UINavigationController检测后退按钮被按下

-(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];
}