Objective-C Fundamentals

Declared properties
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Let’s briefly look at how to use the optional @property attributes listed in table 5.2 to
alter the behavior of the declared properties.
METHOD NAMING
By default for a property named foo, the Objective-C compiler produces a getter
method named foo and a setter method named setFoo:. You can override these names
by explicitly specifying an alternative name via the optional getter and setter attributes.
As an example, the following property declaration provides a getter called
isSelected and a setter called setSelected:
@property (getter=isSelected) BOOL selected;
This attribute is commonly used with properties with a BOOL data type to rename their
getter methods into the form isXYZ, another Objective-C naming convention.
WRITEABILITY
A property typically indicates the presence of both a getter and setter method. By
adding the readonly attribute to a property declaration, you can ensure that users of
the class can only query the current value of the property and can’t change its value.
For example, the following declaration specifies that the age property has only a getter
method:
@property (readonly) int age;
The default behavior of each property having a getter and setter method can also explicitly
be specified using the readwrite attribute. Because a readonly property has no setter,
code in the class would have to modify the associated instance variable directly.
SETTER SEMANTICS
These attributes enable you to specify how a setter method will deal with memory
management. Following are the three mutually exclusive options:
■
■
■
Assign—The setter uses a simple assignment statement (the default option).
Retain—The setter calls retain on the new value and release on the old one.
Copy—A copy of the new value is made and retained.
Memory management can become a complex topic (look out for chapter 9). For now,
it’s enough to understand that assign indicates that no additional memory management
behavior is required (the property assignment is treated as if it were a simple
variable). The copy attribute requests that a “carbon-copy” duplicate of the value
being provided be made and the copy stored, and the retain attribute does something
in between.
THREAD SAFETY
By default, properties are atomic. This is fancy-speak for indicating that in a multithreaded
environment, Objective-C ensures the value obtained via a getter or set via a
setter is always consistent and not corrupted by concurrent access by other threads.
This protection, however, doesn’t come for free and may have a performance cost
associated with it due to the time taken to acquire the locks needed to protect against