Outlets

An outlet is a property of an object that references another object. The reference is archived through Interface Builder. The connections between the containing object and its outlets are reestablished every time the containing object is unarchived from its nib file. The containing object holds an outlet declared as a property with the type qualifier of IBOutlet and a weak option. For example:

@interface AppController : NSObject

{

}

@property (weak) IBOutlet NSArray *keywords;

Because it is a property, an outlet becomes part of an object’s encapsulated data and is backed by an instance variable. But an outlet is more than a simple property. The connection between an object and its outlets is archived in a nib file; when the nib file is loaded, each connection is unarchived and reestablished, and is thus always available whenever it becomes necessary to send messages to the other object. The type qualifier IBOutlet is a tag applied to an property declaration so that the Interface Builder application can recognize the property as an outlet and synchronize the display and connection of it with Xcode.

An outlet is declared as a weak reference (weak) to prevent strong reference cycles.

You create and connect an outlet in the Interface Builder feature of Xcode.The property declaration for the outlet must be tagged with the IBOutlet qualifier.

An application typically sets outlet connections between its custom controller objects and objects on the user interface, but they can be made between any objects that can be represented as instances in Interface Builder, even between two custom objects. As with any item of object state, you should be able to justify its inclusion in a class; the more outlets an object has, the more memory it takes up. If there are other ways to obtain a reference to an object, such as finding it through its index position in a matrix, or through its inclusion as a function parameter, or through use of a tag (an assigned numeric identifier), you should do that instead.

Outlets are a form of object composition, which is a dynamic pattern that requires an object to somehow acquire references to its constituent objects so that it can send messages to them. It typically holds these other objects as properties backed by instance variables. These variables must be initialized with the appropriate references at some point during the execution of the program.