Print xaringan slides to PDF, even the complicated ones

xaringan-chromote-print.R

#' Print xaringan slides to PDF
#'
#' Prints xaringan slides to a PDF file, even complicated slides
#' with panelsets or other html widgets or advanced features.
#' Requires a local installation of Chrome.
#'
#' @param input Path to Rmd or html file of xaringan slides.
#' @param output_file The name of the output file. If using NULL then
#'   the output filename will be based on filename for the input file.
#'   If a filename is provided, a path to the output file can also be provided.
#' @param delay Seconds of delay between advancing to and printing
#'   a new slide.
#' @param include_partial_slides Should partial (continuation) slides be
#'   included in the output? If `FALSE`, the default, only the complete slide
#'   is included in the PDF.
xaringan_to_pdf <- function(
  input,
  output_file = NULL,
  delay = 1,
  include_partial_slides = FALSE
) {
  if (!requireNamespace("chromote", quietly = TRUE)) {
    stop("`chromote` is required: devtools::install_github('rstudio/chromote')")
  }
  required_packages <- c("progress", "jsonlite", "pdftools", "digest", "fs")
  for (pkg in required_packages) {
    if (!requireNamespace(pkg, quietly = TRUE)) {
      stop("`", pkg, "` is required: install.packages('", pkg, "')")
    }
  }

  is_url <- grepl("^(ht|f)tp", tolower(input))

  if (is.null(output_file)) {
    if (is_url) {
      output_file <- fs::path_ext_set(fs::path_file(input), "pdf")
    } else {
      output_file <- fs::path_ext_set(input, "pdf")
    }
  }

  if (!is_url && !grepl("^file://", input)) {
    if (!tolower(fs::path_ext(input)) %in% c("htm", "html")) {
      stop("`input` must be the HTML version of the slides.")
    }
    input <- paste0("file://", fs::path_abs(input))
  }

  b <- chromote::ChromoteSession$new()
  on.exit(b$close(), add = TRUE)

  b$Page$navigate(input, wait_ = TRUE)
  b$Page$loadEventFired()

  has_remark <- b$Runtime$evaluate("typeof slideshow !== 'undefined'")$result$value
  if (!has_remark) {
    stop("Input does not appear to be xaringan slides: ", input)
  }

  current_slide <- function() {
    x <- b$Runtime$evaluate("slideshow.getCurrentSlideIndex()")$result$value
    as.integer(x) + 1L
  }

  slide_is_continuation <- function() {
    b$Runtime$evaluate(
      "document.querySelector('.remark-visible').matches('.has-continuation')"
    )$result$value
  }

  hash_current_slide <- function() {
    digest::digest(b$Runtime$evaluate(
      "document.querySelector('.remark-visible').innerHTML"
    )$result$value)
  }

  get_ratio <- function() {
    r <- b$Runtime$evaluate('slideshow.getRatio()')$result$value
    r <- lapply(strsplit(r, ":"), as.integer)
    width <- r[[1]][1]
    height <- r[[1]][2]
    page_width <- 8/width * width
    list(
      width = as.integer(908 * width / height),
      height = 681L,
      page = list(width = page_width, height = page_width * height / width)
    )
  }

  slide_size <- get_ratio()

  expected_slides <- as.integer(
    b$Runtime$evaluate("slideshow.getSlideCount()")$result$value
  )

  max_slides <- expected_slides * 4

  b$Browser$setWindowBounds(1, bounds = list(
    width = slide_size$width,
    height = slide_size$height
  ))

  b$Emulation$setEmulatedMedia("print")
  b$Runtime$evaluate(paste0(
    "let style = document.createElement('style')\n",
    "style.innerText = '@media print { ",
    ".remark-slide-container:not(.remark-visible){ display:none; }",
    if (include_partial_slides) " .has-continuation { display: block }",
    "}'\n",
    "document.head.appendChild(style)"
  ))

  pb <- progress::progress_bar$new(
    format = "Slide :slide (:part) [:bar] Eta: :eta",
    total = expected_slides
  )

  idx_slide <- current_slide()
  last_hash <- ""
  idx_part <- 0L
  pdf_files <- c()
  for (i in seq_len(max_slides)) {
    if (i > 1) {
      b$Input$dispatchKeyEvent(
        "rawKeyDown",
        windowsVirtualKeyCode = 39,
        code = "ArrowRight",
        key = "ArrowRight",
        wait_ = TRUE
      )
    }

    if (current_slide() == idx_slide) {
      step <- 0L
      idx_part <- idx_part + 1L
    } else {
      step <- 1L
      idx_part <- 1L
    }
    idx_slide <- current_slide()
    pb$tick(step, tokens = list(slide = idx_slide, part = idx_part))

    if (!isTRUE(include_partial_slides) && slide_is_continuation()) next
    Sys.sleep(delay)

    this_hash <- hash_current_slide()
    if (identical(last_hash, this_hash)) break
    last_hash <- this_hash

    pdf_file_promise <- b$Page$printToPDF(
      landscape = TRUE,
      printBackground = TRUE,
      paperWidth = 12,
      paperHeight = 9,
      marginTop = 0,
      marginRight = 0,
      marginBottom = 0,
      marginLeft = 0,
      pageRanges = "1",
      preferCSSPageSize = TRUE,
      wait_ = FALSE
    )$then(function(value) {
      filename <- tempfile(fileext = ".pdf")
      writeBin(jsonlite::base64_dec(value$data), filename)
      filename
    })
    pdf_files <- c(pdf_files, b$wait_for(pdf_file_promise))
  }

  pdftools::pdf_combine(pdf_files, output = output_file)
  fs::file_delete(pdf_files)

  invisible(output_file)
}

Thanks for this feature, Garrick! I would like to try it to see if it prints correctly your panelset feature. Nonetheless, I am getting this error message:

Error in find_chrome() : 
  `google-chrome` and `chromium-browser` were not found. Try setting the CHROMOTE_CHROME environment variable or adding one of these executables to your PATH.

My guess is that it's failing to find Chromium on Ubuntu, which would be linked to "chromium" instead of "chromium-browser". Could you please take a look at it? Thanks once again!

@bttomio The issue is with the chromote package; there is already a pull request, but it hasn't been merged in the main branch. See rstudio/chromote#27

Until this has been fixed, you can change chrome.R of the chromote package by setting

  path <- Sys.which("google-chrome")
    if (nchar(path) == 0) {
      path <- Sys.which("chromium-browser")
    }
    if (nchar(path) == 0) {
      path <- Sys.which("chromium")
    }

@lbelzile, thanks for your quick answer. I wish I could find chrome.R to change it. Any idea of how to find it in Ubuntu?

It's in the R package chromote, see this directory. You can download the source package, change chrome.R and build the package afterwards yourself.

I'm running the latest R and Rstudio on Garuda (Arch-derivative), with everything updated. I'm getting the following error on running your code.

google-chrome and chromium-browser were not found. Try setting the CHROMOTE_CHROME environment variable or adding one of these executables to your PATH.`

But I most definitely have Google Chrome installed!

Thanks a lot for this really helpful gist! When I source and run the function once in RStudio, everything works fine. However, when I want to execute the function again, I get the following error message: Error in onRejected(reason) : code: -32000 message: Browser window not found . Restarting my R session solves this issue.