Learning Core Data for iOS (2014)

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Update Grocery Dude as follows to configure object change notifications:

1. Add the method from Listing 11.3 to the bottom of the VIEW section of LocationAtHomeVC.m, LocationAtShopVC.m, and UnitVC.m.

The application is now ready to test background save:

1. Delete Grocery Dude from the iOS Simulator and click Product > Clean in Xcode.

2. Run Grocery Dude on the iOS Simulator and wait for the test data import process to finish. You can track progress in the console log, as the table view will remain blank until the import completes.

3. Create a new item by clicking + on the Prepare tab. Give the new item any name and press Done. A save will be triggered when the ItemVC view disappears.

Whenever a save triggers, you should see the main queue _context and private queue _parentContext log a SAVED result to the console, as shown in Figure 11.2.

Figure 11.2 Background save

With background save now implemented, you may be wondering how much benefit it has really added. Saving in the foreground in Grocery Dude was fast to begin with, so the benefits are arguably small. The benefits of background save are more obvious when a larger amount of data needs to be saved. A scenario where background save is beneficial is during an import process.

Background Processing

With the introduction of a parent context, there are implications on the way data is imported. You have already seen how the imported test data was not reflected in the table views during the import. The problem is that the fetched results controller of the Items table view has no idea that the new data is available in the persistent store. Although you could set up notification observers to trigger a merge, there’s no need to when you can instead configure _context as the parent of _importContext. Figure 11.3 illustrates this concept.

Figure 11.3 _context as a parent of _importContext

Update Grocery Dude as follows to configure _importContext with a parent:

1. Remove the following line of code from the init method of CoreDataHelper.m:

[_importContext setPersistentStoreCoordinator:_coordinator];

2. Add the following code in its place:

[_importContext setParentContext:_context];

Although it won’t be used in this chapter, the _sourceContext should also be updated with _context as its parent.

Update Grocery Dude as follows to configure _sourceContext with a parent:

1. Remove the following line of code from the init method of CoreDataHelper.m:

[_sourceContext setPersistentStoreCoordinator:_sourceCoordinator];

2. Add the following code in its place:

[_sourceContext setParentContext:_context];

3. Comment out the following code from the init method of CoreDataHelper.m:

_sourceCoordinator =
[[NSPersistentStoreCoordinator alloc] initWithManagedObjectModel:_model];

Now that _importContext is a child of _context, imported data will be immediately visible on table views watching _context after _importContext is saved. This is a benefit of using a fetched results controller–backed table view.

Introducing Faulter

The process of generating thumbnails will impact a potentially large number of objects. Any time a large number of objects are processed, care should be taken to ensure memory bloating and interface lag is avoided. As explained in Chapter 10, you should turn objects into faults once you’re finished with them using refreshObject:mergeChanges:NO after saving the containing context. With a parent and child context hierarchy, you’ll need to do this for every context in the hierarchy. That means for every object imported, a save and fault are required in all three contexts.

The process of saving a context and turning a particular object into a fault is a repeatable one. The code needed to perform this task can be reused regardless of the context involved. As such, a new class called Faulter will be implemented that can be used to turn an object into a fault given itsobjectID. The given context will be saved prior to the object being turned into a fault. Faulter will automatically fault the object through the parent context hierarchy if the given context has a parent. Listing 11.4 shows the header of Faulter.

Listing 11.4 Faulter.h