- 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];