Correct management of addObserverForName:object:queue:usingBlock:

Question

I'm still new to blocks in objective-c and wondering if I have this psuedo code correct. I'm not sure if it's enough to just remove the observer or if i have to call removeObserver:name:object:

-(void) scan {
    Scanner *scanner = [[Scanner alloc] init];
    id scanComplete = [[NSNotificationCenter defaultCenter] addObserverForName:@"ScanComplete" 
                        object:scanner 
                        queue:nil 
                        usingBlock:^(NSNotification *notification){
                            /*
                             do something
                             */
                            [[NSNotificationCenter defaultCenter] removeObserver:scanComplete];
                            [scanner release];
                        }];
    [scanner startScan];
}

Update: I'm receiving intermittent EXC_BAD_ACCESS from this block, so this can't be right.

Answer 1

Declare the scanComplete variable before defining the block itself.

The reason why you need to do this is because you're trying to access a variable that doesn't exist within the block at the time of definition since the variable itself has not been assigned yet.

What is EXC_BAD_ACCESS? Well, it's an exception that is thrown when you try to access a reference that doesn't exist. So that is exactly the case in your example.

So if you declare the variable before the block itself, then it should work:

-(void) scan {
    Scanner *scanner = [[Scanner alloc] init];
    __block id scanComplete;
    scanComplete = [[NSNotificationCenter defaultCenter] addObserverForName:@"ScanComplete" 
                        object:scanner 
                        queue:nil 
                        usingBlock:^(NSNotification *notification){
                           /*
                           do something
                           */
                           [[NSNotificationCenter defaultCenter] removeObserver:scanComplete];
                           [scanner release];
                    }];
    [scanner startScan];
}