iphone project note

Property Declaration Attributes

You can decorate a property with attributes by using the form @property(attribute [, attribute2, ...]) . Like methods, properties are scoped to their enclosing interface declaration. For property declarations that use a comma-delimited list of variable names, the property attributes apply to all of the named properties.

If you use the @synthesize directive to tell the compiler to create the accessor methods (see “Property Implementation Directives” ), the code it generates matches the specification given by the keywords. If you implement the accessor methods yourself, you should ensure that it matches the specification (for example, if you specify copy you must make sure that you do copy the input value in the setter method).

Accessor Method Names

The default names for the getter and setter methods associated with a property are propertyName and set PropertyName : respectively—for example, given a property “foo”, the accessors would be foo and setFoo: . The following attributes allow you to specify custom names instead. They are both optional and can appear with any other attribute (except for readonly in the case of setter= ).

getter=getterName

Specifies the name of the get accessor for the property. The getter must return a type matching the property’s type and take no parameters.

setter=setterName

Specifies the name of the set accessor for the property. The setter method must take a single parameter of a type matching the property’s type and must return void .

If you specify that a property is readonly and also specify a setter with setter= , you get a compiler warning.

Typically you should specify accessor method names that are key-value coding compliant (see Key-Value Coding Programming Guide )—a common reason for using the getter decorator is to adhere to the is PropertyName convention for Boolean values.

Writability

These attributes specify whether or not a property has an associated set accessor. They are mutually exclusive.

readwrite

Indicates that the property should be treated as read/write. This attribute is the default.

Both a getter and setter method are required in the @implementation block. If you use the @synthesize directive in the implementation block, the getter and setter methods are synthesized.

readonly

Indicates that the property is read-only.

If you specify readonly , only a getter method is required in the @implementation block. If you use the @synthesize directive in the implementation block, only the getter method is synthesized. Moreover, if you attempt to assign a value using the dot syntax, you get a compiler error.

Setter Semantics

These attributes specify the semantics of a set accessor. They are mutually exclusive.

strong

Specifies that there is a strong (owning) relationship to the destination object.

weak

Specifies that there is a weak (non-owning) relationship to the destination object.

If the destination object is deallocated, the property value is automatically set to nil .

(Weak properties are not supported on OS X v10.6 and iOS 4; use assign instead.)

copy

Specifies that a copy of the object should be used for assignment.

The previous value is sent a release message.

The copy is made by invoking the copy method. This attribute is valid only for object types, which must implement the NSCopying   protocol .

assign

Specifies that the setter uses simple assignment. This attribute is the default.

You use this attribute for scalar types such as NSInteger and CGRect .

retain

Specifies that retain should be invoked on the object upon assignment.

The previous value is sent a release message.

In OS X v10.6 and later, you can use the __attribute__ keyword to specify that a Core Foundation property should be treated like an Objective-C object for memory management:

@property(retain) __attribute__((NSObject)) CFDictionaryRef myDictionary;

Atomicity

You can use this attribute to specify that accessor methods are not atomic. (There is no keyword to denote atomic.)

nonatomic

Specifies that accessors are nonatomic. By default, accessors are atomic.

Properties are atomic by default so that synthesized accessors provide robust access to properties in a multithreaded environment—that is, the value returned from the getter or set via the setter is always fully retrieved or set regardless of what other threads are executing concurrently.

If you specify strong , copy , or retain and do not specify nonatomic , then in a reference-counted environment, a synthesized get accessor for an object property uses a lock and retains and autoreleases the returned value—the implementation will be similar to the following:

[_internal lock]; // lock using an object-level lock

id result = [[value retain] autorelease];

[_internal unlock];

return result;

If you specify nonatomic , a synthesized accessor for an object property simply returns the value directly.