DashR does not currently have the `get_asset_url` method for loading asset URLs like Dash for Python

[HammadTheOne]

As originally referenced in a DashR docs issue, Dash for R currently lacks this method which is featured in the Python version. Based on the Python implementation, it would grab the asset URL from the assets directory without needing to specify the exact path.

@rpkyle @HammadTheOne - Do you think we could get a get_asset_url into Dash for R before the next release? Here is the implementation in Python:
https://github.com/plotly/dash/blob/e622f2a31202302d0c305c56e738d1ea369403e6/dash/dash.py#L1559-L1566

If it's not in Dash for R, then we can include it in this example as a helper function.

Originally posted by @chriddyp in https://github.com/plotly/dash-enterprise-docs/pull/95

[rpkyle]

Thanks @HammadTheOne -- I meant to reply earlier; just did that now.

It's correct to say that Dash for R doesn't have this method, because the function isn't associated with the Dash R6 class, but rather an unexported function in utils.R:

dashR/R/utils.R

Lines 583 to 598 in 5f2942c

	get_asset_url <- function(asset_path, prefix = "/") {
	# the subpath is stored in the names attribute
	# of the return object from get_asset_path, so
	# we can retrieve it using names()
	asset <- names(asset_path)
	# strip one or more trailing slashes, since we'll
	# introduce one when we concatenate the prefix and
	# asset path
	prefix <- gsub(pattern = "/+$",
	replacement = "",
	x = prefix)
	# prepend the asset name with the route prefix
	return(paste(prefix, asset, sep="/"))
	}

@HammadTheOne I can make this change via PR, and will ask you for review.

[rpkyle]

Thanks @HammadTheOne -- I've responded in the original thread, but I think I'll have to add in a check for the names attribute to make this usable by developers, even if exported. If there is no name associated with the path, the function will infer it from the asset map, otherwise it will use the one passed.

[rpkyle]

Closed by way of #160.