Add shadow_clone() to use themes

NAMESPACE

@@ -4,6 +4,7 @@ export(decktape)
 export(inf_mr)
 export(infinite_moon_reader)
 export(moon_reader)
+export(shadow_clone)
 export(summon_remark)
 export(tsukuyomi)
 import(stats)

R/ninjutsu.R

@@ -0,0 +1,72 @@
+#' Use a new xaringan theme.
+#'
+#' This function clones an xaringan theme for you to use. Include it in a code chunk in the xaringan slide source.
+#'
+#'
+#' @param theme Either a built-in theme shipped with xaringan, or the repository address of the theme in the format of username/repo
+#' @param ref desired git reference. could be a commit SHA, or a branch name. Defauts to master
+#'
+#'
+#' @return The cloned theme
+#' @export
+#'
+#' @examples
+#' # built-in
+#' shadow_clone("kunoichi")
+#' # remote
+#' shadow_clone("tcgriffith/xaringan_theme_example")
+#'
+shadow_clone = function(theme=NULL, ref="master"){
+
+  ## retrieve built-in themes
+  builtin_css = list_css()
+  builtin_theme = names(builtin_css)
+
+  ## similar to remotes::install_github, convert repo into full url on github
+  ## theme=tcgriffith/xaringan_theme_example ->
+  ## css_remote = https://raw.githubusercontent.com/tcgriffith/xaringan_theme_example/master/xaringan_theme_example.css
+
+  if (is.null(theme)) return(NULL)
+
+  bn = basename(theme)
+  css_remote = sprintf("https://raw.githubusercontent.com/%s/%s/%s.css",
+                       theme,
+                       ref,
+                       bn)
+  css_local = basename(css_remote)
+
+  ## built-in theme
+  if (theme %in% builtin_theme) {
+    message("Theme: ", theme)
+    message(sprintf("To use the same theme, try shadow_clone(%s)",theme))
+    css_local = builtin_css[theme]
+  }
+  ## remote theme
+  else {
+    ## extract git hash as ref and print out as message
+    repo = paste0("https://github.com/", theme)
+    cmd = paste0("git ls-remote ",repo," refs/heads/master")
+    rslt = system(command=cmd, intern=TRUE)
+    ref = substr(rslt, 1,8)
+
+    message("Theme repo:", repo)
+    message("ref: ", ref)
+    message(sprintf("To use the same theme, try shadow_clone(%s,%s)",theme, ref))
+
+    ## download remote css when css_local doesn't exist
+    if (!file.exists(css_local)){
+      try_dl = try(xfun::download_file(css_remote))
+      ## if download failed, fallback to default CSS. warning
+      if(try_dl != 0 ) {
+        warning("# Shadow clone failed. Theme not found.")
+        css_local = NULL
+      }
+    }
+  }
+
+  ## include CSS in the final xaringan html.
+
+  if(!is.null(css_local)){
+    htmltools::includeCSS(css_local)
+  }
+}