Returns a unique identifier for the given URL.
- iOS 8.0–13.0Deprecated
The URL of a shared document.
A unique identifier for the item specified by the URL, or
nil if the document is not in the File Provider's shared container.
Override this method to define a static mapping between URLs and their persistent identifiers. A document's identifier should remain constant over time; it should not change when the document is edited, moved, or renamed.
For example, if you already have a unique key for the document in your cloud database, you can use that key as the document’s identifier.