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];
}