R/to-vega.R

In vegawidget: 'Htmlwidget' for 'Vega' and 'Vega-Lite'

#' Convert to Vega specification
#'
#' If you have  **[V8](https://CRAN.R-project.org/package=V8)** installed,
#' you can use this function to compile a Vega-Lite specification
#' into a Vega specification.
#'
#' @inheritParams as_vegaspec
#'
#' @return S3 object of class `vegaspec_vega` and `vegaspec`
#' @examples
#'   vw_spec_version(spec_mtcars)
#'   vw_spec_version(vw_to_vega(spec_mtcars))
#' @export
#'
vw_to_vega <- function(spec) {
  .vw_to_vega(as_vegaspec(spec))
}

# use internal S3 generic
.vw_to_vega <- function(spec, ...) {
  UseMethod(".vw_to_vega")
}

.vw_to_vega.default <- function(spec, ...) {
  stop(".vw_to_vega(): no method for class ", class(spec), call. = FALSE)
}

.vw_to_vega.vegaspec_vega_lite <- function(spec, ...) {

  assert_packages("V8")

  # determine versions of vega, vega-lite
  version_all <- vega_version_all()
  spec_version <- vw_spec_version(spec)
  widget <-
    get_widget_string(
      spec_version[["library"]],
      spec_version[["version"]],
      version_all
    )

  version_widget <- version_all[version_all[["widget"]] == widget, ]
  version_vega <- version_widget[["vega"]]
  version_vega_lite <- version_widget[["vega_lite"]]

  # fire up v8
  ct <- V8::v8()

  # polyfill structuredClone, ref: https://stackoverflow.com/questions/73607410
  # I think that because Vega(-Lite) specs are designed to be JSON, the
  # "stringify/parse" method will be sufficient.
  #
  # TODO: remove this block of code when {v8} supports structuredClone
  #
  ct$source(bin_file("polyfill-structuredClone.js"))

  ct$source(widgetlib_file("vega", glue::glue("vega@{version_vega}.min.js")))
  ct$source(
    widgetlib_file("vega-lite", glue::glue("vega-lite@{version_vega_lite}.min.js"))
  )

  # convert to vega
  ct$eval(glue::glue("var vs = vegaLite.compile({vw_as_json(spec)})"))

  # don't let V8 convert to JSON; send as string
  ct$eval("var strSpec = JSON.stringify(vs.spec)")
  str_spec <- ct$get("strSpec")

  as_vegaspec(str_spec)
}

.vw_to_vega.vegaspec_vega <- function(spec, ...) {
   # do nothing, already a Vega spec
   spec
}