在苹果的WWDC2011大会视频的 《Session 101 - What’s New in Cocoa》 和 《Session 102 - Implementing UIViewController Containment》 中介绍了苹果在iOS5中给UIViewController新增加的5方法以及一个属性:
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit;font-size:undefined; font-style: inherit; font-variant: inherit; font-weight: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">1</span><span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit;font-size:undefined; font-style: inherit; font-variant: inherit; font-weight: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">2</span><span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit;font-size:undefined; font-style: inherit; font-variant: inherit; font-weight: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">3</span><span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit;font-size:undefined; font-style: inherit; font-variant: inherit; font-weight: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">4</span><span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit;font-size:undefined; font-style: inherit; font-variant: inherit; font-weight: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">5</span><span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit;font-size:undefined; font-style: inherit; font-variant: inherit; font-weight: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">6</span><span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit;font-size:undefined; font-style: inherit; font-variant: inherit; font-weight: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">7</span><span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit;font-size:undefined; font-style: inherit; font-variant: inherit; font-weight: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">8</span>
// 方法addChildViewController:removeFromParentViewController:transitionFromViewController:toViewController:duration:options:animations:completion:willMoveToParentViewController:didMoveToParentViewController:// 属性@property(nonatomic,readonly) NSArray *childViewControllers
原来的问题
这些新增的方法和属性用于改进我们的编程方式。那么让我们先看看以前的对于UIViewController的使用有什么潜在的问题，认清问题，我们才能理解苹果改变的意义。
在以前，一个UIViewController的View可能有很多小的子view。这些子view很多时候被盖在最后，我们在最外层ViewController的viewDidLoad方法中，用addSubview增加了大量的子view。这些子view大多数不会一直处于界面上，只是在某些情况下才会出现，例如登陆失败的提示view，上传附件成功的提示view，网络失败的提示view等。但是虽然这些view很少出现，但是我们却常常一直把它们放在内存中。另外，当收到内存警告时，我们只能自己手工把这些view从super view中去掉。
改变
苹果新的API增加了addChildViewController方法，并且希望我们在使用addSubview时，同时调用[self addChildViewController:child]方法将sub view对应的viewController也加到当前ViewController的管理中。对于那些当前暂时不需要显示的subview，只通过addChildViewController把subViewController加进去。需要显示时再调用transitionFromViewController:toViewController:duration:options:animations:completion方法。
另外，当收到系统的Memory Warning的时候，系统也会自动把当前没有显示的subview unload掉，以节省内存。
参考资料
关于这个，这儿有一篇不错的文章介绍了一段sample代码用于演示新API的使用.
我也将其代码稍加修改，增加了view load, unload, appear, disappear的事件Log，以及收到Memory Warning时的Log。代码放在了github上，地址是这里，感兴趣的同学可以自己下载下来看看源码。
可以看到，这些view在没有使用时，是不会被load的，并且当有Memory Warning时，当前没有显示的view自动被unload掉了。所以新的方法确实能有效地节省内存，也能方便地处理内存不足时的资源回收。运行Log如下：
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit;font-size:undefined; font-style: inherit; font-variant: inherit; font-weight: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">1</span><span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit;font-size:undefined; font-style: inherit; font-variant: inherit; font-weight: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">2</span><span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit;font-size:undefined; font-style: inherit; font-variant: inherit; font-weight: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">3</span><span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit;font-size:undefined; font-style: inherit; font-variant: inherit; font-weight: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">4</span><span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit;font-size:undefined; font-style: inherit; font-variant: inherit; font-weight: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">5</span><span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit;font-size:undefined; font-style: inherit; font-variant: inherit; font-weight: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">6</span><span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit;font-size:undefined; font-style: inherit; font-variant: inherit; font-weight: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">7</span><span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit;font-size:undefined; font-style: inherit; font-variant: inherit; font-weight: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">8</span><span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit;font-size:undefined; font-style: inherit; font-variant: inherit; font-weight: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">9</span><span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit;font-size:undefined; font-style: inherit; font-variant: inherit; font-weight: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">10</span><span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit;font-size:undefined; font-style: inherit; font-variant: inherit; font-weight: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">11</span><span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit;font-size:undefined; font-style: inherit; font-variant: inherit; font-weight: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">12</span><span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit;font-size:undefined; font-style: inherit; font-variant: inherit; font-weight: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">13</span><span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit;font-size:undefined; font-style: inherit; font-variant: inherit; font-weight: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">14</span><span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit;font-size:undefined; font-style: inherit; font-variant: inherit; font-weight: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">15</span><span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit;font-size:undefined; font-style: inherit; font-variant: inherit; font-weight: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">16</span><span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit;font-size:undefined; font-style: inherit; font-variant: inherit; font-weight: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">17</span><span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit;font-size:undefined; font-style: inherit; font-variant: inherit; font-weight: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">18</span><span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit;font-size:undefined; font-style: inherit; font-variant: inherit; font-weight: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">19</span><span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit;font-size:undefined; font-style: inherit; font-variant: inherit; font-weight: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">20</span><span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit;font-size:undefined; font-style: inherit; font-variant: inherit; font-weight: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">21</span>
[7397:f803] -[FirstViewController willMoveToParentViewController:][7397:f803] -[SecondViewController willMoveToParentViewController:][7397:f803] -[ThirdViewController willMoveToParentViewController:][7397:f803] -[ThirdViewController viewDidLoad][7397:f803] -[ThirdViewController viewWillAppear:][7397:f803] -[ThirdViewController viewDidAppear:][7397:f803] 生日提醒[7397:f803] -[SecondViewController viewDidLoad][7397:f803] -[ThirdViewController viewWillDisappear:][7397:f803] -[SecondViewController viewWillAppear:][7397:f803] -[SecondViewController viewDidAppear:][7397:f803] -[ThirdViewController viewDidDisappear:][7397:f803] 留言及回复[7397:f803] -[FirstViewController viewDidLoad][7397:f803] -[SecondViewController viewWillDisappear:][7397:f803] -[FirstViewController viewWillAppear:][7397:f803] -[FirstViewController viewDidAppear:][7397:f803] -[SecondViewController viewDidDisappear:][7397:f803] Received memory warning.[7397:f803] -[SecondViewController viewDidUnload][7397:f803] -[ThirdViewController viewDidUnload]
