CoreData is a framework for managing object graphs and storing them on disk. It is much more than just offering a persistent storage, therefore the rich API it provides, is meant to be used app-wide. In this blog post we’ll look into how to initialise CoreData storage, store some data in it and fetching it.
Core data model
First step is to add a Core Data model which will describe model objects, their relationships and properties. In Xcode we’ll go to
File -> New -> File… and then select Data Model.
We are going to continue working on sample app named Planets (from previous post). It needs an entity named Planet which has 3 properties: name, url and position (defines sort order in the app). By default Core Data will autogenerate a model object for our entity Planet we just defined. Therefore all we need to do is creating an entity and adding three properties to the entity.
Initialising CoreData stack
The simplest way for initialising Core Data stack is to use NSPersistentContainer and initialising it with a name of the Core Data model file we just created in the previous step. After creating the container, it needs to be loaded which will read the Core Data model file and sets up a persistent store.
In the sample app we are going to use subclass of NSPersistentContainer named CoreDataStore which contains extra methods used in a moment.
Adding data contains of two steps: making changes in NSManagedObjectContext and then saving it. NSManagedObjectContext is a scratch pad where to make changes in your object graph. All the changes will be stored in memory until save is called.
For adding a new entity we will use a method in our CoreDataStore.
This will add a new empty entity to managed object context. After that, we’ll fill properties with appropriate values and then call save.
In our sample app, there is a simple table view displaying a list of planets. For displaying data efficiently, Core Data has a class NSFetchedResultsController. It uses NSFetchRequest objects for fetching, sorting and filtering results. Our class CoreDataStore has a convenience method for creating fetched results controller for any type of entities and PlanetManager has a getter for returning controller for Planet entities.
Let’s see how to hook up fetched results controller to table view controller. Before fetched controller is used for fetching data, it needs to perform a fetch. For a simple list view it is pretty straight-forward: getting count and fetching an object for index path.
In this blog post we looked into how to initialise Core Data stack with NSPersistentContainer, storing some data in it with NSManagedObjectContext and fetching results with NSFetchedResultsController. It is the most basic usage of CoreData and covers a tiny bit what Core Data can do. Apple has a pretty good documentation what covers much more compared to what was described here (see links to Core Data classes in the previous sections).