View Controller中可以添加多個(gè)sub view，在需要的時(shí)候顯示出來(lái)；

可以通過(guò)viewController(parent)中可以添加多個(gè)child viewController;來(lái)控制頁(yè)面中的sub view，降低代碼耦合度；

通過(guò)切換，可以顯示不同的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回調(diào)用[child willMoveToParentViewController:self] ，但是不會(huì)調(diào)用didMoveToParentViewController，所以需要顯示調(diào)用
    [thirdViewController didMoveToParentViewController:self];
    currentViewController=thirdViewController;
    
　　//切換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前不會(huì)調(diào)用[self willMoveToParentViewController:nil] ，所以需要顯示調(diào)用
    [currentViewController willMoveToParentViewController:nil];
    [currentViewController removeFromSuperview];
    [currentViewController removeFromParentViewController];