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Data: advanced techniques
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display a table view of to-do objects that will be saved and retrieved using Core Data. You ll see how to set up Core Data, how to initialize Core Data objects, how to add those objects to the database, and then how to access and manipulate them. Although this won t be an in-depth discussion about Core Data, it will give you the knowledge necessary to use Core Data for storage in your own applications.
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Background information about Core Data
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Let s briefly look at the concepts and terminology we ll be using. We ll start by discussing the heart of Core Data, the managed object.
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A managed object is a representation of an object you want to store in a database. Think of it as a record in SQL. It generally contains fields that match up with the properties of an object being saved in your application. After you create a managed object, you must insert it into a managed object context before you can save it to the data store.
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The managed object context holds all of your managed objects until they re ready to be committed to the database. Inside this context, managed objects can be added, modified, and deleted. This is like a buffer between your application and the database.
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This object describes the schema of your database. It s used when interfacing the managed object context with the database. A managed object table contains a collection of entity descriptions. Each of these entities describes a table in your database and is used when mapping managed objects to database entries.
Setting up Core Data in your application
Integrating Core Data into your application is simple. It requires less code than SQLite and offers a much simpler interface. The sample application we ll look at in this section is a basic journal application. It will allow you to post entries and view them by date. To start using the Core Data API, be sure to add CoreData.framework to your project. The first thing you must do to integrate Core Data is add the data model to your project. The data model file is where you do all the creation of your Core Data database. To add it, choose File > New File. Then, select Data Model under Resource. Name it something appropriate to your application. In this example, name it CDJournal.Xcdatamodel, and click Finish. Now that you ve added the data model to your project, you must define your database entities. Click the Xcdatamodel to open the table editor. Now, follow these steps to add the table for the journal entries:
Click the + button in the Entity box. Doing so adds a new entity with a default name. Change the name of this field to Entry. Figure 9.4 shows what this box should look like.
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An introduction to Core Data
Figure 9.4 Entry box
Properties of an Entry
Now that you ve created an entity named Entry, you must create the properties that go along with it. To do this, click + in the Property box and add your attribute properties: in this case, body, title, and creationDate. Notice that you can specify the type of each property, similarly to how you d do it in SQLite. The Property box should look like figure 9.5. At this point you may also create any other entities needed for your application. If one entity contains another, you can drag and drop to create relationships. This is similar to a foreign key in SQL. In this case, you could have authors, and an author could have many entries.
Now that you ve created your database, you must generate the classes that represent your database objects. This allows you to get a code representation of your entities. To do this, click your Xcdatamodel file. Then, choose File > New File. You should see a new class type called Managed Object Class under Cocoa Touch Class. If you don t see this, make sure you clicked the Xcdatamodel file prior to opening the New File dialog box. Figure 9.6 shows what this window should look like.
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