What Is a Core Data Fault
NSFetchRequest *fetch = [[NSFetchRequest alloc] init]; NSEntityDescription * entity *sort = [[NSSortDescriptor alloc] initWithKey:@"name" ascending:YES]; [ fetch fetchedResultsController; // goal: tell the FRC to fetch all property objects that. its attributes are equivalent to table fields and the relationships with other Our first goal is to display the data using table views and NSFetchedResultsController. a NSFetchRequest returning an array of Publisher managed objects. NSFetchedResultsController is able to sort the data by one or more. So, that creates the NSFetchRequest, gives it a sort descriptor to arrange the newest So I'm addressing to the relationship with an index, and I end up unfaulting Some of this is designed toward a specific end goal, but much of it is very.
In other words, the Note and Category entities share a one-to-many relationship. A note can also have tags and a tag can be linked to multiple notes. The Note and Tag entities share a many-to-many relationship. Build and run the application to give it a try. Make sure Notes contains a few notes, a few categories, and a few tags. That is important for our experiment. Execute the fetch request and print the first note to the console. Run the application to see the result. We fetched a record of the Note entity.
But notice that the data associated with the record is missing. Instead we see the word fault. As you know, Core Data can only operate on records of the persistent store once they are loaded into memory. This is only possible because Core Data is heavily optimized to keep its memory footprint as low as possible. One of the techniques Core Data uses to accomplish this is faulting.
But the team at Apple didn't invent faulting. Several other frameworks use a similar strategy to accomplish similar goals. Ruby on Rails and Ember come to mind. Even though faulting may look mysterious at first, the idea is simple. Core Data only fetches the data it absolutely needs to satisfy the needs of your application. That is faulting in a nutshell. The idea of faulting is simple, but the underlying implementation is an advanced bit of programming. Fortunately, we don't have to worry about that.
That is the responsibility of the framework. Let me show you how it works with another example. Below the print statement, we safely unwrap the value of the title property and print it to the console and we add another print statement for the note. We print the note to the console, ask the value of one of the properties of the note, and print the note again.
Why we do that becomes clear when we inspect the results in the console. Remember, this only happens if you check the Use Core Data checkbox at the beginning. This default managed object context lives as a property of the NSPersistentContainer in the application delegate. To access it, you first get a reference to the app delegate. You create a new managed object and insert it into the managed object context. You may be wondering what an NSEntityDescription is all about. Recall earlier, NSManagedObject was called a shape-shifter class because it can represent any entity.
An entity description is the piece linking the entity definition from your Data Model with an instance of NSManagedObject at runtime. With an NSManagedObject in hand, you set the name attribute using key-value coding.
Ios nsfetchrequest order
You must spell the KVC key name in this case exactly as it appears in your Data Model, otherwise, your app will crash at runtime. You commit your changes to person and save to disk by calling save on the managed object context. Note save can throw an error, which is why you call it using the try keyword within a do-catch block. Finally, insert the new managed object into the people array so it shows up when the table view reloads. Some of the code here, such as getting the managed object context and entity, could be done just once in your own init or viewDidLoad then reused later.
Build and run the app, and add a few names to the table view: If the names are actually stored in Core Data, the HitList app should pass the persistence test. With the app in the foreground, go to the fast app switcher and then terminate it. From Springboard, tap the HitList app to trigger a fresh launch. The table view is empty: You saved to Core Data, but after a fresh app launch, the people array is empty!
Fetching from Core Data To get data from your persistent store into the managed object context, you have to fetch it. Before you can do anything with Core Data, you need a managed object context. Fetching is no different! Like before, you pull up the application delegate and grab a reference to its persistent container to get your hands on its NSManagedObjectContext. Fetch requests are both powerful and flexible.
Getting Started with Core Data Tutorial
You can use fetch requests to fetch a set of objects meeting the provided criteria i. Fetch requests have several qualifiers used to refine the set of results returned. For now, you should know NSEntityDescription is one of these required qualifiers. This is what you do here to fetch all Person entities. Also note NSFetchRequest is a generic type. You hand the fetch request over to the managed object context to do the heavy lifting.
If an error occurred during the fetch, you can inspect the error inside the catch block and respond appropriately. Build and run the application. Immediately, you should see the list of names you added earlier: Add a few more names to the list and restart the app to verify saving and fetching are working.
Short of deleting the app, resetting the Simulator or throwing your phone off a tall building, the names will appear in the table view no matter what. There were a few rough edges in this sample app: Key Points Core Data provides on-disk persistence, which means your data will be accessible even after terminating your app or shutting down your device.
This is different from in-memory persistence, which will only save your data as long as your app is in memory, either in the foreground or in the background. Xcode comes with a powerful Data Model editor, which you can use to create your managed object model. A managed object model is made up of entities, attributes and relationships An entity is a class definition in Core Data.
An attribute is a piece of information attached to an entity. You can read and write to its attributes using Key-Value Coding. Where to Go From Here? Data Models, entities, attributes, managed objects, managed object contexts and fetch requests. If you enjoyed what you learned in this tutorial, why not check out the complete Core Data by Tutorials bookavailable in our store? This chapter covers the basics of setting up your data model and then adding and fetching records.
The project bundle contains a pre-populated SQLite file that will be used as the persistent store for the Core Data stack. Because the Core Data entities are linked to each other to form a data tree, we can embed the table views in a navigation controller and browse the data one level at a time.
On the main screen we will display the list of Publishers and, for each Publisher, we will show on a second screen the list of books they published.
Open Xcode and create a single view application with support for Core Data: If you create the project from scratch, feel free to replace the default data model file with the one you copy from the sample project. Xcode adds a navigation controller to the storyboard and initializes its root view controller with the table view controller we previously added.
Finally, select the navigation controller and make it the Initial view controller in the Attributes inspector: Create the PublishersTableViewController class which inherits from UITableViewController; remove the commented code from the default implementation file and associate the class to the table view controller in the storyboard: