childViewController

//默认显示在self.view中
[self addChildViewController:firstVC];
// addChildViewController回调[child willMoveToParentViewController:self] 
// 但是不会调用didMoveToParentViewController，所以需要显示调用
[firstVC didMoveToParentViewController:self];

_currentVC = firstVC;

//切换childViewController 
[self transitionFromViewController:currentViewController toViewController:firstViewController duration:4 options:UIViewAnimationOptionTransitionFlipFromLeft 
animations:^{ } 
completion:^(BOOL finished) { //...... }];

//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
[_currentVC willMoveToParentViewController:nil]; 
[_currentVC removeFromSuperview]; 
[_currentVC removeFromParentViewController];