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@interface XYZToDoItem : NSObject

@end

This example declares a class named XYZToDoItem, which inherits from NSObject.

The public properties and behavior are defined inside the @interface declaration. In this example, nothing

is specified beyond the superclass, so the only behavior expected to be available on instances of XYZToDoItem

is the behavior inherited from NSObject. All objects are expected to have a minimum behavior, so by default,

they must inherit from NSObject (or one of its subclasses).

Implementation

The Objective-C syntax used to declare a class implementation looks like this:

#import "XYZToDoItem.h"

@implementation XYZToDoItem

@end

If you declare any methods in the class interface, you’ll need to implement them inside this file.

2013-10-22 | Copyright © 2013 Apple Inc. All Rights Reserved.

87

Writing a Custom Class

Properties Store an Object’s Data

Properties Store an Object’s Data

Consider what information the to-do item needs to hold. You probably need to know its name, when it was

created, and whether it’s been completed. In your custom XYZToDoItem class, you’ll store this information in

properties.

Declarations for these properties reside inside the interface file (XYZToDoItem.h). Here’s what they look like:

@interface XYZToDoItem : NSObject

@property NSString *itemName;

@property BOOL completed;

@property NSDate *creationDate;

@end

In this example, the XYZToDoItem class declares three public properties. These properties are available for

full public access. With public access, other objects can both read and change the values of the properties.

You may decide to declare that a property shouldn’t be changed (that is, that it should be read-only). To indicate

whether a property is intended to be read-only—among other things—Objective-C property declarations

include property attributes. For example, if you don’t want the creation date of an XYZToDoItem to be

changeable, you might update the XYZToDoItem class interface to look like this:

@interface XYZToDoItem : NSObject

@property NSString *itemName;

@property BOOL completed;

@property (readonly) NSDate *creationDate;

@end

Properties can be private or public. Sometimes it makes sense to make a property private so that other classes

can’t see or access it. For example, if you want to keep track of a property that represents the date an item was

marked as completed without giving other classes access to this information, make the property private by

putting it in a class extension at the top of your implementation file (XYZToDoItem.m).

#import "XYZToDoItem.h"

2013-10-22 | Copyright © 2013 Apple Inc. All Rights Reserved.

88

Writing a Custom Class

Methods Define an Object’s Behavior

@interface XYZToDoItem ()

@property NSDate *completionDate;

@end

@implementation XYZToDoItem

@end

You access properties using getters and setters. A getter returns the value of a property, and a setter changes

it. A common syntactical shorthand for accessing getters and setters is dot notation. For properties with read

and write access, you can use dot notation for both getting and setting a property’s value. If you have an object

toDoItem of class XYZToDoItem, you can do the following:

toDoItem.itemName = @"Buy milk";

//Sets the value of itemName

NSString *selectedItemName = toDoItem.itemName; //Gets the value of itemName