AFNetworking, UITableView and Blocks

Question

So I was reading How do I avoid capturing self in blocks when implementing an API? about how memory management works in regards to referencing self within a completion block and that got me thinking: will the following cause a retain cycle?

Pseudocode:

[AFnetworking requestGet:@"http://www.website.com" completionBlock:(^)(RequestObj* request, NSError* error){
      [self.tableView reloadData];
}];

Ignoring syntax problems, does self.tableView cause a retain cycle? Is it necessary to do the following instead?

__weak id weakSelf = self;
[AFnetworking requestGet:@"http://www.website.com" completionBlock:(^)(RequestObj* request, NSError* error){
      [weakSelf.tableView reloadData];
}];

Or is there some sort of memory magic that AFNetworking does to prevent this?

Edit courtesy of Aaron Brager

Here you don't have a retain cycle. But if you did, in the completion block, you should convert weakSelf back into a strong reference so it won't get deallocated halfway through your completion block.

id strongSelf = weakSelf; 
[strongSelf.tableView reloadData];

Here you don't have a retain cycle. But if you did, in the completion block, you should convert `weakSelf` back into a strong reference so it won't get deallocated halfway through your completion block. (`id strongSelf = weakSelf; [strongSelf.tableView reloadData];`). — Aaron Brager, Oct 09 '13 at 19:31

Hejazi · Accepted Answer · 2013-10-09T19:45:47.590

6

A retain cycle occurs when two or more objects have strong references to each other.

In this case, the block will have a strong reference to self, but self doesn't have a strong reference to the block, so you are fine here and no need to use weakSelf.

The case when you'll have a retain cycle and need to break it by using the weakSelf is when the class has a strong reference to the block too like in the following example:

typedef void (^CompletionCallback)(RequestObj* request, NSError* error);

@interface SomeClass() {
    /// Strong reference to the block
    CompletionCallback completionBlock;
}
@end

@implementation SomeClass()

- (void)someMethod {
    completionBlock = ^(RequestObj* request, NSError* error) {
        /// Strong reference to the class
        [self.tableView reloadData];
    };

    [AFnetworking requestGet:@"http://www.website.com" completionBlock:completionBlock];
}

@end

In this example, both completionBlock and self have strong references to each other, and therefore you'll have a retain cycle here and need to break it.

edited Oct 09 '13 at 19:45

answered Oct 09 '13 at 19:24

Hejazi

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It's people like you, Hejazi, that make me love stackoverflow. Thank you for being awesome. – Endama Oct 09 '13 at 19:27
Glad to be helpful. I updated the answer to give an example where you'll have a retain cycle. – Hejazi Oct 09 '13 at 19:47
3

Keep in mind the strong reference cycle may not be this straightforward. For example, if your controller has a strong reference to an object, that object has a strong reference to a completion block, and then the completion block strongly references your controller, you'll still have a retain cycle, just with 3 objects instead of 2. – Aaron Brager Oct 09 '13 at 20:35
Yes, that's why I said "when two or *more objects* have strong references to each other", but in our case the controller doesn't have a strong reference to an instance of `AFnetworking` for example. – Hejazi Oct 09 '13 at 20:39

AFNetworking, UITableView and Blocks

1 Answers1