feat: Speed up get_color_contrast() (#1140)

NEWS.md

@@ -4,6 +4,8 @@
 
 * Fixed a bug in `bs_themer()` (and `bs_theme_preview()`) that caused it to stop applying changes if a Sass variable was `NULL`. (@meztez, #1112)
 
+* Optimized for better performance the internal functions that compile Sass to call the `color-contrast()` algorithm. (#1140)
+
 # bslib 0.8.0
 
 ## Breaking changes

R/utils.R

@@ -132,14 +132,37 @@ rename2 <- function(x, ...) {
 # Get an accessible color contrast for a specified bg_color
 # (and return NULL+warn on failure)
 get_color_contrast <- function(bg_color) {
-  # Use a specific name that won't clash with other variables
-  nm <- "__bslib-custom-bg-color__"
-  theme <- bs_add_variables(bs_theme(), !!nm := bg_color)
-  tryCatch(
-    bs_get_contrast(theme, nm)[[1]],
-    error = function(e) {
-      warning("Failed to compute a contrasting color for '", bg_color, "'", call. = FALSE)
-      NULL
-    }
+  utils_layer <- sass::sass_layer(
+    functions = sass::sass_file(
+      path_inst("sass-utils", "color-contrast.scss")
+    ),
+    defaults = c(
+      "$black: #000000;",
+      "$white: #FFFFFF;"
+    ),
+    rules = sprintf(
+      "._ {--RET: #{color-contrast(%s)}}",
+      bg_color
+    )
   )
+
+  tryCatch({
+    css <- sass::sass(
+      utils_layer,
+      cache_key_extra = get_package_version("bslib"),
+      # Don't listen to global Sass options so we can be sure
+      # that stuff like source maps won't be included
+      options = sass::sass_options(source_map_embed = FALSE)
+    )
+    # example: css <- "._ {\n  --RET: #fff;\n}"
+    # we'll split to get value:     ^     ^
+    ret <- strsplit(css, "--RET:")[[1]][2]
+    trimws(strsplit(ret, ";")[[1]][1])
+  }, error = function(err) {
+    warning(
+      "Failed to compute a contrasting color for '", bg_color, "'", 
+      call. = FALSE
+    )
+    NULL
+  })
 }