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Methods Define an Object’s Behavior

Methods define what an object can do. A method is a piece of code that you define to perform a task or

subroutine in a class. Methods have access to data stored in the class and can use that information to perform

some kind of operation.

For example, to give a to-do item (XYZToDoItem) the ability to get marked as complete, you can add a

markAsCompleted method to the class interface. Later, you’ll implement this method’s behavior in the class

implementation, as described in “Implementing Methods” (page 91).

@interface XYZToDoItem : NSObject

@property NSString *itemName;

@property BOOL completed;

@property (readonly) NSDate *creationDate;

- (void)markAsCompleted;

@end

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

89

Writing a Custom Class

Methods Define an Object’s Behavior

The minus sign (-) at the front of the method name indicates that it is an instance method, which can be

called on an object of that class. This minus sign differentiates it from class methods, which are denoted with

a plus sign (+). Class methods can be called on the class itself. A common example of class methods are class

factory methods, which you learned about in “Working with Foundation” (page 75). You can also use class

methods to access some piece of shared information associated with the class.

The void keyword is used inside parentheses at the beginning of the declaration to indicate that the method

doesn’t return a value. In this case, the markAsCompleted method takes in no parameters. Parameters are

discussed in more detail in “Method Parameters” (page 90).

Method Parameters

You declare methods with parameters to pass along some piece of information when you call a method.

For example, you can revise the markAsCompleted method from the previous code snippet to take in a single

parameter that will determine whether the item gets marked as completed or uncompleted. This way, you can

toggle the completion state of the item instead of setting it only as completed.

@interface XYZToDoItem : NSObject

@property NSString *itemName;

@property BOOL completed;

@property (readonly) NSDate *creationDate;

- (void)markAsCompleted:(BOOL)isComplete;

@end

Now, your method takes in one parameter, isComplete, which is of type BOOL.

When you refer to a method with a parameter by name, you include the colon as part of the method name,

so the name of the updated method is now markAsCompleted:. If a method has multiple parameters, the

method name is broken up and interspersed with the parameter names. If you wanted to add another parameter

to this method, its declaration would look like this:

- (void)markAsCompleted:(BOOL)isComplete onDate:(NSDate *)date;

Here, the method’s name is written as markAsCompleted:onDate:. The names isComplete and date are

used in the implementation to access the values supplied when the method is called, as if these names were

variables.

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

90

Writing a Custom Class

Methods Define an Object’s Behavior

Implementing Methods

Method implementations use braces to contain the relevant code. The name of the method must be identical

to its counterpart in the interface file, and the parameter and return types must match exactly.

Here is a simple implementation of the markAsCompleted: method you added to your XYZToDoItem class

interface:

@implementation XYZToDoItem

- (void)markAsCompleted:(BOOL)isComplete {

self.completed = isComplete;

}

@end

Like properties, methods can be private or public. Public methods are declared in the public interface and so

can be seen and called by other objects. Their corresponding implementation resides in the implementation

file and can’t be seen by other objects. Private methods have only an implementation and are internal to the

class, meaning they’re only available to call inside the class implementation. This is a powerful mechanism for