How to fetch the CKShare object of an already existing share (to get share URL)

I am trying to allow users of my app to be able to retrieve the iCloud sharing link to a record that they have ALREADY shared. I can get the URL when creating the share by using the let share = CKShare(rootRecord: CKRecord) followed by Apple's UISharingController. However, every time I use this method, the old link becomes invalid and kicks others out of the share they have already joined. How can I allow the users (the record owners) to fetch a shareable URL for the records CKShare object at any time?

//Initially shares the record
let share = CKShare(rootRecord: newRecord)
share[CKShare.SystemFieldKey.title] = "Group" as CKRecordValue?        
let modifyRecordsOperation = CKModifyRecordsOperation( recordsToSave: [newRecord, share], recordIDsToDelete: nil)

//Activates the UISharingController (code not shown)

Solution

Assuming the record has already been shared, you can get the URL to invite additional participants with the following:

if let shareReference = record.share {
    database.fetch(withRecordID: shareReference.recordID) { (share, error) in
        let shareURL = (share as? CKShare)?.url
    }
}

If you wanted to show the UICloudSharingController again for an existing share (without it generating a new URL), you could do:

let sharingController = UICloudSharingController { (_, prepareCompletionHandler) in

    func createNewShare() {
        let operationQueue = OperationQueue()
        operationQueue.maxConcurrentOperationCount = 1

        let share = CKShare(rootRecord: record)
        share[CKShare.SystemFieldKey.title] = "Title" as CKRecordValue
        share.publicPermission = .readWrite

        let modifyRecordsOp = CKModifyRecordsOperation(recordsToSave: [share, record], recordIDsToDelete: nil)
        modifyRecordsOp.modifyRecordsCompletionBlock = { records, recordIDs, error in
            prepareCompletionHandler(share, self, error)
        }
        modifyRecordsOp.database = database
        operationQueue.addOperation(modifyRecordsOp)
    }

    if let shareReference = record.share {
        database.fetch(withRecordID: shareReference.recordID) { (share, error) in
            guard let share = share as? CKShare else {
                createNewShare()
                return
            }
            prepareCompletionHandler(share, self, nil)
        }
    } else {
        createNewShare()
    }
}