How to filter an NSArray of file urls by Spotlight File Metadata Attributes / NSMetadataQuery?

Question

The NSMetadataQuery class seems to be how Finder/Spotlight searches for files via their metadata.

NSMetadataQuery class provided by the Foundation framework. Queries can be run in two modes: asynchronous, and asynchronous with live updates. The first simply performs the search on the files that exist at the time of the initial search. The latter continues to search. updating the data as the files that fulfill or no longer fulfill the search parameters update.

https://developer.apple.com/library/content/documentation/Carbon/Conceptual/SpotlightQuery/Concepts/Introduction.html#//apple_ref/doc/uid/TP40001843-BBCFBCAG

However, it seems oriented around providing a directory (searchScopes), and then asynchronously returning results that were found in those directories (NSMetadataQueryDidFinishGathering).

I already have an NSArray containing file urls. I would like to construct a filter/search of those NSURLs using the same metadata and query syntax as a Spotlight Search. But I will provide a list of files to quickly filer, rather than a provide a directory with and receive asynchronous results.

// Something like this...
let imageFileTypePredicate = NSPredicate(fromMetadataQueryString: "(kMDItemGroupId = 13)")
let imageURLs = allURLs.filter{ imageFileTypePredicate.evaluate(with:$0) };

However, that is using a standard NSPredicate search rather than a file metadata filter and is throwing the error:

this class is not key value coding-compliant for the key _kMDItemGroupId.

The Spotlight Metadata Attributes I'm interested in filtering by are listed here:
https://developer.apple.com/library/content/documentation/CoreServices/Reference/MetadataAttributesRef/Reference/CommonAttrs.html#//apple_ref/doc/uid/TP40001694-SW1

How can an array of file urls be filtered by Spotlight metadata?

Answer 1

Create an MDItem for each url to get the file's spotlight attributes.

MDItem is a CF-compliant object that represents a file and the metadata associated with the file.

https://developer.apple.com/reference/coreservices/1658213-mditem

MDItemRef item = MDItemCreateWithURL(kCFAllocatorDefault, url);
CFArrayRef attributes = MDItemCopyAttributeNames(item);
NSDictionary *attributeValues = CFBridgingRelease(MDItemCopyAttributes(item, attributes));