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];}