Outlets
Object composition 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 instance variables. These variables must be initialized with the appropriate references at some point during the execution of the program.
An outlet is such an object instance variable, but it is special in that the reference is configured and 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 as an instance variable with the type qualifier of IBOutlet. For example:
@interface AppController : NSObject |
{ |
IBOutlet NSArray *keywords; |
} |
Because it is an instance variable, an outlet becomes part of an object’s encapsulated data. But an outlet is more than a simple instance variable. 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 instance-variable declaration so that the Interface Builder application can recognize the instance variable as an outlet and synchronize the display and connection of it with Xcode.
You connect an outlet in Interface Builder, but the procedure starts in Xcode. The following steps describe the procedure:
When defining your custom class, declare an outlet by tagging the instance variable with the
IBActionqualifier.In Interface Builder, drag a generic object into the top level of the nib file window (if one doesn’t already exist for your custom class).
If an instance of your custom class is to be the File’s Owner for the nib file, this step is not necessary. Also, if you’re defining a custom NSView object, select that object instead.
Import the custom class into Interface Builder.
With the generic object (or File’s Owner) selected, type the name of your custom class in the Class field of the Identify pane of the Interface Builder inspector. This assigns your custom class as the class of the selected object. This step has to be done only once.
Select your custom instance (or File’s Owner).
Right-click or Control-click this object to display the connections panel.
Find your outlet under Outlets and drag a connection line from the circle next to the outlet to the object on the user interface that you want that outlet to reference.
Figure 5-1 illustrates what an outlet connection looks like in the connections panel when you’ve completed these steps. (Note that the circle next to the outlet is filled in and the type of the object is specified.)
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 instance variable, you should be able to justify its inclusion in a class; the more instance variables 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.
Last updated: 2007-10-31
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