Container View Controller

有时候，我们的Controler中包含有另一个controler view的view时，可以使用这种方式。
https://developer.apple.com/library/ios/featuredarticles/ViewControllerPGforiPhoneOS/ImplementingaContainerViewController.html
把一个controller view 加入到 controler中

[self addChildViewController:content];  //1
content.view.frame = [self frameForContentController];
[self.view addSubview:self.currentClientView]; //2
[content didMoveToParentViewController:self]; //3

移除一个controller view

[content willMoveToParentViewController:nil]; //1
[content.view removeFromSuperview]; //2
[content removeFromParentViewController]; //3