Example shiny dynamic download

app.R

# Author: dkulp2
#
# Shiny example for dynamically downloading a file related to a specific row.
#
# A DT datatable is created with options that adds a 'download-control' class to a column with a download icon 
#   and a callback to set up the click event on the 'download-control' cells.
# The javascript click event determines which page of the datatable is being displayed and which row the click event occurred.
#   This approach fails if rows are reordered or the table is filtered, so both are disabled. An alternative is to place the 
#   row ID in a "data-" attribute, which would allow reordering and filtering.
#   It then sends a Shiny event, 'download-row', with the index of the row in the table.
# The shiny event then creates the data file based on the row number and sends the raw data to javascript via the 'download-data' message.
#   In this example I create a data frame with the row index in it and send it as an RDS.
# The javascript message handler for 'download-data' decodes the raw data and triggers the client to download the data as a file.
#   This approach will not perform well for large data due to latency and memory. An alternative is to store the file
#   dynamically in the www/ folder and send the URL to the client.

library(shiny)
library(DT)

JS.row.dl <- htmlwidgets::JS("
                    table.on('click', 'td.download-control',
                              function() {
                                start=table.page.info().start
                                i=$(this).closest('tr')[0].rowIndex;
                                Shiny.setInputValue('download_row', {row:start+i}, {priority:'event'})
                              });")

ui <- fluidPage(
  div(style="display: none", icon("")), # hack to trigger inclusion of icon library
  dataTableOutput("myTable"),
  includeScript("download.js")
)

server <- function(input, output, session) {
  output$myTable <- renderDataTable(DT::datatable(data.frame(' '=as.character(icon('download')), 'alphabet'=LETTERS),
                                                  escape=FALSE,
                                                  selection='single',
                                                  options=list(ordering=FALSE,
                                                               dom="ltip", # disable filtering
                                                               columnDefs=list(list(className='download-control',targets=1))),
                                                  callback=JS.row.dl))
  
  observeEvent(input$download_row, {
    row.i <- input$download_row$row
    message("clicked row: ",row.i)
    rc <- rawConnection(raw(0), "r+")
    saveRDS(data.frame(x=row.i), rc)
    data <- rawConnectionValue(rc)
    close(rc)
    session$sendCustomMessage("download-data", 
                              list(row=row.i, data=data, filename='df.RDS'))
  }, ignoreInit=TRUE)
  
}

shinyApp(ui = ui, server = server)

download.js

// https://stackoverflow.com/a/20151856
function base64toBlob(base64Data, contentType) {
  contentType = contentType || '';
  var sliceSize = 1024;
  var byteCharacters = atob(base64Data);
  var bytesLength = byteCharacters.length;
  var slicesCount = Math.ceil(bytesLength / sliceSize);
  var byteArrays = new Array(slicesCount);
  
  for (var sliceIndex = 0; sliceIndex < slicesCount; ++sliceIndex) {
    var begin = sliceIndex * sliceSize;
    var end = Math.min(begin + sliceSize, bytesLength);
    
    var bytes = new Array(end - begin);
    for (var offset = begin, i = 0; offset < end; ++i, ++offset) {
      bytes[i] = byteCharacters[offset].charCodeAt(0);
    }
    byteArrays[sliceIndex] = new Uint8Array(bytes);
  }
  return new Blob(byteArrays, { type: contentType });
}

function saveData(blob, fileName) // does the same as FileSaver.js - https://stackoverflow.com/a/33664602
{
  var a = document.createElement("a");
  document.body.appendChild(a);
  a.style = "display: none";
  
  var url = window.URL.createObjectURL(blob);
  a.href = url;
  a.download = fileName;
  a.click();
  window.URL.revokeObjectURL(url);
}

Shiny.addCustomMessageHandler('download-data', function(args) {
  console.log("Downloading table for row " + args.row);
  saveData(base64toBlob(args.data), args.filename);
});

wow such long code! :)