标签：style blog io color ar os sp for strong
这段时间在研究关于ARC得循环引用导致变量不能释放，在此先介绍一本书英文书：
《Pro Multithreading and Memory Management for iOS and OS X with ARC, Grand Central Dispatch, and Blocks》
(《iOS与OS X多线程和内存管理》)
建议读英文原版，中文版看的我稀里糊涂的，后来被迫去看原版.这本书介绍了关于ARC的自动引用机制，这里的机制类似：当C语言的局部变量离开他的作用域之后就会被清除，这里的ARC也类似是这样的原理,如下面的代码一样,当离开｛｝之后obj会被nil。默认id是强指针
{ /* * You create an object and have ownership. */ id __strong obj = [[NSObject alloc] init]; /* * The variable obj is qualified with __strong. * Which means, it has ownership of the object. */ } /* * Leaving the scope of variable obj, its strong reference disappears. * The object is released automatically. * Because no one has ownership, the object is disposed of. */
不过有一种情况会导致在｛｝内的变量再离开作用域之后不会释放，这就是循环引用，下面我先贴一份没有循环引用的代码
//Test Class @interface Test : NSObject @property (nonatomic) id obj; - (void) setObj:(id)obj; @end @implementation Test - (void) setObj:(id)obj{ _obj = obj; } @end //Use Test Class - (void)viewDidLoad { [super viewDidLoad]; // Do any additional setup after loading the view, typically from a nib. id test0 = [[Test alloc]init]; id test1 = [[Test alloc]init]; self.t0 = test0; self.t1 = test1; } - (void) viewDidAppear:(BOOL)animated{ [super viewDidAppear:animated]; NSLog(@"test0 is %p",self.t0); NSLog(@"test1 is %p",self.t1); NSLog(@"test0 obj is %p",self.t0.obj); NSLog(@"test1 obj is %p",self.t1.obj); NSLog(@"viewDidApperar!"); } /* 这里的输出结果： 2014-11-18 11:39:31.654 circular[27065:60b] test0 is (null) 2014-11-18 11:39:31.654 circular[27065:60b] test1 is (null) 2014-11-18 11:39:31.655 circular[27065:60b] test0 obj is (null) 2014-11-18 11:39:31.655 circular[27065:60b] test1 obj is (null) 2014-11-18 11:39:31.655 circular[27065:60b] viewDidApperar! */
现在我贴一下存在循环引用的代码，这个时候局部变量的强指针相互引用，这里我们可以看见Test0的obj指向Test1，而Test1的obj指向Test0,导致释放不了.
//Test Class @interface Test : NSObject @property (nonatomic) id obj; - (void) setObj:(id)obj; @end @implementation Test - (void) setObj:(id)obj{ _obj = obj; } @end //Use Test Class - (void)viewDidLoad { [super viewDidLoad]; // Do any additional setup after loading the view, typically from a nib. id test0 = [[Test alloc]init]; id test1 = [[Test alloc]init]; [test0 setObj:test1]; [test1 setObj:test0]; self.t0 = test0; self.t1 = test1; } - (void) viewDidAppear:(BOOL)animated{ [super viewDidAppear:animated]; NSLog(@"test0 is %p",self.t0); NSLog(@"test1 is %p",self.t1); NSLog(@"test0 obj is %p",self.t0.obj); NSLog(@"test1 obj is %p",self.t1.obj); NSLog(@"viewDidApperar!"); } /*这里的输出结果： 2014-11-18 11:42:35.555 circular[27654:60b] test0 is <Test: 0x8c8fe20> 2014-11-18 11:42:35.556 circular[27654:60b] test1 is <Test: 0x8cf7c20> 2014-11-18 11:42:35.556 circular[27654:60b] test0 obj is <Test: 0x8cf7c20> 2014-11-18 11:42:35.556 circular[27654:60b] test1 obj is <Test: 0x8c8fe20> 2014-11-18 11:42:35.558 circular[27654:60b] viewDidApperar! */
一般的解决办法就是weak obj,代码如下：
//Test Class @interface Test : NSObject @property (nonatomic,weak) id obj; - (void) setObj:(id)obj; @end @implementation Test - (void) setObj:(id)obj{ _obj = obj; } @end //Use Test Class - (void)viewDidLoad { [super viewDidLoad]; // Do any additional setup after loading the view, typically from a nib. id test0 = [[Test alloc]init]; id test1 = [[Test alloc]init]; [test0 setObj:test1]; [test1 setObj:test0]; self.t0 = test0; self.t1 = test1; } - (void) viewDidAppear:(BOOL)animated{ [super viewDidAppear:animated]; NSLog(@"test0 is %p",self.t0); NSLog(@"test1 is %p",self.t1); NSLog(@"test0 obj is %p",self.t0.obj); NSLog(@"test1 obj is %p",self.t1.obj); NSLog(@"viewDidApperar!"); } /*这里的输出结果： 2014-11-18 11:46:19.803 circular[28360:60b] test0 is (null) 2014-11-18 11:46:19.804 circular[28360:60b] test1 is (null) 2014-11-18 11:46:19.804 circular[28360:60b] test0 obj is (null) 2014-11-18 11:46:19.805 circular[28360:60b] test1 obj is (null) 2014-11-18 11:46:19.805 circular[28360:60b] viewDidApperar! */
标签：style blog io color ar os sp for strong
原文地址：http://www.cnblogs.com/zuopeng/p/4105356.html
