A demonstration of some potential issues around the use of -[NSNotificationCenter addObserver:...usingBlock:].

gistfile1.m

@implementation MyObject

@synthesize observer;

- (void) storeObserverVariable
{
    self.observer = [[NSNotificationCenter defaultCenter]
                     addObserverForName: @"TheNotification"
                                 object: nil
                                  queue: [NSOperationQueue mainQueue]
                             usingBlock: ^(NSNotification * note) {
        // this reference to 'self' within the block causes 'observer'
        // to retain 'self' which retains 'observer', which retains the
        // block, which retains 'self' ...
        if ( [self validateContents] == NO )
            return;
        [self sortContents];
        [self performLayout];
    }];
}

- (void) stopObservingInsideBlock
{
    id tmpObserver = [[NSNotificationCenter defaultCenter]
                      addObserverForName: @"TheNotification"
                                  object: nil
                                   queue: [NSOperationQueue mainQueue]
                              usingBlock: ^(NSNotification * note) {
        if ( [self validateContents] == NO )
        {
            // must remember to remove observer if there are multiple
            // exit paths (which are often a bad thing in themselves,
            // but...)
            [[NSNotificationCenter defaultCenter] removeObserver: tmpObserver];
            return;
        }
        
        [self sortContents];
        [self performLayout];

        // we must remove observer from within this block, because
        // nothing else has a reference to it otherwise.
        [[NSNotificationCenter defaultCenter] removeObserver: tmpObserver];
    }];
}

- (void) repeatObservationsWithoutMemberVariable
{
    id tmpObserver = [[NSNotificationCenter defaultCenter]
                      addObserverForName: @"TheNotification"
                                  object: nil
                                   queue: [NSOperationQueue mainQueue]
                              usingBlock: ^(NSNotification * note) {
        if ( [self validateContents] == NO )
        {
            // must remember to remove observer if there are multiple
            // exit paths (which are often a bad thing in themselves,
            // but...)
            [[NSNotificationCenter defaultCenter] removeObserver: tmpObserver];

            // I want to sign up again, getting another observer object
            // to release on the next invocation -- but how can I
            // reference the block that's currently executing to pass
            // it into this function?
            return;
        }

        [self sortContents];
        [self performLayout];

        // we must remove observer from within this block, because
        // nothing else has a reference to it otherwise.
        [[NSNotificationCenter defaultCenter] removeObserver: tmpObserver];

        // see above-- how do I re-register to keep receiving the
        // events?
    }];
}

- (void) onlyFlawlessWayOfDoingIt
{
    typedef void (^notificationObserver_block)(NSNotification *);

    // our observer block needs to reference itself, which means we
    // need a variable to hold it which is defined up front. For
    // clarity and safety (and sanity) we'll make it a __block
    // variable:
    __block notificationObserver_block observer_block = (notificationObserver_block)0;

    // the observer needs to be assigned by the observer block when it
    // signs up again, which means another __block variable:
    __block id tmpObserver = nil;

    // the observer block gets defined and assigned to its variable
    // here:
    observer_block = ^(NSNotification * note) {
        if ( [self validateContents] )
        {
            [self sortContents];
            [self performLayout];
        }

        // remove the existing observer
        [[NSNotificationCenter defaultCenter] removeObserver: tmpObserver];

        // signup again, because we want to keep receiving this
        // notification
        tmpObserver = [[NSNotificationCenter defaultCenter]
                       addObserverForName: @"TheNotification"
                                   object: nil
                                    queue: [NSOperationQueue mainQueue]
                               usingBlock: observer_block]; // references currently-executing block
    };

    // now we need to repeat the last line of our defined block to
    // start the ball rolling:
    tmpObserver = [[NSNotificationCenter defaultCenter]
                   addObserverForName: @"TheNotification"
                               object: nil
                                queue: [NSOperationQueue mainQueue]
                           usingBlock: observer_block];
}

@end

In stopObservingInsideBlock, shouldn't id tmpObserver be __block id tmpObserver ? Otherwise it is released and is nil in the notification block