Stream Download Files to Browser

Push files to browser downloads using 'StreamSaver.js' for efficient streaming of large files. This function sends file data in chunks to the browser, which then writes them directly to disk without buffering the entire file in memory.

Usage

stream_download(
  filepath,
  filename = basename(filepath),
  session = shiny::getDefaultReactiveDomain(),
  chunk_size = 2 * 1024^2,
  cleanup = FALSE,
  method = c("streamsaver", "blob"),
  quiet = FALSE
)

Arguments

filepath: path to the file to download; must be an existing file
filename: the file name to use for the download (defaults to the base name of filepath)
session: the Shiny session; defaults to shiny::getDefaultReactiveDomain()
chunk_size: size in bytes for each chunk; default is 2 MB
cleanup: whether to delete the source file after download completes; default is FALSE
method: download method: "streamsaver" (default, uses service worker for true streaming) or "blob" (fallback that accumulates chunks in memory)
quiet: whether to suppress progress messages; default is FALSE

Value

Invisible NULL; the function is called for its side effects

Details

This function requires the use_shiny_dipsaus() to be called in your Shiny UI to load the necessary 'JavaScript' dependencies.

The "streamsaver" method (default) uses service workers to enable true streaming downloads that start immediately without buffering the entire file. This is ideal for large files (100MB+). The "blob" method is a fallback that accumulates chunks in memory before triggering the download, which works on more browsers but uses more memory.

Service Worker Requirements

'StreamSaver.js' requires the following files to be served from the same origin:

streamsaver/StreamSaver.js - Main library
streamsaver/sw.js - Service worker
streamsaver/mitm.html - Man-in-the-middle page

These files are automatically included when using use_shiny_dipsaus.

Examples

if(interactive()) {
  library(shiny)

  ui <- fluidPage(
    use_shiny_dipsaus(),
    actionButton("download_btn", "Download Large File")
  )

  server <- function(input, output, session) {
    observeEvent(input$download_btn, {
      # Create a temporary file to demonstrate
      temp_file <- tempfile(fileext = ".txt")
      writeLines(rep("Hello World!\n", 100000), temp_file)

      # Stream the file to the browser
      stream_download(
        filepath = temp_file,
        filename = "large_file.txt",
        cleanup = TRUE  # Delete temp file after download
      )
    })
  }

  shinyApp(ui, server)
}