output = NULL returns css and doesn't write file

style_* returns name of file invisibly

R/style_extra_css.R

 #' @inheritParams style_xaringan
 #' @export
 style_extra_css <- function(css, outfile = "xaringan-themer.css", append = TRUE) {
+  x <- paste("\n\n/* Extra CSS */", list2css(css), sep = "\n")
+  if (is.null(outfile)) return(x)
   cat(
-    "\n\n/* Extra CSS */",
-    list2css(css),
+    x,
     file = outfile,
     append = TRUE,
     sep = "\n"

R/style_xaringan.R

   template <- readLines(tf, warn = FALSE)
   template <- paste(template, collapse = "\n")
   x <- whisker::whisker.render(template)
+  if (!is.null(extra_css)) {
+    x <- c(x, style_extra_css(extra_css, outfile = NULL))
+  }
+  if (is.null(outfile)) {
+    return(x)
+  }
   writeLines(x, con = outfile)
-  if (!is.null(extra_css)) style_extra_css(extra_css, outfile)
-  outfile
+  invisible(outfile)
 }

inst/scripts/style_xaringan_body.R

 template <- readLines(tf, warn = FALSE)
 template <- paste(template, collapse = "\n")
 x <- whisker::whisker.render(template)
+if (!is.null(extra_css)) {
+  x <- c(x, style_extra_css(extra_css, outfile = NULL))
+}
+if (is.null(outfile)) {
+  return(x)
+}
 writeLines(x, con = outfile)
-if (!is.null(extra_css)) style_extra_css(extra_css, outfile)
-outfile
+invisible(outfile)

tests/testthat/helper-session.R

   empty_wd <- tempfile()
   dir.create(empty_wd)
   owd <- setwd(empty_wd)
-  message(owd)
-  on.exit({message(owd); setwd(owd); unlink(empty_wd, TRUE)})
+  on.exit({setwd(owd); unlink(empty_wd, TRUE)})
   callr::r_safe(.f, args)
 }

tests/testthat/test-themes.R

   expect_equal(theme_vars$header_font_family, quote_elements_w_spaces(formals(style_xaringan)$header_font_family))
   expect_true(theme_vars$header_font_is_google)
 })
+
+
+test_that("NULL output returns CSS as text", {
+  expect_false(
+    with_clean_session(function() {
+      xaringanthemer::style_xaringan(outfile = NULL)
+      file.exists("xaringan-themer.css")
+    })
+  )
+
+  xt <- with_clean_session(function() {
+    xaringanthemer::style_xaringan(outfile = NULL)
+  })
+
+  expect_type(xt, "character")
+  expect_true(any(grepl("generated by xaringanthemer", xt)))
+})