UIViewController

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1.在init中给self.view.frame赋值,会调用viewdidload方法,然后再继续init中的代码，可以把给self.view.frame赋值放到init中的最后。

调用顺序 init，loadView，viewDidLoad，viewWillAppear

2. viewDidUnload

   It is called during low-memory conditions，when the view controller needs to release its view and any objects associated with that view to free up memory.
   在栈导航中返回上一层时，本页的dealloc调用了，viewDidUnload也没有调用。
   当内存警告时，会调用viewDidUnload，要在这里选择做一些释放内存的操作。再加载这个页面时，会重新调用loadView，viewDidLoad。viewController.view将会被重新构建。
   不用重写didReceiveMemoryWarning
   调用键盘等耗内存大时，会收到- (void)didReceiveMemoryWarning ，但不用重写didReceiveMemoryWarning，要在- (void)viewDidUnload 中选择释放一些内存。当再进入这个view controller时，系统会重新调用它的loadView，viewDidLoad。
   dealloc后此viewcontroller不会再重用了，但viewDidUnload后此viewcontroller还是要重用的，所以要注意在viewDidUnload中选择释放内存，并置nil。

测试依次调用了applicationDidReceiveMemoryWarning，UITabBarController的didReceiveMemoryWarning，以及第一个tab项ViewController的didReceiveMemoryWarning和viewDidUnload，以及其它ab项ViewController的didReceiveMemoryWarning，及其它子viewController的didReceiveMemoryWarning.
即会调用活着的viewController的didReceiveMemoryWarning，选择调用它们的viewDidUnload。
loadView，viewDidLoad不能认为在viewController生命周期内只会被调用一次。一些内容可以不在viewDidUnload中释放，但要在loadView中做好保护。

3.关于 ios5中UIViewController addChildViewController新方法的使用

http://wangjun.easymorse.com/?p=1630

4.presentModalViewController上升效果与父UIViewController的view的尺寸有关

presentModalViewController的时候，它会以window作为底色，所以设置alpha的时候会出现白色

5.UIInterfaceOrientation interfaceOrientation 属性，可能这个值在使用过程中不准确，还是使用[[UIApplication sharedApplication] statusBarOrientation];替代。

在UIViewController初始化过程中 self.interfaceOrientation;准确，但在方向变化后，self.interfaceOrientation可能没有被更新，这时要使用[[UIApplication sharedApplication] statusBarOrientation];

6.parentViewController 属性

/*
If this view controller is a child of a containing view controller (e.g. a navigation controller or tab bar
controller,) this is the containing view controller. Note that as of 5.0 this no longer will return the
presenting view controller.
*/
@property(nonatomic,readonly) UIViewController *parentViewController;

如果viewController中导航中，parentViewController返回的是navigation controller实例。

7.navigationController属性

self.navigationController可能返回的是self.parentViewController成员，或者返回的是self.parentViewController.parentViewController，直到找到类型UINavigationController。

7.In practice, on any modern (3.0+) iOS, all three methods are sent to the view controller of the actively-shown view.

The methods are called in the order listed:
willRotateToInterfaceOrientation:duration: first

willAnimateRotationToInterfaceOrientation:duration: second,

   didRotateFromInterfaceOrientation: last
   Note: iOS 5 deprecates the other rotation-notification methods (dealing with the two-step animation process), so these three are the only ones that should be used in new projects.
   http://stackoverflow.com/questions/2521273/what-are-the-differences-between-the-interface-orientation-delegate-methods

8.ios6中，当一个ViewController的view赋值给另一个ViewController的view时，运行报错： A view can only be associated with at most one view controller at a time。