View Controller中可以添加多个sub view，在需要的时候显示出来；
可以通过viewController(parent)中可以添加多个child viewController;来控制页面中的sub view，降低代码耦合度；
通过切换，可以显示不同的view，替代之前的addSubView的管理
//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];
