addChildViewController:的正确用法

现在有ParentVC,OneVC,TwoVC三个controller，想在ParentVC上加载OneVC和TwoVC。
1、如果sub VC还没在父VC上加载过的，则先加载到父VC上，如下

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

2、如果已经加载到父VC上，则

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- (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 = [self newViewStartFrame];
   CGRect endFrame = [self oldViewEndFrame];
 
   // Queue up the transition animation.
   [self transitionFromViewController: oldVC toViewController: newVC
        duration: 0.25 options:0
        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];
        }];
}

references:https://developer.apple.com/library/ios/featuredarticles/ViewControllerPGforiPhoneOS/ImplementingaContainerViewController.html