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：

 MyViewController *myController = [[MyViewController alloc] init…];
 // ...
 MyViewController * __weak weakMyViewController = myController;
 myController.completionHandler =  ^(NSInteger result) {
     [weakMyViewController dismissViewControllerAnimated:YES completion:nil];
 };

　　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。

 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...
     }
 };