What Is a Core Data Fault
In the previous article, we learned about NSManagedObject and how easy it is to create, read, update, and delete records using Core Data. The variable that's used to refer to the object where the fetched property is being which tells Core Data that we want to compare to the birthdate value on the object where Make sure variables start with a dollar sign and are not surrounded by quotes. CHAPTER 7: Relationships, Fetched Properties, and Expressions Name the project Quotes and make sure the Use Core Data checkbox The fetched results controller is going to be in charge of managing the.
The fetched results controller is going to be in charge of managing the data for the table view. For example, we want to hide the table view if the application doesn't contain any quotes.
Define a didSet property observer for the quotes property in which we invoke a helper method, updateView. In viewDidLoadwe invoke another helper method, setupView. You should see the message label being displayed. The activity indicator view is also visible. That is the next thing we fix.Learn about Relationships with Core Data - Beginning Core Data Tutorial - changethru.info
What is that about? We initialize the persistent container by passing in the name of the data model we created earlier. Don't forget to add an import statement for the Core Data framework at the top.
If the NSPersistentContainer container class is new to you, then you may want to read more about that first. We need to add the persistent store to the persistent store coordinator. We do this in the viewDidLoad method as shown below. When this operation finishes, the completion handler of the loadPersistentStores completionHandler: Even though the persistent store is added to the persistent store coordinator on a background queue, the completion handler is always invoked on the calling thread.
In the completion handler, we make sure no errors popped up. If everything went smoothly, we invoke setupView. Before we can run the application again to see the result, we need to revisit the main storyboard. Select the activity indicator view and, in the Attributes Inspector on the right, check the Animating and Hides When Stopped checkboxes.
Populate a Table View With NSFetchedResultsController and Swift
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.
What Is a Core Data Fault
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. Run the application and take a look at the output in the console. Despite this fault, we can access the value of the title property and print it to the console.
This is confirmed by the third print statement in which we print the note again. What is happening here? We first asked for the user's notes and Core Data diligently gave us the list of notes. But, as you can see in the console, it is a list of empty records.
From the moment we ask for the value of a property of one of the records, Core Data jumps into action and fetches the data from the persistent store. This is better known as firing a Core Data fault. But it doesn't just fetch the value of the title property. As you can see in the console, Core Data fetches the values of every property of the note with the exception of relationships.
Notice that the value of the tags property is missing.
Instead, Xcode displays relationship fault. Core Data will transparently map those objects back to a persistent store when you save to disk. A managed object holds a copy of data from a persistent store. If you use a database as a persistent store, then a managed object might represent data from a table row in that database. If you use an XML file as a persistent store Mac onlythen a managed object would represent data found within certain data elements.
A managed object context exists in high-speed volatile memory, also known as RAM. One reason a managed object context is required is the overhead involved with transferring data to and from disk. Having a managed object context allows access to data that has been previously retrieved from disk to be very fast. The downside, however, is that you need to call save: The managed object context exists also to track changes to its objects in order to provide full undo and redo support.
Learning Core Data for iOS: Your First Core Data Application | What Is Core Data? | InformIT
Each chapter of this book is headed by a famous Albert Einstein quote. Whenever I write technical tutorials or documentation, I remember this quote and strive for easy-to-read, highly informative material. To help visualize how the main pieces of Core Data fit together, examine Figure 1. The thing to note about Binary and XML stores is that they are atomic. This means that even if you only want to change a small amount of data, you still have to write out the whole file to disk when you save.
Of course, the same issue applies when reading an atomic store into memory in the first place. This can become problematic if you have a lot of data because it consumes valuable memory. An SQLite database, on the other hand, is updated incrementally as change logs, also known as transaction logs, are committed.
As a result, the SQLite database memory footprint is comparably very small.