objc-release-gotcha.m

// Meanwhile, somewhere in a UI-specific non-thread-safe class

- (void) dealloc {
    // glView is not thread-safe and may only be accessed via the main thread.
    [_glView release];

    [super dealloc];
}

- (void) updateUI {
    // Using GCD to execute a heavy operation on a background thread, and then
    // report the results on the main thread.
    // The dispatch calls copy the provided blocks, which increments self's retain count.
    dispatch_async(queue, ^{
        NSData *data = [NSData dataWithContentsOfFile: file];
        dispatch_async(dispatch_get_main_queue(), ^{
            [self displayData: data];
        });
    });

    // Once the blocks have executed, they will be released. If -release/-autorelease is called on a non-main thread and
    // the retain count hits 0, our UI classes' non-threadsafe -dealloc will be called on a background thread.
}