UINavigationController检测后退按钮被按下

Objective-c代码  

-(void) viewWillDisappear:(BOOL)animated
{
    if ([self.navigationController.viewControllers indexOfObject:self]==NSNotFound)
    {
        // back button was pressed.  We know this is true because self is no longer
        // in the navigation stack.
        NSLog(@"viewWillDisappear:(BOOL)animated ");
        if(request)
        {
            [request clearDelegatesAndCancel];
            request.delegate = nil;
            [request release];
            request = nil;
        }
    }
    [super viewWillDisappear:animated];
}