Support returning asset URLs via public method within Dash class

R/dash.R

@@ -702,7 +702,53 @@ Dash <- R6::R6Class(
  }
  private$callback_context_
  },
-
+
+ # ------------------------------------------------------------------------
+ # return asset URLs
+ # ------------------------------------------------------------------------
+ get_asset_url = function(asset_path, prefix = "/") {
+ app_root_path <- Sys.getenv("DASH_APP_PATH")
+
+ if (app_root_path == "" && getAppPath() != FALSE) {
+ # app loaded via source(), root path is known
+ app_root_path <- dirname(private$app_root_path)
+ } else {
+ # app not loaded via source(), env var not set, no reliable way to ascertain root path
+ warning("application not started via source(), and DASH_APP_PATH environment variable is undefined. get_asset_url returns NULL since root path cannot be reliably identified.")
+ return(NULL)
+ }
+
+ asset <- lapply(private$asset_map, 
+ function(x) {
+ # asset_path should be prepended with the full app root & assets path
+ # if leading slash(es) present in asset_path, remove them before
+ # assembling full asset path
+ asset_path <- file.path(app_root_path,
+ private$assets_folder, 
+ sub(pattern="^/+",
+ replacement="",
+ asset_path))
+ return(names(x[x == asset_path]))
+ }
+ )
+ asset <- unlist(asset, use.names = FALSE)
+
+ if (length(asset) == 0)
+ stop(sprintf("the asset path '%s' is not valid; please verify that this path exists within the '%s' directory.",
+ asset_path,
+ private$assets_folder))
+
+ # strip multiple slashes if present, since we'll
+ # introduce one when we concatenate the prefix and
+ # asset path & prepend the asset name with route prefix
+ return(gsub(pattern="/+",
+ replacement="/",
+ paste(prefix, 
+ private$assets_url_path, 
+ asset, 
+ sep="/")))
+ },
+
  # ------------------------------------------------------------------------
  # convenient fiery wrappers
  # ------------------------------------------------------------------------
@@ -1051,7 +1097,7 @@ Dash <- R6::R6Class(
 
  # need to check whether the assets have actually been updated, since
  # this function is also called to generate the asset map
- if (private$asset_modtime > private$app_launchtime) {
+ if (!is.na(private$asset_modtime) && private$asset_modtime > private$app_launchtime) {
  # here we use mapply to make pairwise comparisons for each of the
  # asset classes in the map -- before/after for css, js, and other
  # assets; this returns a list whose subelements correspond to each