//在parent view controller 中添加 child view controller　　 FirstViewController *firstViewController=[[FirstViewController alloc] initWithNibName:@"FirstViewController" bundle:nil];    [self addChildViewController:firstViewController];        SecondViewController *secondViewController=[[SecondViewController alloc] initWithNibName:@"SecondViewController" bundle:nil];    [self addChildViewController:secondViewController];        ThirdViewController *thirdViewController=[[ThirdViewController alloc] initWithNibName:@"ThirdViewController" bundle:nil];    [self addChildViewController:thirdViewController];        [self.view  addSubview:thirdViewController.view];　　// addChildViewController回调用[child willMoveToParentViewController:self] ，但是不会调用didMoveToParentViewController，所以需要显示调用    [thirdViewController didMoveToParentViewController:self];    currentViewController=thirdViewController;    　　//切换child view controller     [self transitionFromViewController:currentViewController toViewController:firstViewController duration:4 options:UIViewAnimationOptionTransitionFlipFromLeft animations:^{            }  completion:^(BOOL finished) {               //......            }];    currentViewController=firstViewController;　　//移除child view controller    // removeFromParentViewController在移除child前不会调用[self willMoveToParentViewController:nil] ，所以需要显示调用    [currentViewController willMoveToParentViewController:nil];    [currentViewController removeFromSuperview];    [currentViewController removeFromParentViewController];