Under what conditions should we use CKContainer.fetchUserRecordID() and then fetch the CKRecord object, and under what conditions should we use CKFetchRecordsOperation.fetchCurrentUserRecordOperation().
Which way to get currently logged in user in iOS?
CKContainer.fetchUserRecordID() executes the operation and fetches the record.
CKFetchRecordsOperation.fetchCurrentUserRecordOperation() is a class function used to generate a CKOperation, which when executed would fetch you the record. But has an additional requirement: You must associate at least one progress handler with the operation object (docs)
You probably want to go with the first one.