addChildViewController

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];