Can't get retain cycle

Question

I found this code of tableview lazyloading from apple.com,but can't get the point of retain cycle,what's the need of creating weak pointer of parser,please help.

ParseOperation *parser = [[ParseOperation alloc] initWithData:self.appListData];

parser.errorHandler = ^(NSError *parseError) {
    dispatch_async(dispatch_get_main_queue(), ^{
        [self handleError:parseError];
    });
};
// Referencing parser from within its completionBlock would create a retain
// cycle.
__weak ParseOperation *weakParser = parser;

parser.completionBlock = ^(void) {
    if (weakParser.appRecordList) {
        // The completion block may execute on any thread.  Because operations
        // involving the UI are about to be performed, make sure they execute
        // on the main thread.
        dispatch_async(dispatch_get_main_queue(), ^{
            // The root rootViewController is the only child of the navigation
            // controller, which is the window's rootViewController.
                RootViewController *rootViewController = (RootViewController*)       [(UINavigationController*)self.window.rootViewController topViewController];

            rootViewController.entries = weakParser.appRecordList;

            // tell our table view to reload its data, now that parsing has completed
            [rootViewController.tableView reloadData];
        });
    }

    // we are finished with the queue and our ParseOperation
    self.queue = nil;
};

[self.queue addOperation:parser]; // this will start the "ParseOperation"

Answer 1

If you reference the parser in the completion block, the block will retain it. And since the parser in turn holds onto the completion block, you get a retain cycle:

       parser
      +---------------------------+
      |                           |
      |                           |
      |                           |
 +----+   completion block        |<-------+
 |    |  +---------------------+  |        |
 |    |  |                     |  |        | holds onto
 |    |  |                     |  |        |
 |    |  |                     +-----------+
 +------>|                     |  |
      |  |                     |  |
      |  |                     |  |
      |  |                     |  |
      |  |                     |  |
      |  +---------------------+  |
      |                           |
      +---------------------------+

When you use a weak pointer in the completion block, you break this cycle, since the completion block no longer keeps the parser from being deallocated.