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。
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.}
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.