[ios2]警告：Block的Retain Cycle的解决方法 【转】

1. <span style="background-color: rgb(248, 248, 248); font-family: 'PT Sans', Geogia, Baskerville, 'Hiragino Sans GB', serif; ">警告：Captureing ‘self’ strongly in this block is likely to lead to a retain cycle</span>

一个使用Block语法的实例变量，在引用另一个实例变量的时候，经常会引起retain cycle。这个问题在使用ASIHTTPRequest的block语法的时候会时不时的碰到。这个问题困扰了我这个小白很久。终于有一天，在 Advanced Mac OS X Programming上，看到了这个问题的解决方案。

先用代码描述一下症状：

1. <span style="font-size:18px;">/* ViewController.h */
2. #import <UIKit/UIKit.h>
3. typedef void (^ABlock)(void); //定义一个简单的Block
4. @interface ViewController : UIViewController {
5. NSMutableArray *_items;
6. ABlock _block;
7. }
8. @end
9. /* ViewController.m */
10. #import "ViewController.h"
11. @implementation ViewController
12. - (void)viewDidLoad
13. {
14. [super viewDidLoad];
15. // Do any additional setup after loading the view, typically from a nib.
16. _items = [[NSMutableArray alloc] init];
17. _block = ^{
18. [_items addObject:@"Hello!"]; //_block引用了_items，导致retain cycle。
19. };
20. }
21. @end</span>

Xcode在编译以上程序的时候会给出一个警告：Captureing ‘self’ strongly in this block is likely to lead to a retain cycle。原因是_items实际上是self->items。_block对象在创建的时候会被retain一次，因此会导致self也被retain一次。这样就形成了一个retain cycle。

解决方法就是，创建一个本地变量blockSelf，指向self，然后用结构体语法访问实例变量。代码如下：

1. __block ViewController *blockSelf = self;
2. _block = ^{
3. [blockSelf->_items addObject:@"Hello!"];
4. };

这么修改之后，blockSelf是本地变量，是弱引用，因此在_block被retain的时候，并不会增加retain count，所以retain cycle就解除了，Xcode也不再出现警告了，问题解决。

注：本文并非原创，详情请参阅Advanced Mac OS X Programming，第92页“Block Retain Cycles”。

n manual reference counting mode, __block id x; has the effect of not retaining x. In ARC mode, __block id x; defaults to retaining x (just like all other values). To get the manual reference counting mode behavior under ARC, you could use __unsafe_unretained __block id x;. As the name __unsafe_unretained implies, however, having a non-retained variable is dangerous (because it can dangle) and is therefore discouraged. Two better options are to either use __weak (if you don’t need to support iOS 4 or OS X v10.6), or set the __block value to nil to break the retain cycle.

The following code fragment illustrates this issue using a pattern that is sometimes used in manual reference counting.

MyViewController *myController = [[MyViewController alloc] init…];

// ...

myController.completionHandler =  ^(NSInteger result) {

   [myController dismissViewControllerAnimated:YES completion:nil];

};

[self presentViewController:myController animated:YES completion:^{

   [myController release];

}];

As described, instead, you can use a __block qualifier and set the myController variable to nil in the completion handler:

MyViewController * __block myController = [[MyViewController alloc] init…];

// ...

myController.completionHandler =  ^(NSInteger result) {

    [myController dismissViewControllerAnimated:YES completion:nil];

    myController = nil;

};

Alternatively, you can use a temporary __weak variable. The following example illustrates a simple implementation:

MyViewController *myController = [[MyViewController alloc] init…];

// ...

MyViewController * __weak weakMyViewController = myController;

myController.completionHandler =  ^(NSInteger result) {

    [weakMyViewController dismissViewControllerAnimated:YES completion:nil];

};

For non-trivial cycles, however, you should use:

MyViewController *myController = [[MyViewController alloc] init…];

// ...

MyViewController * __weak weakMyController = myController;

myController.completionHandler =  ^(NSInteger result) {

    MyViewController *strongMyController = weakMyController;

    if (strongMyController) {

        // ...

        [strongMyController dismissViewControllerAnimated:YES completion:nil];

        // ...

    }

    else {

        // Probably nothing...

    }

};