Support returning asset URLs via public method within Dash class

[rpkyle]

The previous implementation of get_asset_url in Dash for R was not particularly useful -- it required passing an object whose asset paths were set on the names attribute of the object, which is used internally by Dash, but not generally available to application developers. It was also unexported, and not used elsewhere in the package -- effectively dead code.

This revision makes the get_asset_url a public method of the Dash class itself, so it is available to developers for use.

If this PR is merged, the method will permit returning the URL of any file in the assets directory, provided that a valid relative path within an app's assets_folder is passed. If the asset is not found, an error is returned.

[HammadTheOne]

I encountered the following error when running this line to generate the asset_path:

Error in dirname(private$app_root_path) : a character vector argument expected

Digging a bit deeper into it, the app_root_path private field in Dash calls the function getAppPath from utils.R upon initialization of the Dash class, and returns the value FALSE if the app isn't run via source().

A potential fix could be:
ifelse(is.logical(private$app_root_path), "", dirname(private$app_root_path)), but I'm not exactly sure what needs to be there when the condition is TRUE to get the correct path.

A more permanent fix would be to tweak the getAppPath function to deliver a more comprehensive return value for the else condition.

[HammadTheOne]

Hey Ryan, I didn't see any real issues with the code, it's written concisely and makes sense, the comments make it easy to follow.

The get_asset_url method works perfectly fine when running the app from source('app.R'), but has an issue when run from the script. Once that's figured out, it should be good to go.

[rpkyle]

Hey Ryan, I didn't see any real issues with the code, it's written concisely and makes sense, the comments make it easy to follow.

The get_asset_url method works perfectly fine when running the app from source('app.R'), but has an issue when run from the script. Once that's figured out, it should be good to go.

Hmm, yes. This is going to be tricky to resolve. We don't have a reliable way to determine the application root unless code is executed using source(). Basically, source() calls parse(), which populates srcref attributes, as described here:

https://journal.r-project.org/archive/2010-2/RJournal_2010-2_Murdoch.pdf

The srcref attributes are very handy, since they are a relatively safe way of identifying the location of R code.

Otherwise, we're basically making our best guess at the value of app_root_path whenever code is pasted/entered into the console.

@alexcjohnson Any suggestions? We're trying to reproduce the behaviour of get_asset_url in R to mimic that in Python, but running into a snag when code is not loaded via source().

[alexcjohnson]

Otherwise, we're basically making our best guess at the value of app_root_path whenever code is pasted/entered into the console.

This concern isn't new in this PR, right? So we can make an issue for it but I wouldn't try to address it in this PR.

When you paste code in the console there's really no reason to expect it comes from anywhere in the filesystem. So it seems to me in fact a bad idea to try to impute some location to it. If users want to work this way, the root path would need to be provided explicitly in the code; otherwise we should steer folks to a workflow in which the app is invoked explicitly via its path.

[HammadTheOne]

Based on the conversation above, it doesn't seem like that particular use-case needs to be an issue tackled with this PR, it might be helpful to take a look at for debugging and testing purposes in the future, but following the usual workflow of a deployed Dash application, this shouldn't come up. Apart from that, the functionality works as intended and I was able to follow along with the same usage as with the Python implementation.

💃