Fix warning "Capturing [an object] strongly in this block is likely to lead to a retain cycle" in ARC-enabled code

Question

In ARC enabled code, how to fix a warning about a potential retain cycle, when using a block-based API?

The warning:
Capturing 'request' strongly in this block is likely to lead to a retain cycle

produced by this snippet of code:

ASIHTTPRequest *request = [[ASIHTTPRequest alloc] initWithURL:...

[request setCompletionBlock:^{
    NSDictionary *jsonDictionary = [[CJSONDeserializer deserializer] deserialize:request.rawResponseData error:nil];
    // ...
    }];

Warning is linked to the use of the object request inside the block.

Answer 1

Replying to myself:

My understanding of the documentation says that using keyword block and setting the variable to nil after using it inside the block should be ok, but it still shows the warning.

__block ASIHTTPRequest *request = [[ASIHTTPRequest alloc] initWithURL:...

[request setCompletionBlock:^{
    NSDictionary *jsonDictionary = [[CJSONDeserializer deserializer] deserialize:request.responseData error:nil];
    request = nil;
// ....

    }];

Update: got it to work with the keyword '_weak' instead of '_block', and using a temporary variable:

ASIHTTPRequest *_request = [[ASIHTTPRequest alloc] initWithURL:...
__weak ASIHTTPRequest *request = _request;

[request setCompletionBlock:^{
    NSDictionary *jsonDictionary = [[CJSONDeserializer deserializer] deserialize:request.responseData error:nil];
    // ...
    }];

If you want to also target iOS 4, use __unsafe_unretained instead of __weak. Same behavior, but the pointer stays dangling instead of being automatically set to nil when the object is destroyed.

Answer 2

The issue occurs because you're assigning a block to request that has a strong reference to request in it. The block will automatically retain request, so the original request won't deallocate because of the cycle. Make sense?

It's just weird because you're tagging the request object with __block so it can refer to itself. You can fix this by creating a weak reference alongside it.

ASIHTTPRequest *request = [[ASIHTTPRequest alloc] initWithURL:...];
__weak ASIHTTPRequest *wrequest = request;

[request setCompletionBlock:^{
    NSDictionary *jsonDictionary = [[CJSONDeserializer deserializer] deserialize:wrequest.rawResponseData error:nil];
    // ...
    }];

Answer 3

It causes due to retaining the self in the block. Block will accessed from self, and self is referred in block. this will create a retain cycle.

Try solving this by create a weak refernce of self

__weak typeof(self) weakSelf = self;

operationManager = [[AFHTTPRequestOperation alloc] initWithRequest:request];
operationManager.responseSerializer = [AFJSONResponseSerializer serializer];
[operationManager setCompletionBlockWithSuccess:^(AFHTTPRequestOperation *operation, id responseObject) {

    [weakSelf requestFinishWithSucessResponseObject:responseObject withAFHTTPRequestOperation:operation andRequestType:eRequestType];

} failure:^(AFHTTPRequestOperation *operation, NSError *error) {
    [weakSelf requestFinishWithFailureResponseObject:error withAFHTTPRequestOperation:operation andRequestType:eRequestType];
}];
[operationManager start];

Answer 4

Some times the xcode compiler has problems for identifier the retain cycles, so if you are sure that you isn't retain the completionBlock you can put a compiler flag like this:

#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Warc-retain-cycles"
#pragma clang diagnostic ignored "-Wgnu"

-(void)someMethod {
}

Answer 5

When I try the solution provided by Guillaume, everything is fine in Debug mode but it crashs in Release mode.

Note that don't use __weak but __unsafe_unretained because my target is iOS 4.3.

My code crashs when setCompletionBlock: is called on object "request" : request was deallocated ...

So, this solution works both in Debug and Release modes :

// Avoiding retain cycle :
// - ASIHttpRequest object is a strong property (crashs if local variable)
// - use of an __unsafe_unretained pointer towards self inside block code

self.request = [ASIHttpRequest initWithURL:...
__unsafe_unretained DataModel * dataModel = self;

[self.request setCompletionBlock:^
{
    [dataModel processResponseWithData:dataModel.request.receivedData];        
}];