Avoiding The mdls Command Line Round Trip With swiftr::swift_function()

This article is originally published at https://rud.is/b

The last post showed how to work with the macOS mdls command line XML output, but with {swiftr} we can avoid the command line round trip by bridging the low-level Spotlight API (which mdls uses) directly in R via Swift.

If you’ve already played with {swiftr} before but were somewhat annoyed at various boilerplate elements you’ve had to drag along with you every time you used swift_function() you’ll be pleased that I’ve added some SEXP conversion helpers to the {swiftr} package, so there’s less cruft when using swift_function().

Let’s add an RSwift bridge function to retrieve all available Spotlight attributes for a macOS file:

library(swiftr)

swift_function('

  // Add an extension to URL which will retrieve the spotlight 
  // attributes as an array of Swift Strings
  extension URL {

  var mdAttributes: [String]? {

    get {
      guard isFileURL else { return nil }
      let item = MDItemCreateWithURL(kCFAllocatorDefault, self as CFURL)
      let attrs = MDItemCopyAttributeNames(item)!
      return(attrs as? [String])
    }

  }

}

@_cdecl ("file_attrs")
public func file_attrs(path: SEXP) -> SEXP {

  // Grab the attributres
  let outAttr = URL(fileURLWithPath: String(path)!).mdAttributes!

  // send them to R
  return(outAttr.SEXP!)

}
')

And, then try it out:

fil <-  "/Applications/RStudio.app"

file_attrs(fil)
##  [1] "kMDItemContentTypeTree"                 "kMDItemContentType"                    
##  [3] "kMDItemPhysicalSize"                    "kMDItemCopyright"                      
##  [5] "kMDItemAppStoreCategory"                "kMDItemKind"                           
##  [7] "kMDItemDateAdded_Ranking"               "kMDItemDocumentIdentifier"             
##  [9] "kMDItemContentCreationDate"             "kMDItemAlternateNames"                 
## [11] "kMDItemContentModificationDate_Ranking" "kMDItemDateAdded"                      
## [13] "kMDItemContentCreationDate_Ranking"     "kMDItemContentModificationDate"        
## [15] "kMDItemExecutableArchitectures"         "kMDItemAppStoreCategoryType"           
## [17] "kMDItemVersion"                         "kMDItemCFBundleIdentifier"             
## [19] "kMDItemInterestingDate_Ranking"         "kMDItemDisplayName"                    
## [21] "_kMDItemDisplayNameWithExtensions"      "kMDItemLogicalSize"                    
## [23] "kMDItemUsedDates"                       "kMDItemLastUsedDate"                   
## [25] "kMDItemLastUsedDate_Ranking"            "kMDItemUseCount"                       
## [27] "kMDItemFSName"                          "kMDItemFSSize"                         
## [29] "kMDItemFSCreationDate"                  "kMDItemFSContentChangeDate"            
## [31] "kMDItemFSOwnerUserID"                   "kMDItemFSOwnerGroupID"                 
## [33] "kMDItemFSNodeCount"                     "kMDItemFSInvisible"                    
## [35] "kMDItemFSTypeCode"                      "kMDItemFSCreatorCode"                  
## [37] "kMDItemFSFinderFlags"                   "kMDItemFSHasCustomIcon"                
## [39] "kMDItemFSIsExtensionHidden"             "kMDItemFSIsStationery"                 
## [41] "kMDItemFSLabel"   

No system() (et al.) round trip!

Now, lets make RSwift bridge function to retrieve the value of an attribute.

Before we do that, let me be up-front that relying on debugDescription (which makes a string representation of a Swift object) is a terrible hack that I’m using just to make the example as short as possible. We should do far more error checking and then further check the type of the object coming from the Spotlight API call and return an R-compatible version of that type. This mdAttr() method will almost certainly break depending on the item being returned.

swift_function('
extension URL {

  // Add an extension to URL which will retrieve the spotlight 
  // attribute value as a String. This will almost certainly die 
  // under various value conditions.

  func mdAttr(_ attr: String) -> String? {
    guard isFileURL else { return nil }
    let item = MDItemCreateWithURL(kCFAllocatorDefault, self as CFURL)
    return(MDItemCopyAttribute(item, attr as CFString).debugDescription!)
  }

}

@_cdecl ("file_attr")
public func file_attr(path: SEXP, attr: SEXP) -> SEXP {

  // file path as Swift String
  let xPath = String(cString: R_CHAR(Rf_asChar(path)))

  // attribute we want as a Swift String
  let xAttr = String(cString: R_CHAR(Rf_asChar(attr)))

  // the Swift debug string value of the attribute
  let outAttr = URL(fileURLWithPath: xPath).mdAttr(xAttr)

  // returned as an R string
  return(Rf_mkString(outAttr))
}
')

And try this out on some carefully selected attributes:

file_attr(fil, "kMDItemDisplayName")
## [1] "RStudio.app"

file_attr(fil, "kMDItemAppStoreCategory")
## [1] "Developer Tools"

file_attr(fil, "kMDItemVersion")
## [1] "1.4.1651"

Note that if we try to get fancy and retrieve an attribute value that is something like an array of strings, it doesn’t work so well:

file_attr(fil, "kMDItemExecutableArchitectures")
## [1] "<__NSSingleObjectArrayI 0x7fe1f6d19bf0>(\nx86_64\n)\n"

Again, ideally, we’d make a small package wrapper vs use swift_function() for this in production, but I wanted to show how straightforward it can be to get access to some fun and potentially powerful features of macOS right in R with just a tiny bit of Swift glue code.

Also, I hadn’t tried {swiftr} on the M1 Mini before and it seems I need to poke a bit to see what needs doing to get it to work properly in the arm64 RStudio rsession.

FIN

I hope to be able to carve out some more time in the not-too-distant-future for both {swiftr} and the in-progress guide on using Swift and R, but hopefully this post [re-]piqued interest in this topic for some R and/or Swift users.

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This article is originally published at https://rud.is/b
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