CODEWITHSUNDEEP
iosobjective-cobjective-c-blocks
david
david

Reputation: 157

Does block object cause retain cycle?

I have this code:

@implementation example
{
    NSString *myObject;
}

- (void)viewDidLoad
{
    [super viewDidLoad];

    __block NSString* blockObject = myObject;
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0),^(void){
        blockObject = @"Hello world";    
    });
}

Now because I am using __block, a strong reference is made to self, because I am accessing an instance variable by reference and not by value.

So the above code is the same as:

@implementation example
{
    NSString *myObject;
}

- (void)viewDidLoad
{
    [super viewDidLoad];

    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH,0),^(void)  {
        myObject = @"Hello world";    
    });
}

So dealloc method will not called until the block exit. My compiler use arc!

Upvotes: 1

Views: 138

Answers (3)

Inder Kumar Rathore
Inder Kumar Rathore

Reputation: 39978

__block NSString* blockObject = myObject;
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0),^(void){
        blockObject = @"Hello world";    
    });

The above code will not strongly hold the self and thus your dealloc will be called before the block exits. Why it will not strongly hold the self is because you are creating a new variable blockObject and block will strongly hold this variable not the ivar thus not retaining the self

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH,0),^(void)  {
        myObject = @"Hello world";    
    });

The above code will strongly hold the self and dealloc will not be called until block exits.

Upvotes: 1

Mike Pollard
Mike Pollard

Reputation: 10195

The two example in your question are in fact very different.

The first results in NO CHANGE to your instance variable myObject whereas the second changes it to "Hello world".

Upvotes: 1

YarGnawh
YarGnawh

Reputation: 4654

Usually if I have to access instance variables, I write something like this... fyi. pseudocode 

__weak typeof(self)weakSelf = self;

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH,0),^(void)  {
    __strong typeof(weakSelf)strongSelf = weakSelf;
    strongSelf->myObject = @"Hello world";    
});

Upvotes: 0

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