Returns an array of objects that meet the criteria specified by a given fetch request.
- iOS 3.0+
- macOS 10.4+
- tvOS 9.0+
- watchOS 2.0+
- Core Data
A fetch request that specifies the search criteria for the fetch.
If there is a problem executing the fetch, upon return contains an instance of
NSErrorthat describes the problem.
An array of objects that meet the criteria specified by
request fetched from the receiver and from the persistent stores associated with the receiver’s persistent store coordinator. If an error occurs, returns
nil. If no objects match the criteria specified by
request, returns an empty array.
Returned objects are registered with the receiver.
The following points are important to consider:
If the fetch request has no predicate, then all instances of the specified entity are retrieved, modulo other criteria below.
An object that meets the criteria specified by
request(it is an instance of the entity specified by the request, and it matches the request’s predicate if there is one) and that has been inserted into a context but which is not yet saved to a persistent store, is retrieved if the fetch request is executed on that context.
If an object in a context has been modified, a predicate is evaluated against its modified state, not against the current state in the persistent store. Therefore, if an object in a context has been modified such that it meets the fetch request’s criteria, the request retrieves it even if changes have not been saved to the store and the values in the store are such that it does not meet the criteria. Conversely, if an object in a context has been modified such that it does not match the fetch request, the fetch request will not retrieve it even if the version in the store does match.
If an object has been deleted from the context, the fetch request does not retrieve it even if that deletion has not been saved to a store.
Objects that have been realized (populated, faults fired, “read from”, and so on) as well as pending updated, inserted, or deleted, are never changed by a fetch operation without developer intervention. If you fetch some objects, work with them, and then execute a new fetch that includes a superset of those objects, you do not get new instances or update data for the existing objects—you get the existing objects with their current in-memory state.