Capturing wild widgets with webshot

NOTE: you won’t need to use this function if you use the [development version](https://github.com/yihui/knitr) of `knitr`

Winston Chang released his [`webshot`](https://github.com/wch/webshot) package to CRAN this past week. The package wraps the immensely useful [`phantomjs`](http://phantomjs.org/) utility and makes it dirt simple to capture whole or partial web pages in R. One beautiful bonus feature of `webshot` is that you can install `phamtomjs` with it (getting `phantomjs` to work on Windows is a pain).

You can do many things with the `webshot` package but I hastily drafted this post to put forth a means to generate a static image from an `htmlwidget`. I won’t elaborate much since I included a fully `roxygen`-doc’d function below, but the essence of `capture_widget()` is to pass in an `htmlwidget` object and have it rendered for you to a `png` file and get back either:

– a file system `path` reference (e.g. `/path/to/widget.png`)
– a `markdown` image reference (e.g. `![](file:///path/to/widget.png)`)
– an `html` image reference (e.g. ``), or
– an `inline` base64 encoded HTML imgage reference (e.g. ``)

which you can then use in R markdown documents knitted to PDF (or in any other context).

Take a look at the function, poke the tyres and drop suggestions in the comments. I’ll add this to one of my widgets soon so folks can submit complaints or enhancements via issues & PRs on github).

To use the function, just pipe a sized widget to it and use the output from it.

#' Capture a static (png) version of a widget (e.g. for use in a PDF knitr document)
#'
#' Widgets are generally interactive beasts rendered in an HTML DOM with
#' javascript. That makes them unusable in PDF documents. However, many widgets
#' initial views would work well as static images. This function renders a widget
#' to a file and make it usable in a number of contexts.
#'
#' What is returned depends on the value of \code{output}. By default (\code{"path"}),
#' the full disk path will be returned. If \code{markdown} is specified, a markdown
#' string will be returned with a \code{file:///...} URL. If \code{html} is
#' specified, an \code{<img src='file:///...'/>} tag will be returned and if
#' \code{inline} is specified, a base64 encoded \code{<img>} tag will be returned
#' (just like you'd see in a self-contained HTML file from \code{knitr}).
#'
#' @importFrom webshot webshot
#' @importFrom base64 img
#' @param wdgt htmlwidget to capture
#' @param output how to return the results of the capture (see Details section)
#' @param height,width it's important for many widget to be responsive in HTML
#'        documents. PDFs are static beasts and having a fixed image size works
#'        better for them. \code{height} & \code{width} will be passed into the
#'        rendering process, which means you should probably specify similar
#'        values in your widget creation process so the captured \code{<div>}
#'        size matches the size you specify here.
#' @param png_render_path by default, this will be a temporary file location but
#'        a fully qualified filename (with extension) can be specified. It's up to
#'        the caller to free the storage when finished with the resource.
#' @return See Details
#' @export
capture_widget <- function(wdgt,
                           output=c("path", "markdown", "html", "inline"),
                           height, width,
                           png_render_path=tempfile(fileext=".png")) {
 
  wdgt_html_tf <- tempfile(fileext=".html")
 
  htmlwidgets::saveWidget(vl, wdgt_html_tf)
 
  webshot::webshot(url=sprintf("file://%s", wdgt_html_tf),
                   selector="#htmlwidget_container",
                   file=wdgt_png_tf,
                   vwidth=width, vheight=height)
 
  # done with HTML
  unlink(wdgt_html_tf)
 
  switch(match.arg(output, c("path", "markdown", "html", "inline")),
             `path`=png_render_path,
         `markdown`=sprintf("![widget](file://%s)", png_render_path),
             `html`=sprintf("<img src='file://%s'/>", png_render_path),
           `inline`=base64::img(wdgt_png_tf))
 
}

rud.is