Common block-related retain cycles in iOS

Summarize some common block-related retain cycle patterns.

1. retain local variables

- (void)presentNewViewController {
  UIViewController *newVC = [UIViewController new];
  newVC.completion = ^ {
    newVC.title = @"some_title_after_completion";  
  };
  [self presentViewController: newVC animated: YES];
}

In the above example, the retain cycle is newVC -> completion -> newVC

2. Superficial class method

@implementation CallingViewController {
  - (void)loadData {
    [DataSource fetchDataWithCompletion: ^{
      self.dataReady = YES;
    }];
  }
}
// However, in the data source it uses instance method instead of class method as it appears.
// in DataSource
- (void)fetchDataWithCompletion:(void(^)(id data, NSError *error))completion {
  [[NetworkingLayer sharedInstance] getRequest:request completion:completion];
}

In the above example, CallingViewController loadData method scope -> NetworkingLayer instance -> completionBlock -> CallingViewController

Rule of thumb

1. Always use weakSelf with block (for pattern 2)
2. Double check local variable references