diff
Calculate the diff of the provided items against the last set of items that were passed to this differ.
The returned DittoDiff identifies changes from the old list of items to the new list of items using indices into both lists.
Initially, the Differ has no items, so the first call to this method will always return a diff showing all items as insertions.
The identity of items is determined by their "_id" field.
Return
A DittoDiff describing the changes.
Parameters
items
The query result items to compare against the last list of items.