ios - How to fix "Capturing 'block' strongly in this block is likely to lead to a retain cycle"

Question

I am working on this code, which does some lengthy asyncronous operation on the net and when it finishes it triggers a completion block where some test is executed and if a variable get a certain value another lengthy operation should start immediately:

-(void) performOperation
{

    void(^completionBlock) (id obj, NSError *err, NSURLRequest *request)= ^(id obj,NSError *err, NSURLRequest *request){


        int variable=0;

        // Do completion operation A
        //...
        //...

        // Do completion operation B                
        //Get the variable value

        if(variable>0){
            [self doLengthyAsynchronousOperationWithCompletionBlock: completionBlock];
        }

    };

//Perform the lenhgty operation with the above completionBlock
    [self doLengthyAsynchronousOperationWithCompletionBlock: completionBlock];

}

-(void) doLengthyAsynchronousOperationWithCompletionBlock: completionBlock
{
    //Do some lengthy asynchronous stuff
}

With this code I get this warning from the compiler:

WARNING: Block pointer variable 'completionBlock' is uninitialized when caputerd by the block

I changed:

void(^completionBlock) (id obj, NSError *err, NSURLRequest *request)= ^(id obj,NSError *err, NSURLRequest *request)

in:

__block void(^completionBlock) (id obj, NSError *err, NSURLRequest *request)= ^(id obj,NSError *err, NSURLRequest *request)

but I get this other warning:

WARNING 2: Capturing 'completionBlock' strongly in this block is likely to lead to a retain cycle

How can I fix this?

Thanks

Nicola

Answer 1

WARNING: Block pointer variable 'completionBlock' is uninitialized when captured by the block

This happens because block variables initialized to a recursive block need __block storage.

• Variables within a block are copied unless declared with __block, in which case they are passed as reference.
• When a recursive block is assigned to a block variable, the creation happens before the assignment, and such creation triggers a variable copy. Given that the variable hasn't been assigned yet, the copied variable will be a bad value, and it will produce a crash when the block is ran.
• But if we add __block, the block will be created with a reference to the variable instead. Then the variable will be initialized to the created block, and the block will be ready to use.

WARNING: Capturing 'completionBlock' strongly in this block is likely to lead to a retain cycle

This happens because a block variable is a strong reference to the block, and the block is itself referencing the variable (because as we saw before, the variable has a __block so it is referenced instead copied).

So we need

• A weak reference to the strong variable inside the block.
• And a strong reference outside to prevent the block from being deallocated during the scope of the method where it is created.

    void(^ completionBlock) (id obj, NSError *err, NSURLRequest *request);
    void(^ __block __weak weakCompletionBlock) (id obj, NSError *err, NSURLRequest *request);
    weakCompletionBlock = completionBlock = ^(id obj,NSError *err, NSURLRequest *request){
        [self lengthyAsyncMethod:weakCompletionBlock];
    };

The name doLengthyAsynchronousOperationWithCompletionBlock suggests that the method may outlive the method scope where the block is created. Given that the compiler doesn't copy a block passed as an argument, it's responsibility of this method to copy this block. If we are using this block with block aware code (eg: dispatch_async()), this happens automatically.

Had we been assigning this block to an instance variable, we would need a @property(copy) and a weak reference to self inside the block, but this is not the case, so we just use self.