Model View Controller (MVC)
iPhone application development uses MVC architecture where M stands for Model, V stands for View, and C for Controller.
- Model represents the backend data—data model
- View represents the user interface elements through which the user looks at the contents displayed by the application and can interact with them
- Controller represents the application logic that decides the type of view to be displayed on the basis of actions taken by the user
Core Data organizes the data model in terms of objects that are easy to handle and manipulate. The finalized objects are stored on a persistent storage. The usual way of representing data models is through classes that contains variables and accessor methods. We don't have to create classes by hand, (for our data models) as Core Data framework provides a special Data Model Design tool (also known as Data Model Editor) for quickly creating an entity relationship model. The terms that we will be frequently using from now onwards are Managed Object Model, Managed Objects, and Managed Object Context. Let us see what these terms mean:
- Managed Object Model: The data model created by the Data Model Design tool (Data Model Editor) is also known as Managed Object Model.
- Managed Objects: Managed objects are instances of the
NSManagedObjectclass (or its subclass) that represent instances of an entity that are maintained (managed) by the Core Data framework. In a managed object model, an entity is defined by an entity name and the name of the class that is used at runtime to represent it. TheNSManagedObjectclass implements all of the functionality required by a managed object. - A managed object is associated with an entity description (an instance of
NSEntityDescription)that describes the object; for example, the name of the entity, its attributes, relationships, and so on. In other words, anNSEntityDescriptionobject may consist ofNSAttributeDescriptionandNSRelationshipDescriptionobjects that represent the properties of the entity. At runtime, the managed object is associated with a managed object context. - Managed Object Context: The objects when fetched from the persistent storage are placed in managed object context. It performs validations and keeps track of the changes made to the object's attributes so that undo and redo operations can be applied to it, if required. In a given context, a managed object provides a representation of a record in a persistent store. Depending on a situation, there may be multiple contexts—each containing a separate managed object representing that record.
Note
All managed objects are registered with managed object context.
For an application, we need the information represented by the Managed Object (instance of an entity) to be stored on the disk (persistent store) via managed object context. To understand the concepts of managed object context and its relation with data persistence, we need to understand the components of Core Data API, so let us go ahead and look at what Core Data API is all about.