Strongify: Preventing Block Retain Cycles in Objective-C

Strongify is an open-source GitHub project that offers a macro library to prevent block retain cycles. It provides two macros, @weakify and @strongify, designed to handle weak and strong references within blocks.

Retain Cycles

In Objective-C, when referencing self within a block, and self retains the block while the block retains self, it leads to a retain cycle, causing memory leaks. For example:

@interface TestViewController : UIViewController

@property (nonatomic, strong) void (^block)(void);

@end

@implementation TestViewController

- (void)viewDidLoad {
    [super viewDidLoad];

    self.block = ^{
        [self doSomething];
    };
}

- (void)doSomething {
    // ...
}

@end

In the code above, a retain cycle is formed as self retains the block, and the block retains self. When self is deallocated, the block still holds a reference to self, preventing it from being released and resulting in a memory leak.

Strongify and Weakify

@weakify and @strongify macros are used to create weak references and then strong references within blocks, respectively.

• @weakify(self): Weakly references self.
• @strongify(self): Strongly references the weak reference to self.

Using strongify and weakify effectively prevents block retain cycles. For example:

@import strongify;

@interface TestViewController : UIViewController

@property (nonatomic, strong) void (^block)(void);

@end

@implementation TestViewController

- (void)viewDidLoad {
    [super viewDidLoad];

    self.block = ^{
        @weakify(self);

        [self doSomething];

        @strongify(self);

        // ...
    };
}

- (void)doSomething {
    // ...
}

@end

In the code above, @weakify(self) creates a weak reference to self, and @strongify(self) creates a strong reference to the weak reference. This ensures that when the block execution is complete, self can be released correctly, preventing a retain cycle.

Usage Notes

• strongify and weakify should only be used inside blocks.
• They cannot be used with class methods.
• They cannot be used with static variables.

Conclusion

Strongify is a simple and easy-to-use macro library that effectively prevents block retain cycles. It helps developers avoid memory leaks, thereby improving code quality.