Updating the Data Source
One problem with the app at the moment is that fetchInterestingPhotos(completion:) only returns the newly inserted photos. Now that the application supports saving, it should return all of the photos – the previously saved photos as well as the newly inserted ones. You need to ask Core Data for all of the Photo entities, and you will accomplish this using a fetch request.
Fetch requests and predicates
To get objects back from the NSManagedObjectContext, you must prepare and execute an NSFetchRequest. After a fetch request is executed, you will get an array of all the objects that match the parameters of that request.
A fetch request needs an entity description that defines which entity you want ...
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