How do you manage retain counts in nested dispatch queue blocks

Question

In the following code, how would you avoid nested blocks increasing the retain count of 'self'.

This is how I avoid nested blocks

-(void)openSession {
        [self.loginManager logInWithReadPermissions:@[@"user_photos"]
                                 fromViewController:[self.datasource mediaAccountViewControllerForRequestingOpenSession:self]
                                            handler:[self loginHandler]];
}

-(void(^)(FBSDKLoginManagerLoginResult *result, NSError *error))loginHandler {

    __weak typeof (self) weakSelf = self;

    return ^ (FBSDKLoginManagerLoginResult *result, NSError *error) {

        __strong typeof(weakSelf) strongSelf = weakSelf;

        if (error) {
            [strongSelf.delegate mediaAccount:strongSelf failedOpeningSessionWithError:error];
        } else if (result.isCancelled) {
            [strongSelf.delegate mediaAccountSessionOpeningCancelledByUser:strongSelf];
        } else {
            [strongSelf.delegate mediaAccountDidOpenSession:strongSelf];
        }

        [strongSelf notifyWithCompletion:[strongSelf completionHandler]]
    };
}

-(void)notifyWithCompletion:(void(^)(void))completion {

    [self notify];
    completion();

}

-(void(^)(void))completionHandler {

    return ^ {
        //do something
    };

}

But how do you avoid many nested blocks, which is often the case when you use GCD within a block ?

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{

                [self parseLoadsOfData];

                dispatch_async(dispatch_get_main_queue(), ^{

                    [self updateUI];

                });

            });

Are there retain cycles here ?

why do you think there will be a retain cycle in any of these cases? — newacct, Apr 12 '16 at 07:43
Nope, there is no *cycle*. cycle = A -> B -> A Where is the cycle? — newacct, Apr 12 '16 at 18:30
the cycle does exist in certain situations. i will explain with an example. if 'self' was a view controller and the view controller was dismissed by it's parent, I would expect that view controller to be deallocated, if nothing is holding on to it strongly. If this block is waiting on a queue holding onto self strongly, then the view controller will still be in memory. — , Apr 12 '16 at 20:29
this is a more clear explanation maybe. hope that helps. http://stackoverflow.com/a/20032131/1951992 — , Apr 12 '16 at 20:32
i have answered this question, with an example that might help you understand http://stackoverflow.com/a/36583600/1951992 — , Apr 12 '16 at 20:39
But that is still not a "cycle". The view controller is not strongly referencing the block or the queue. — newacct, Apr 13 '16 at 17:45

score 0 · Answer 1 · answered Apr 08 '16 at 18:35

0

__weak typeof(self) *weakSelfOuter = self;
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
    __strong typeof(self) *strongInnerSelf1 = weakSelfOuter;
    [strongInnerSelf1 parseLoadsOfData];
    __weak typeof(self) *weakInnerSelf = strongInnerSelf1;
    dispatch_async(dispatch_get_main_queue(), ^{
        __strong typeof(self) *strongInnerSelf2 = weakInnerSelf;
        [strongInnerSelf2 updateUI];
    });
});

answered Apr 08 '16 at 18:35

Steve Smith

48
5

A couple of comments about this: - once you recapture `weakSelfOuter` into `strongInnerSelf1`, it's generally good practice to check `strongInnerSelf1` for nil in case you actually need to dereference an ivar on self directly (not strictly necessary in this case) - you also don't need to re-weaken `strongInnerSelf1` to `weakInnerSelf`; in the inner dispatch back to main, you can just recapture `weakSelfOuter`. – fullofsquirrels Apr 08 '16 at 22:06
you are referencing 'self' within a block twice here. __weak typeof(self) and __strong typeof (self) – Apr 09 '16 at 23:20

How do you manage retain counts in nested dispatch queue blocks

1 Answers1