Objective-C属性参数

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属性参数有以下三种Writability、Setter、Semantics Atomicity

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.

readonly 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
weak

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.

指定应该使用对象的副本（深度复制），前一个值发送一条release消息。基本上像retain，但是没有增加引用计数，是分配一块新的内
存来放置它。

assgin Specifies that the setter uses simple assignment. This attribute is the default. You use this attribute for scalar types such as NSInteger and CGRect.

setter方法直接赋值，而不进行retain操作，比如NSInteger和CGRect，它们不是对象，所以不用release。

retain Specifies that retain should be invoked on the object upon assignment.The previous value is sent a release message.

setter方法对参数进行release旧值，再retain新值例如：

　   1.-(void) setObj:(ClassX*) value
         2.{
　      3.         if (obj != value)
　      4.         {
　      5.                   [obj release];
　      6.                   obj = [value retain];
　      7.         }
　      8.}