Breaking ARC Retain Cycle in Objective-C Blocks

In a recent client project, we noticed its iOS app often received low memory warnings. The iOS app is developed with ARC-enabled (Automatic Reference Counting).
When profiling the app using Instruments > Allocations, we found that a lot of unused ViewController objects were not being released from memory.

Retain Cycle

The codebase uses a lot of Objective-C blocks as shown below, and self is often being called within the blocks:

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@interface DetailPageViewController : UIViewController @property(nonatomic, strong) UIButton *backButton; ... @end @implementation DetailPageViewController @synthesize backButton; - (void)loadView { ... [self.doneButton onTouch:^(id sender) { [self doSomething]; self.isDone = YES; }]; } ... @end

In this example code, the controller object holds a strong reference to a doneButton object. But because the doneButton onTouch: block is referencing self, now the button object holds a strong reference back to the controller object.
When object A points strongly to object B, and B points strongly to A, a retain cycle is created, and both objects cannot be released from memory.

Use Lifetime Qualifiers

Apple Developer’s guide on ARC transition suggested a few ways to break the retain cycle using different lifetime qualifiers. It is definitely a must read for iOS development.
If your app is targeted for iOS 5, you can use the __weak lifetime qualifier to break the cycle:

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@interface DetailPageViewController : UIViewController - (void)loadView { ... __weak DetailPageViewController *controller = self; [self.doneButton onTouch:^(id sender) { [controller doSomething]; controller.isDone = YES; }]; } @end

Because we are using the __weak qualifier here, the doneButton onTouch: block only has a weak reference to the controller object. Now, the controller object will be released from memory when its reference count drops to 0.