建立父子关系后,容器设置子视图控制器的视图大小，并且把它添加到自己的视图层级关系中。设定子视图控制器的视图大小是很重要的,确保视图正确显示在父控制器的视图中。添加视图后,父控制器调用子视图控制器的didMoveToParentViewController:方法，父视图控制器给孩子一个机会来响应视图所属关系的改变。

//添加子控制器
- (void)displayContentController:(UIViewController*)content contentFrame:(CGRect)frame {

    [self addChildViewController:content];
    content.view.frame = frame;
    [self.view addSubview:content.view];
    [content didMoveToParentViewController:self];

}

//移除子控制器
- (void) hideContentController: (UIViewController*) content {

[content willMoveToParentViewController:nil];
[content.view removeFromSuperview];
[content removeFromParentViewController];

}

//子控制器之间的切换
- (void)cycleFromViewController: (UIViewController*) oldVC
               toViewController: (UIViewController*) newVC {
    // Prepare the two view controllers for the change.
    [oldVC willMoveToParentViewController:nil];
    [self addChildViewController:newVC];
    
    // Get the start frame of the new view controller and the end frame
    // for the old view controller. Both rectangles are offscreen.
    newVC.view.frame = CGRectMake(100, 200, 100, 100);
    CGRect endFrame = CGRectMake(200, 100, 100, 100);
    
    // Queue up the transition animation.
    [self transitionFromViewController: oldVC toViewController: newVC
                              duration: 1.25 options:UIViewAnimationOptionCurveEaseIn
                            animations:^{
                                // Animate the views to their final positions.
                                newVC.view.frame = oldVC.view.frame;
                                oldVC.view.frame = endFrame;
                            }
                            completion:^(BOOL finished) {
                                // Remove the old view controller and send the final
                                // notification to the new view controller.
                                [oldVC removeFromParentViewController];
                                [newVC didMoveToParentViewController:self];
                            }];
}