避免Block中的强引用环

【避免Block中的强引用环】

　　In 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.

　　例如，在MRC&ARC下，以下代码会存在强引用环：

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

// ...

myController.completionHandler =  ^(NSInteger result) {

   [myController dismissViewControllerAnimated:YES completion:nil];

};

　　在MRC下，解决上述强引用环的方法如下：

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

// ...

myController.completionHandler =  ^(NSInteger result) {

    [myController dismissViewControllerAnimated:YES completion:nil];

    myController = nil;

};

　　在ARC下，解决上述强引用环的方法如下：

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

2 // ...

3 MyViewController * __weak weakMyViewController = myController;

4 myController.completionHandler =  ^(NSInteger result) {

5     [weakMyViewController dismissViewControllerAnimated:YES completion:nil];

6 };

　　更好的方式是在使用__weak变量前，先用__strong变量把该值锁定，类似使用weak_ptr一样。

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

 2 // ...

 3 MyViewController * __weak weakMyController = myController;

 4 myController.completionHandler =  ^(NSInteger result) {

 5     MyViewController *strongMyController = weakMyController;

 6     if (strongMyController) {

 7         // ...

 8         [strongMyController dismissViewControllerAnimated:YES completion:nil];

 9         // ...

10     }

11     else {

12         // Probably nothing...

13     }

14 };