Selector from string for unexposed properties

What's the alternative to constructing a slector from a string for properties that have not been exposed?

E.g. for getting the file name and file size of a PHAssetResource:

if resource.responds(to: Selector("originalFilename")) {
    let fileName = resource.value(forKey: "originalFilename") as? String
    NSLog("resource filename: %@", fileName ?? "<unknown>")
}

if resource.responds(to: Selector("fileSize")) {
    let fileSize = resource.value(forKey: "fileSize") as? CLong
    NSLog("resource fileSize: %d", fileSize ?? -1)
}

These two properties, originalFilename and fileSize, are defined in an internal subclass of PHAssetResource, PHInternalAssetResource.