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浅谈 关于ARC循环引用得问题

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!*/

 

浅谈 关于ARC循环引用得问题