Getting extensions of a given UTType

Question

I know I can get the UTType of a given extension using UTTypeCreatePreferredIdentifierForTag(kUTTagClassFilenameExtension, extension, NULL) but I can't find a way to do the opposite, that is an array of extensions for a given UT... What can I do?

Thank you

Faludi

Answer 1

UTTypeCopyPreferredTagWithClass is used to convert a UTI to another tag, like file extensions:

NSString *extension = (__bridge_transfer NSString *)UTTypeCopyPreferredTagWithClass(myUTI, kUTTagClassFilenameExtension);

Algid

Very cool. I've not tested it yet because I have an extra question: the class reference says "Translates a uniform type identifier to a list of tags" but the return type is CFStringRef... Shouldn't it be CFArrayRef? – Faludi

Definitely true. The documentation is contradictory regarding the return type of the function. You should file a documentation bug. Nevertheless, the function returns a string, not an array. – Algid

Answer 2

Swift 5

import MobileCoreServices

static func fileExtension(for dataUTI: String) -> String? {
    guard let fileExtension = UTTypeCopyPreferredTagWithClass(dataUTI as CFString, kUTTagClassFilenameExtension) else {
        return nil
    }

    return String(fileExtension.takeRetainedValue())
}

Catharina

You need to use .takeRetainedValue() instead to avoid a memory leak... see this answer and this more general answer for more information. Also this article has more general info on Swift interactions with Unmanaged return types from C APIs. – Brindled

Good catch @justinpawela, thanks for improving my answer. Just edited it based on your factual suggestion. – Catharina