block循环引用的解决办法

Use Lifetime Qualifiers to Avoid Strong Reference Cycles

You can use lifetime qualifiers to avoid strong reference cycles. For example, typically if you have a graph of objects arranged in a parent-child hierarchy and parents need to refer to their children and vice versa, then you make the parent-to-child relationship strong and the child-to-parent relationship weak. Other situations may be more subtle, particularly when they involve block objects.

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.

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...

}

};

In some cases you can use __unsafe_unretained if the class isn’t __weak compatible. This can, however, become impractical for nontrivial cycles because it can be hard or impossible to validate that the __unsafe_unretained pointer is still valid and still points to the same object in question.