An example showing why you should never perform coordinated file accesses in a file presentation handler.
This example employs two file presenters. Both are handling -relinquishPresentedItemToReader: and -presentedItemDidChange. Whenever -presentedItemDidChange is called, the presenters perform a coordinated read. By that, both presenters are querying each other for relinquishing the file. This is done by enqueuing a block on the sequential -presentedItemOperationQueue of each other presenter. By that, the request will be never confirmed: the queue is already blocked by the -presentedItemDidChange notification that waits for the confirmation request of the other presenter.