ref – https://digitalleaves.com/blog/2015/05/demystifying-retain-cycles-in-arc/

Examples of non-cycles

Using dispatch_async as the example is misleading as self does not hold a strong reference to it so, there is no threat of a retain cycle.
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  [myArray enumerateObjectsUsingBlock:^(id obj, NSUInteger idx, BOOL *stop){    [self doSomethingWithObject:obj]; }];

  The block retains self, but self doesn’t retain the block. If one or the other is released, no cycle is created and everything gets deallocated as it should.

How you get into trouble

Where you get into trouble is something like:

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//In the interface: @property (strong) void(^myBlock)(id obj, NSUInteger idx, BOOL *stop);   //In the implementation: [self setMyBlock:^(id obj, NSUInteger idx, BOOL *stop) {   [self doSomethingWithObj:obj];     }];

Now, your object (self) has an explicit strong reference to the block. And the block has an implicit strong reference to self. That’s a cycle, and now neither object will be deallocated properly.