barcode using vb.net Core Data in Brief in Objective-C

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Core Data in Brief
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Core Data is a framework used to easily manage an application s data objects Core Data consists of managed object models, managed object contexts, and persistent data stores A managed object model contains an object graph The object graph is a collection of objects and their relationships with one another You create a graph using Xcode s data modeler The data modeler is where you visually add entities and create relationships between them
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Core Data
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Managed Objects Managed Objects Managed Object Context
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Entities Entities Persistent Store Coordinator Managed Object Model
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Persistent Store
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Figure 17-1 Core Data s architecture (simplified)
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A managed object context contains the objects created from the entities in the managed object model A managed object context has a persistent store coordinator that manages one or more persistent stores A persistent data store persists objects created by the managed object context Although an application can have multiple persistent stores, in this chapter, you restrict yourself to one persistent store Figure 17-1 illustrates Core Data s architecture
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Creating a Model
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A model contains entities Entities contain attributes Relationships model how one or more entities relate to one another You model these concepts using Xcode s data modeler You add a model to your application by creating a file with an xcdatamodel extension through Xcode s New File dialog (Figure 17-2) It is best to place the model file in a new Models folder After creating the model, when you select the file, Xcode should automatically display the data modeler in the Editor window
Entities
Entities, represented by the NSEntityDescription class, are patterns describing NSManagedObject instances NSManagedObjects are what you persist For Core Data to know how to instantiate new object instances and persist the instances, it needs a pattern Entities in the model provide those patterns For instance, you might want to create a handy reference application that lists all of the American Kennel Club (AKC) dog breeds The AKC divides all of the dog breeds into categories like herding group and sporting group, so we ll need an AKCGroup class to represent these groups Within each group there are a number of dog breeds, so we also have
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Figure 17-2 Creating a new Core Data model
a Breed class to represent the breeds These straightforward Objective-C classes will be subclasses of NSManagedObject, and you tell Core Data how to persist them using entities
Attributes
Entities tell Core Data which classes we will need to persist In order to actually save objects in a persistent store and reconstitute them later, Core Data also needs to know what information must be saved for each object Therefore, in our data model, entities have attributes that describe each of the properties that will need to be persisted
Relationships
In all but the simplest of applications, there will also be relationships between the classes you define For instance, each AKCGroup will contain one or more Breed objects and every Breed will be in exactly one group The objects were modeled with entities, and how they are interrelated is modeled with relationships Entities can have relationships with other entities For instance, an apple and an orange are both types of fruit, a crate might hold one or more apples and oranges, and a fruit stand might contain one or more crates
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Core Data
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Adding Entities and Relationships to a Core Data Model
Reference applications are particularly popular in the App Store They typically contain hierarchical information that is easily viewed with a navigation-based iOS application In fact, this pattern is so common that Xcode provides a template that creates a navigationbased application that is tied to a Core Data model In this first task, you will first create a new navigation-based application and tell Xcode that it uses Core Data Then you create the entities needed for a dog breeds reference application and add attributes and relationships Later tasks will generate Objective-C classes that correspond to those entities, and then write the code to create, delete, and navigate those objects
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