Read and load css variables from xaringan css files

R/css.R

+read_css_vars <- function(file = NULL) {
+  if (is.null(file)) {
+    css_candidates <- find_xaringan_themer_css()
+    if (!length(css_candidates)) {
+      stop("Unable to locate a xaringanthemer css file.")
+    } else if (length(css_candidates) == 1) {
+      file <- css_candidates
+    } else if (length(css_candidates) > 1) {
+      is_xaringan_themer_css <- grepl("xaringan-themer.css", css_candidates, fixed = TRUE)
+      if (any(is_xaringan_themer_css)) {
+        file <- css_candidates[is_xaringan_themer_css][1]
+      } else {
+        file <- css_candidates[1]
+        message(glue::glue("Using xaringanthemer theme in {file}"))
+      }
+    }
+  }
+
+  css_get_root(file)
+}
+
+find_xaringan_themer_css <- function() {
+  css_files <- list.files(pattern = "css$", recursive = TRUE, full.names = TRUE)
+  is_xt <- css_files %>%
+    purrr::map(readLines, n = 5) %>%
+    grepl(pattern = "generated by xaringanthemer", fixed = TRUE)
+
+  css_files[is_xt]
+}
+
+css_get_root <- function(file) {
+  x <- readLines(file, warn = FALSE)
+  x <- paste(x, collapse = "\n")
+  where <- regexpr(":root\\s*\\{[^}]+\\}", x)
+  if (where < 0) return(NULL)
+  x <- substr(x, where, where + attr(where, "match.length"))
+  x <- strsplit(x, "\n")[[1]]
+  m <- regexec("--(.+):\\s*(.+?);", x)
+  x <- regmatches(x, m)
+  x <- purrr::compact(x)
+  vars <- gsub("-", "_", purrr::map_chr(x, `[`, 2))
+  values <- purrr::map(x, `[`, 3)
+  names(values) <- vars
+  for (font_type in c("text", "header", "code")) {
+    font_is_google <- paste0(font_type, "_font_is_google")
+    values[[font_is_google]] <- if (!is.null(values[[font_is_google]])) {
+      values[[font_is_google]] %in% c("1", "TRUE", "true", "yes")
+    } else FALSE
+  }
+  values
+}

R/ggplot2.R

 #'   `header_color`
 #' @param accent_secondary_color Color for secondary accents, inherits from
 #'   `text_bold_color`
+#' @param css_file Path to a \pkg{xaringanthemer} CSS file, from which the
+#'   theme variables and values will be inferred. In general, if you use the
+#'   \pkg{xaringathemer} defaults, you will not need to set this. This feature
+#'   lets you create a \pkg{ggplot2} theme for your \pkg{xaringan} slides, even
+#'   if you have only saved your theme CSS file and you aren't creating your
+#'   CSS theme with \pkg{xaringanthemer} in your slides' source file.
 #' @inheritDotParams theme_xaringan_base
 #'
 #' @examples
   background_color = NULL,
   accent_color = NULL,
   accent_secondary_color = NULL,
+  css_file = NULL,
   ...
 ) {
-  requires_xaringanthemer_env()
+  requires_xaringanthemer_env(css_file = css_file, try_css = TRUE)
   requires_package(fn = "xaringan_theme")
 
   background_color <- background_color %||% xaringanthemer_env$background_color
 #'   `header_color`
 #' @param accent_secondary_color Color for secondary accents, inherits from
 #'   `text_bold_color`
+#' @inheritParams theme_xaringan
 #' @inheritDotParams theme_xaringan_base
 #'
 #' @examples
   family <- gsub("['\"]", "", family)
 
   if (!identical(xaringanthemer_env$showtext_auto, TRUE)) {
-    if (requires_package(pkg = "showtext", fn, required = FALSE)) {
-      showtext::showtext_auto()
-    } else {
+    if (!requires_package(pkg = "showtext", fn, required = FALSE)) {
       return(family)
     }
+    showtext::showtext_auto()
     xaringanthemer_env$showtext_auto <- TRUE
   }
 
+  if (family %in% theme_xaringan_get_value("registered_font_families") %||% "") {
+    return(family)
+  }
+
   if (!requires_package(pkg = "sysfonts", fn, required = FALSE)) {
     return(family)
   } else if (!family %in% sysfonts::font_families()) {
       "Source Code Pro",
       "Yanone Kaffeesatz"
     )
-    if (identical(google, TRUE) || is_default_font) {
+    font_found <- family %in% sysfonts::font_families()
+    is_google_font <- identical(google, TRUE) || is_default_font
+    if (is_google_font) {
       tryCatch(
-        sysfonts::font_add_google(family, ...),
-        error = function(e) warning(e$message),
-        warning = function(w) warning(w$message)
+        {
+          sysfonts::font_add_google(family, ...)
+          font_found <- TRUE
+        },
+        error = function(e) {},
+        warning = function(w) {}
       )
-    } else {
-      warning(paste(
-        "Please manually register fonts not served by Google Fonts.",
-        "See `sysfonts::font_add()` for more information."
-      ))
+    }
+    if (!font_found) { # warn user if font still not found
+      msg <- if (is_google_font) glue::glue(
+        "Font '{family}' not found in Google Fonts. ",
+        "Please manually register the font using `sysfonts::font_add()`."
+      ) else {
+        glue::glue(
+          "Font '{family}' must be manually registered using `sysfonts::font_add()`."
+        )
+      }
+      warning(msg, call. = FALSE)
     }
   }
+  xaringanthemer_env[["registered_font_families"]] <- c(
+    xaringanthemer_env[["registered_font_families"]],
+    family
+  )
   family
 }
 
   invisible(TRUE)
 }
 
-requires_xaringanthemer_env <- function() {
-  if (!exists("xaringanthemer_env") || is.null(xaringanthemer_env$header_color)) {
+requires_xaringanthemer_env <- function(css_file = NULL, try_css = TRUE) {
+  reload <- !is.null(css_file) && isTRUE(try_css)
+  if (reload || !exists("xaringanthemer_env") || is.null(xaringanthemer_env$header_color)) {
+    if (try_css) {
+      css_vars <- read_css_vars(css_file)
+      for (css_var in names(css_vars)) {
+        xaringanthemer_env[[css_var]] <- css_vars[[css_var]]
+      }
+      return(requires_xaringanthemer_env(try_css = FALSE))
+    } else {
       stop("Please call a xaringanthemer theme function first.")
     }
   }
+}
 
 #' Get the Value of xaringanthemer Style Setting
 #'
 #' - `title_slide_text_color`
 #'
 #' @param setting A xaringanthemer style setting
+#' @inheritParams theme_xaringan
 #' @export
-theme_xaringan_get_value <- function(setting) {
-  requires_xaringanthemer_env()
+theme_xaringan_get_value <- function(setting, css_file = NULL) {
+  requires_xaringanthemer_env(css_file = css_file)
   if (length(setting) > 1) {
     xaringanthemer_env[setting]
   } else {

man/theme_xaringan.Rd

   background_color = NULL,
   accent_color = NULL,
   accent_secondary_color = NULL,
+  css_file = NULL,
   ...
 )
 }
 \item{accent_secondary_color}{Color for secondary accents, inherits from
 \code{text_bold_color}}
 
+\item{css_file}{Path to a \pkg{xaringanthemer} CSS file, from which the
+theme variables and values will be inferred. In general, if you use the
+\pkg{xaringathemer} defaults, you will not need to set this. This feature
+lets you create a \pkg{ggplot2} theme for your \pkg{xaringan} slides, even
+if you have only saved your theme CSS file and you aren't creating your
+CSS theme with \pkg{xaringanthemer} in your slides' source file.}
+
 \item{...}{
   Arguments passed on to \code{\link[=theme_xaringan_base]{theme_xaringan_base}}
   \describe{

man/theme_xaringan_get_value.Rd

 \alias{theme_xaringan_get_value}
 \title{Get the Value of xaringanthemer Style Setting}
 \usage{
-theme_xaringan_get_value(setting)
+theme_xaringan_get_value(setting, css_file = NULL)
 }
 \arguments{
 \item{setting}{A xaringanthemer style setting}
+
+\item{css_file}{Path to a \pkg{xaringanthemer} CSS file, from which the
+theme variables and values will be inferred. In general, if you use the
+\pkg{xaringathemer} defaults, you will not need to set this. This feature
+lets you create a \pkg{ggplot2} theme for your \pkg{xaringan} slides, even
+if you have only saved your theme CSS file and you aren't creating your
+CSS theme with \pkg{xaringanthemer} in your slides' source file.}
 }
 \description{
 A helper function to retrieve the value of style settings as set by a

tests/testthat/test-css.R

+test_that("read theme settings from css variables", {
+  expected_vars <- list(
+    background_color = "#E9EBEE",
+    text_color = "#18273F",
+    header_color = "#23395b",
+    text_bold_color = "#23395b",
+    inverse_background_color = "#23395b",
+    inverse_text_color = "#E9EBEE",
+    inverse_header_color = "#E9EBEE",
+    text_font_size = "20px",
+    header_h3_font_size = "35px",
+    text_font_family = "'Noto Serif'",
+    text_font_is_google = TRUE,
+    header_font_family = "'Yanone Kaffeesatz'",
+    header_font_is_google = FALSE,
+    code_font_family = "'Source Code Pro'",
+    code_font_is_google = FALSE
+  )
+
+  css_vars <- read_css_vars(testthat::test_path("css/mono_light.css"))
+  for (var in names(expected_vars)) {
+    expect_equal(!!css_vars[[var]], !!expected_vars[[var]])
+  }
+})