“原子"和“非原子"之间有什么区别?

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问题描述

我只想知道这两行代码之间的区别:

I just want to know what is the differneve between theses two lines of code :

@property(nonatomic, retain) NSString *str;

和

@property(atomic, retain) NSString *str;

感谢，问候，tek3

Thanx,Regards,tek3

推荐答案

在引用计数的多线程环境中，原子属性是必需的，以便在线程有机会保留它们之前阻止对象消失.

Atomic properties are necessary in a reference counted multi threaded environment in order to stop objects from disappearing before a thread has a chance to retain them.

考虑一下get访问器的简单实现:

Consider the naive implementation of a get accessor:

@interface MyObject : NSObject
{
    id myPropertyIVar;
}
-(id) myProperty;

@end

@implementation MyObject

-(id) myProperty
{
    return myPropertyIvar;
}

// other stuff

@end

这很好，除了如果您在保留-myProperty的返回值之前释放MyObject实例，则返回值很可能会被释放.因此，像这样实现-myProperty更安全:

This is all fine except that if you release the instance of MyObject before retaining the returned value from -myProperty the returned value may well be deallocated. For this reason, it is safer to implement -myProperty like this:

-(id) myProperty
{
    return [[myPropertyIvar retain] autorelease];
}

现在在单线程环境中这是完全安全的.

This is now completely safe in a single threaded environment.

不幸的是，在多线程环境中存在竞争条件.如果线程在保留增加保留计数之前的任何时间被中断，则以下任一情况都将导致您收到垃圾指针:

Unfortunately, in a multithreaded environment there is a race condition. If the thread is interrupted at any time before the retain has incremented the retain count, either of the following will cause you to receive a garbage pointer:

MyObject的实例由另一个线程释放和释放，导致ivar被释放和释放
myProperty由另一个线程重新分配，导致旧版本被释放并释放

由于这个原因，对属性的所有访问都必须受到锁的保护. get访问器看起来像这样.

For this reason, all accesses to the property must be protected by a lock. The get accessor looks something like this.

-(id) myProperty
{
    // lock
    return [[myPropertyIvar retain] autorelease];
    // unlock
}

集合访问器受到类似的保护，-dealloc中的发行版也受到保护

The set accessor is similarly protected and so is the release in -dealloc

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