Returns the object previously associated with the current transaction using the specified key.
- iOS 8.0+
- macOS 10.8+
- tvOS 9.0+
- watchOS 3.0+
The unique string identifying an object previously associated with the transaction.
The object previously associated with the transaction (or an enclosing transaction) using the specified key, or
nil if no value for that key could be found.
Nested transactions have nested data scope. Setting a value for a key associates it with the current transaction (or innermost nested transaction) only, but reading the value for a key searches through nested transactions (starting from the innermost).