Returns an object that encapsulates the current record of document changes at the beginning of a save operation.
- macOS 10.7+
The type of save operation.
An object encapsulating the document changes.
The returned object is meant to be passed to
update at the end of the save operation. For example,
save invokes this method, on the main thread, before it does any actual saving. This method facilitates asynchronous saving, during which a user can change a document while it is being saved.