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 %@",self.t0);
    NSLog(@"test1 is %@",self.t1);
    NSLog(@"test0 obj is %@",self.t0.obj);
    NSLog(@"test1 obj is %@",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!
    */

/*

• *  Leaving the scope of variable test0, its strong reference disappears.

• *  object A is released automatically. *

• *  Leaving the scope of variable test1, its strong reference disappears.

• *  object B is released automatically.

  *

• *  At this moment, obj_ of object B has a strong reference to object A. *

• *  At this moment, obj_ of object A has a strong reference to object B. *

• *  memory leaked!! */ 

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 %@",self.t0);
    NSLog(@"test1 is %@",self.t1);
    NSLog(@"test0 obj is %@",self.t0.obj);
    NSLog(@"test1 obj is %@",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 %@",self.t0);
    NSLog(@"test1 is %@",self.t1);
    NSLog(@"test0 obj is %@",self.t0.obj);
    NSLog(@"test1 obj is %@",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!
*/

那么我们现在来了解一下为什么使用weak之后就会解除循环引用呢？

当我们直接写  id __weak obj = [[NSObject alloc] init];  

会提示警告：因为使用了weak，所以obj并不持有该对象，那么[[NSObject alloc] init]被创建之后没有人持有它，那么它就会被释放，所以这里应该修改成，当执行｛｝之后 obj0 和 obj1 都会被释放，因为当obj0被释放之后，obj1会变成nil因为weak引用不持有对象，当引用对象的强指针消失时，weak声明的指针也会自动null

{
/*
* You create an object and have ownership. */
id __strong obj0 = [[NSObject alloc] init];
/*
* The variable obj0 is qualified with __strong.
* Which means, it has ownership of the object. */
id __weak obj1 = obj0;
/*
* variable obj1 has a weak reference of the created object */
} /*
* Leaving the scope of variable obj0, its strong reference disappears.
* The object is released automatically.
* Because no one has ownership, the object is discarded. */

那么下面这个例子就说明了weak会自动null

id __weak obj1; //__strong ,__weak 声明的变量会自动设置成为nil，这里等同于 id __weak obj1 = nil; 

{
id __strong obj0 = [[NSObject alloc] init];
obj1 = obj0;
NSLog(@"A: %@", obj1); 
}
NSLog(@"B: %@", obj1);
The result is:
A: <NSObject: 0x753e180> B: (null)