barcode using vb.net Create a new Navigation-based Application named DogBreeds Be certain to select the Use in Objective-C

Make QR Code 2d barcode in Objective-C Create a new Navigation-based Application named DogBreeds Be certain to select the Use

1 Create a new Navigation-based Application named DogBreeds Be certain to select the Use
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Core Data check box (Figure 17-3) Xcode will create an almost functional Navigationbased Application with Core Data support, including a default xcdatamodel file
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Figure 17-3 Creating a new Navigation-based Application
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2 The default xcdatamodel contains a single Event entity If there were corresponding
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Eventh and Eventm class files for the Event entity, you would be able to run the new application that was just created Fortunately, Xcode will create the class definition for you based on the attributes defined in the xcdatamodel Select the Entity and then select Create NSManagedObject Subclass from the Editor menu
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3 Eventh and Eventm will be generated (Listings 17-1 and 17-2) Run the application You
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will notice that you can create new events in the list, and edit the list to delete events If you quit the application and later relaunch it, the events have all been persisted
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4 Take a few minutes to look at the generated source code for the AppDelegate and
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RootViewController and familiarize yourself with the methods that were automatically generated for you Listing 17-1 Definition of a simple Event class
#import <Cocoa/Cocoah> @interface Event : NSManagedObject { @private } @property (nonatomic, retain) NSDate * timeStamp; @end
Listing 17-2
Implementation of a simple Event class
#import "Eventh" @implementation Event @dynamic timeStamp; @end 5 Now we ll change the default application to reflect our dog breeds data model Delete the
Eventh and Eventm files Select the DogBreedsxcdatamodel file Delete the Event entity by selecting it and pressing the DELETE key
6 Click the Add Entity button (at the bottom of the pane) and create a new entity called
AKCGroup (Figure 17-4)
7 Click the Add Attribute button and add an attribute called groupDescription with type
String Create another attribute called name, also with type String
8 Click the Add Entity button again and create a new entity called Breed Give the Breed
entity three String attributes: name, breedDescription, and photoURL (Figure 17-5)
17:
Core Data
Figure 17-4 Adding the AKCGroup entity
9 With the Breed entity selected, click and hold on the Add Attribute button and select Add
Relationship Name the new relationship group and select AKCGroup from the Destination pull-down menu
10 Select View | Utilities | Core Data Model to display the relationship details Uncheck the
Optional check box
11 Select the AKCGroup entity and add a new relationship to it called breeds Set its
destination to Breed and Inverse to group Select the To-Many Relationship check box and set the delete rule to Cascade (Figure 17-6)
12 Save the model You are finished describing the objects we need to persist and how they
relate to each other Core Data will use this information to determine how and when to save your objects in an underlying SQLite database
13 Now you need to create the underlying classes for the AKCGroup and Breed entities
Highlight AKCGroup and Breed and then select Editor | Create NSManagedObject Subclass from the main menu
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Figure 17-5 Breed entity with its attributes
14 Xcode will generate four files for you, Breedh, Breedm, AKCGrouph, and AKCGroupm,
based on the entity descriptions we created earlier
15 Select Breedh and take a look at what was generated for you You will notice that the
Breed class has properties defined for each of the attributes in the entity You will also notice that the Breed class doesn t contain instance variables for the properties Instead, for Core Data managed classes, they are defined with the @dynamic directive in the implementation file
NOTE
NSManagedObjects have no dealloc methods, as the Core Data framework manages their life cycle Core Data is also responsible for generating NSManagedObjects accessor methods at runtime 16 Select the AKCGrouph file and take a look at what was generated for you There are
properties for the two attributes and the relationship that we defined in the xcdatamodel file
17:
Core Data
Figure 17-6 Relationship between AKCGroup and Breeds
17 We have changed the object model since you ran the default application in Step 2, so you
need to completely delete the application from the iPhone Simulator so that the database containing the Event data will also be deleted Otherwise, when you run the modified application in the next task, you will get an error because the model changed from Event entities to Breeds and AKCGroups
Notice you specified that the relationship between AKCGroup and Breed has a Cascade Delete Rule This rule informs the model that when an AKCGroup object is deleted, then any Breed objects that it references should also be deleted Other delete rules you might specify include Nullify, Deny, and No Action
NOTE
For more information on creating models using Xcode, refer to Apple s Creating a Managed Object Model with Xcode
iOS SDK Programming: A Beginner s Guide
Model, Context, and Store
The preceding task created the model used to create entities and their relationships in a managed object context When an application runs, it needs a model instance, the context, and the persistent store The persistent store and the model are largely transparent to you when coding your application You simply obtain both these items and then set properties referencing them in the managed object context Because we started from a navigation-based application template set to use Core Data, Xcode added code to create and manage these for us
NSManagedObjectModel
As discussed earlier, an application s managed object model contains entities and their relationships It serves as an application s schema by describing the entities used in an application s managed object context Take a look at DogBreedsAppDelegate and you will see that Xcode added a property to the AppDelegate to hold the NSManagedObjectModel and an accessor method that creates the managedObjectModel on the first access The easiest way to obtain the model is through the mergedModelFromBundles: class method
managedObjectModel = [[NSManagedObjectModel mergedModelFromBundles: nil] retain];
This method creates a data model by merging all the models it finds into a bundle Because the previous code specifies nil, the method simply finds all the models in the application s Resources folder and merges them into one NSManagedObjectModel instance
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