淺談 關於ARC循環引用得問題

這段時間在研究關於ARC得循環引用致使變量不能釋放，在此先介紹一本書英文書：

《Pro Multithreading and Memory Management for iOS and OS X with ARC, Grand Central Dispatch, and Blocks》

(《iOS與OS X多線程和內存管理》)

建議讀英文原版，中文版看的我稀裏糊塗的，後來被迫去看原版.這本書介紹了關於ARC的自動引用機制，這裏的機制相似：當C語言的局部變量離開他的做用域以後就會被清除，這裏的ARC也相似是這樣的原理,以下面的代碼同樣,當離開｛｝以後obj會被nil。默認id是強指針

{
/*
* 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.
*/

 

不過有一種狀況會致使在｛｝內的變量再離開做用域以後不會釋放，這就是循環引用，下面我先貼一份沒有循環引用的代碼

//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!");
}
/*  這裏的輸出結果：
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!! */ 

 

如今我貼一下存在循環引用的代碼，這個時候局部變量的強指針相互引用，這裏咱們能夠看見Test0的obj指向Test1，而Test1的obj指向Test0,致使釋放不了.

//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!");
}

/*這裏的輸出結果：
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!
*/

 通常的解決辦法就是weak obj,代碼以下：

//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!");
}

/*這裏的輸出結果：
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!
*/

 

 

那麼咱們如今來了解一下爲何使用weak以後就會解除循環引用呢？

當咱們直接寫  id __weak obj = [[NSObject alloc] init];  

會提示警告：由於使用了weak，因此obj並不持有該對象，那麼[[NSObject alloc] init]被建立以後沒有人持有它，那麼它就會被釋放，因此這裏應該修改爲，當執行｛｝以後 obj0 和 obj1 都會被釋放，由於當obj0被釋放以後，obj1會變成nil由於weak引用不持有對象，當引用對象的強指針消失時，weak聲明的指針也會自動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. */

 

那麼下面這個例子就說明了weak會自動null

id __weak obj1; //__strong ,__weak 聲明的變量會自動設置成爲nil，這裏等同於 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)