Warning solution for block's retain cycle

First, the block's retain cycle compilation warning only appears in the ARC project.

A solution here: http://blog.csdn.net/itianyi/article/details/8715857

I personally don't feel very good, so I checked the apple documentation and described it as follows:

In manual reference counting mode,__block id x;Has the effect of not retainingx. In Arc
Mode,__block id x;Defaults to retainingx(Just like all other values). To get the manual
Reference counting mode behavior under Arc, you cocould use__unsafe_unretained __block id x;. As the name__unsafe_unretainedImplies,
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
Support IOS 4 or OS X v10.6), or set__blockValuenilTo break the retain cycle.

The following code fragment extends strates 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__blockQualifier and setmyControllerVariable
TonilIn 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__weakVariable. The following example extends strates 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 shoshould 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...

}

};

I personally prefer the method in the Apple documentation: using a temporary variable with weak reference, or using a block variable with a post-Nil