements actionButton as tag instead of , and it binds click event on it. Therefore, in order to fully disable it, in addition to add a disabled attribute to make it appear to be disabled, you also need to override its click event by adding an inline onclick attribute. Otherwise the user can still accidentally click the (seemingly disabled) download button and triggers the download.