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This chapter is from the book

Adding Core Data to an Existing Application

When you create an iOS Application project in Xcode, you can choose from various starting-point templates. Using Core Data in your project is as easy as ticking the Use Core Data check box during creation of a Master-Detail, Utility Application, or Empty Application template-based project. Adding Core Data manually is more educational, so the “Grocery Dude” project is created based on the Single View Application template, which doesn’t include Core Data. To use the Core Data Framework, you’ll need to link it to the project.

Update Grocery Dude as follows to link to the Core Data Framework:

  1. Select the Grocery Dude Target, as shown in Figure 1.2.

    Figure 1.2

    Figure 1.2 Linking the Core Data Framework

  2. Click the + found in the Linked Frameworks and Libraries section of the General tab and then link to the CoreData.framework, as shown in Figure 1.2.

Introducing Core Data Helper

If you’ve ever examined the built-in Core Data–enabled templates, you may have noticed a lot of Core Data setup is done in the application delegate. So that you may apply the approach used in this book to your own projects, Core Data will be set up using a helper class. This keeps the Core Data components modular and portable. The application delegate will be used to lazily create an instance of the CoreDataHelper class. An instance of this class will be used to do the following:

  • Initialize a managed object model
  • Initialize a persistent store coordinator with a persistent store based on the managed object model
  • Initialize a managed object context based on the persistent store coordinator

Update Grocery Dude as follows to create the CoreDataHelper class in a new Xcode group:

  1. Right-click the Grocery Dude group in Xcode and then create a new group called Generic Core Data Classes, as shown in Figure 1.3.

    Figure 1.3

    Figure 1.3 Xcode group for generic Core Data classes

  2. Select the Generic Core Data Classes group.
  3. Click File > New > File....
  4. Create a new iOS > Cocoa Touch > Objective-C class and then click Next.
  5. Set Subclass of to NSObject and Class name to CoreDataHelper and then click Next.
  6. Ensure the Grocery Dude target is ticked and then create the class in the Grocery Dude project directory.

Listing 1.1 shows new code intended for the CoreDataHelper header file.

Listing 1.1 CoreDataHelper.h

#import <Foundation/Foundation.h>
#import <CoreData/CoreData.h>

@interface CoreDataHelper :NSObject

@property (nonatomic, readonly) NSManagedObjectContext       *context;
@property (nonatomic, readonly) NSManagedObjectModel         *model;
@property (nonatomic, readonly) NSPersistentStoreCoordinator *coordinator;
@property (nonatomic, readonly) NSPersistentStore            *store;

- (void)setupCoreData;
- (void)saveContext;
@end

As an Objective-C programmer, you should be familiar with the purpose of header (.h) files. CoreDataHelper.h is used to declare properties for the context, model, coordinator and the store within it. The setupCoreData method will be called once an instance of CoreDataHelper has been created in the application delegate. The saveContext method may be called whenever you would like to save changes from the managed object context to the persistent store. This method can cause interface lag if there are a lot of changes to be written to disk. It is recommended that it only be called from the applicationDidEnterBackground and applicationWillTerminate methods of AppDelegate.m—at least until background save is added in Chapter 11.

Update Grocery Dude as follows to configure the CoreDataHelper header:

  1. Replace all code in CoreDataHelper.h with the code from Listing 1.1. If you select CoreDataHelper.m, Xcode will warn that you haven’t implemented the setupCoreData and saveContext methods, which is okay for now.

Core Data Helper Implementation

The helper class will start out with four main sections. These sections are FILES, PATHS, SETUP, and SAVING. For easy navigation and readability, these areas are separated by pragma marks. As shown in Figure 1.4, the pragma mark feature of Xcode allows you to logically organize your code and automatically provides a nice menu for you to navigate with.

Figure 1.4

Figure 1.4 Pragma mark generated menu

Files

The FILES section of CoreDataHelper.m starts out with a persistent store filename stored in an NSString. When additional persistent stores are added later, this is where you’ll set their filenames. Listing 1.2 shows the code involved along with a new #define statement, which will be used in most of the classes in Grocery Dude to assist with debugging. When debug is set to 1, debug logging will be enabled for that class. Most NSLog commands will be wrapped in an if (debug == 1) statement, which will only work when debugging is enabled.

Listing 1.2 CoreDataHelper.m: FILES

#define debug 1

#pragma mark - FILES
NSString *storeFilename = @"Grocery-Dude.sqlite";

Update Grocery Dude as follows to add the FILES section:

  1. Add the code from Listing 1.2 to the bottom of CoreDataHelper.m before @end.

Paths

To persist anything to disk, Core Data needs to know where in the file system persistent store files should be located. Three separate methods help provide this information. Listing 1.3 shows the first method, which is called applicationDocumentsDirectory and returns an NSString representing the path to the application’s documents directory. You’ll also notice the first use of an if (debug==1) statement wrapping a line of code that shows what method is running. This NSLog statement is useful for seeing the order of execution of methods in the application, which is great for debugging.

Listing 1.3 CoreDataHelper.m: PATHS

#pragma mark - PATHS
- (NSString *)applicationDocumentsDirectory {
if (debug==1) {
    NSLog(@"Running %@ '%@'", self.class,NSStringFromSelector(_cmd));
}
return [NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask,YES) lastObject];
}

Update Grocery Dude as follows to add the PATHS section:

  1. Add the code from Listing 1.3 to the bottom of CoreDataHelper.m before @end.

The next method, applicationStoresDirectory, appends a directory called Stores to the application’s documents directory and then returns it in an NSURL. If the Stores directory doesn’t exist, it is created as shown in Listing 1.4.

Listing 1.4 CoreDataHelper.m: applicationStoresDirectory

- (NSURL *)applicationStoresDirectory {
if (debug==1) {
    NSLog(@"Running %@ '%@'", self.class, NSStringFromSelector(_cmd));
}

NSURL *storesDirectory =
[[NSURL fileURLWithPath:[self applicationDocumentsDirectory]]
                                    URLByAppendingPathComponent:@"Stores"];

NSFileManager *fileManager = [NSFileManager defaultManager];
if (![fileManager fileExistsAtPath:[storesDirectory path]]) {
    NSError *error = nil;
    if ([fileManager createDirectoryAtURL:storesDirectory
              withIntermediateDirectories:YES
                               attributes:nil
                                    error:&error]) {
        if (debug==1) {
            NSLog(@"Successfully created Stores directory");}
        }
        else {NSLog(@"FAILED to create Stores directory: %@", error);}
    }
    return storesDirectory;
}

Update Grocery Dude as follows to add to the PATHS section:

  1. Add the code from Listing 1.4 to the bottom of CoreDataHelper.m before @end.

The last method, which is shown in Listing 1.5, simply appends the persistent store filename to the store’s directory path. The end result is a full path to the persistent store file.

Listing 1.5 CoreDataHelper.m: storeURL

- (NSURL *)storeURL {
if (debug==1) {
    NSLog(@"Running %@ '%@'", self.class, NSStringFromSelector(_cmd));
}
return [[self applicationStoresDirectory]
              URLByAppendingPathComponent:storeFilename];
}

Update Grocery Dude as follows to add to the PATHS section:

  1. Add the code from Listing 1.5 to the bottom of CoreDataHelper.m before @end.

Setup

With the files and paths ready to go, it’s time to implement the three methods responsible for the initial setup of Core Data. Listing 1.6 shows the first method, called init, which runs automatically when an instance of CoreDataHelper is created.

Listing 1.6 CoreDataHelper.m: SETUP

#pragma mark - SETUP
- (id)init {
if (debug==1) {
    NSLog(@"Running %@ '%@'", self.class, NSStringFromSelector(_cmd));
}
    self = [super init];
    if (!self) {return nil;}

    _model = [NSManagedObjectModel mergedModelFromBundles:nil];
    _coordinator = [[NSPersistentStoreCoordinator alloc]
                            initWithManagedObjectModel:_model];
    _context = [[NSManagedObjectContext alloc]
                            initWithConcurrencyType:NSMainQueueConcurrencyType];
    [_context setPersistentStoreCoordinator:_coordinator];
    return self;
}

The _model instance variable points to a managed object model. The managed object model is initiated from all available data model files (object graphs) found in the main bundle by calling mergedModelFromBundles and passing nil. At the moment, there are no model files in the project; however, one will be added in Chapter 2. It is possible to pass an NSArray of NSBundles here in case you wanted to merge multiple models. Usually you won’t need to worry about this.

The _coordinator instance variable points to a persistent store coordinator. It is initialized based on the _model pointer to the managed object model that has just been created. So far, the persistent store coordinator has no persistent store files because they will be added later by the setupCoreData method.

The _context instance variable points to a managed object context. It is initialized with a concurrency type that tells it to run on a “main thread” queue. You’ll need a context on the main thread whenever you have a data-driven user interface. Once the context has been initialized, it is configured to use the existing _coordinator pointer to the persistent store coordinator. Chapter 8 will demonstrate how to use multiple managed object contexts, including a background (private queue) concurrency type. For now, the main thread context will do.

Update Grocery Dude as follows to add the SETUP section:

  1. Add the code from Listing 1.6 to the bottom of CoreDataHelper.m before @end.

The next method required in the SETUP section is loadStore and is shown in Listing 1.7.

Listing 1.7 CoreDataHelper.m: loadStore

- (void)loadStore {
if (debug==1) {
NSLog(@"Running %@ '%@'", self.class, NSStringFromSelector(_cmd));
}
    if (_store) {return;} // Don't load store if it's already loaded
    NSError *error = nil;
    _store = [_coordinator addPersistentStoreWithType:NSSQLiteStoreType
                                        configuration:nil
                                                  URL:[self storeURL]
                                              options:nil error:&error];
    if (!_store) {NSLog(@"Failed to add store. Error: %@", error);abort();}
    else         {if (debug==1) {NSLog(@"Successfully added store: %@", _store);}}
}

The loadStore method is straightforward. Once a check for an existing _store has been performed, a pointer to a nil NSError instance is created as error. This is then used when setting the _store instance variable to capture any errors that occur during setup. If _store is nil after an attempt to set it up fails, an error is logged to the console along with the content of the error.

When the SQLite persistent store is added via addPersistentStoreWithType, a pointer to the persistent store is held in _store. The storeURL of the persistent store is the one returned by the methods created previously.

Update Grocery Dude as follows to add to the SETUP section:

  1. Add the code from Listing 1.7 to the bottom of CoreDataHelper.m before @end.

Finally, it’s time to create the setupCoreData method. With the other supporting methods in place, this is a simple task. Listing 1.8 shows the contents of this new method, which at this stage only calls loadStore. This method will be expanded later in the book as more functionality is added.

Listing 1.8 CoreDataHelper.m: setupCoreData

- (void)setupCoreData {
if (debug==1) {
    NSLog(@"Running %@ '%@'", self.class, NSStringFromSelector(_cmd));
}
    [self loadStore];
}

Update Grocery Dude as follows to add to the SETUP section:

  1. Add the code from Listing 1.8 to the bottom of CoreDataHelper.m before @end.

Saving

The next puzzle piece is a method called whenever you would like to save changes from the _context to the _store. This is as easy as sending the context a save: message, as shown in Listing 1.9. This method will be placed in a new SAVING section.

Listing 1.9 CoreDataHelper.m: SAVING

#pragma mark - SAVING
- (void)saveContext {
if (debug==1) {
    NSLog(@"Running %@ '%@'", self.class, NSStringFromSelector(_cmd));
}
    if ([_context hasChanges]) {
        NSError *error = nil;
        if ([_context save:&error]) {
            NSLog(@"_context SAVED changes to persistent store");
        } else {
            NSLog(@"Failed to save _context: %@", error);
        }
    } else {
        NSLog(@"SKIPPED _context save, there are no changes!");
    }
}

Update Grocery Dude as follows to add the SAVING section:

  1. Add the code from Listing 1.9 to the bottom of CoreDataHelper.m before @end.

The Core Data Helper is now ready to go! To use it, a new property is needed in the application delegate header. The CoreDataHelper class also needs to be imported into the application delegate header, so it knows about this new class. The bold code shown in Listing 1.10 highlights the changes required to the application delegate header.

Listing 1.10 AppDelegate.h

#import <UIKit/UIKit.h>
#import "CoreDataHelper.h"
@interface AppDelegate : UIResponder <UIApplicationDelegate>
@property (strong, nonatomic) UIWindow *window;
@property (nonatomic, strong, readonly) CoreDataHelper *coreDataHelper;
@end

Update Grocery Dude as follows to add CoreDataHelper to the application delegate:

  1. Replace all code in AppDelegate.h with the code from Listing 1.10.

The next step is to update the application delegate implementation with a small method called cdh, which returns a non-nil CoreDataHelper instance. In addition, a #define debug 1 statement needs to be added for debug purposes, as shown in Listing 1.11.

Listing 1.11 AppDelegate.m: cdh

#define debug 1

- (CoreDataHelper*)cdh {
if (debug==1) {
    NSLog(@"Running %@ '%@'", self.class, NSStringFromSelector(_cmd));
}
    if (!_coreDataHelper) {
        _coreDataHelper = [CoreDataHelper new];
        [_coreDataHelper setupCoreData];
    }
    return _coreDataHelper;
}

Update Grocery Dude as follows to add the cdh method to the application delegate:

  1. Add the code from Listing 1.11 to AppDelegate.m on the line after @implementation AppDelegate.

The final step required is to ensure the context is saved each time the application enters the background or is terminated. This is an ideal time to save changes to disk because the user interface won’t lag during save as it is hidden. Listing 1.12 shows the code involved in saving the context.

Listing 1.12 AppDelegate.m: applicationDidEnterBackground

- (void)applicationDidEnterBackground:(UIApplication *)application {
    [[self cdh] saveContext];
}
- (void)applicationWillTerminate:(UIApplication *)application {
    [[self cdh] saveContext];
}

Update Grocery Dude as follows to ensure the context is saved when the application enters the background or is terminated:

  1. Add [[self cdh] saveContext]; to the bottom of the applicationDidEnterBackground method in AppDelegate.m.
  2. Add [[self cdh] saveContext]; to the bottom of the applicationWillTerminate method in AppDelegate.m.

Run Grocery Dude on the iOS Simulator and examine the debug log window as you press the home button (Shift+cmd.jpg+H or Hardware > Home). The log is initially blank because Core Data is set up on demand using the cdh method of the application delegate. The first time Core Data is used is during the save: that’s triggered when the application enters the background. As the application grows, the cdh method will be used earlier. Figure 1.5 shows the order of method execution once you press the home button.

Figure 1.5

Figure 1.5 The debug log window showing order of execution

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