【iOS6后的内存警告处理】
The memory used by a view to draw itself onscreen is potentially quite large. However, the system automatically releases these expensive resources when the view is not attached to a window. The remaining memory used by most views is small enough that it is not worth it for the system to automatically purge and recreate the view hierarchy.
iOS6后UIKit框架会自动将view中的图片资源给释放掉，而不会将view对象本身给释放掉。因为和位图内存相比，view对象本身所占内存小到可以忽略。所以iOS6起后，didReceiveMemoryWarning默认会做的事情是把把位图资源释放掉，简而言之就是程序员不用再考虑了，UIKit会自动做。而view对象都会被保留。
如果程序员需要自己释放资源，可以这像下面这样写。
- (void)didReceiveMemoryWarning { [super didReceiveMemoryWarning]; // Add code to clean up any of your own resources that are no longer necessary. if (self.isViewLoaded && [self.view window] == nil) { // Add code to preserve data stored in the views that might be // needed later. // Add code to clean up other strong references to the view in // the view hierarchy. self.view = nil; }
iOS5默认会释放位图资源与卸载view对象，iOS6默认只释放位图资源。而为了统一处理，可以用上述代码，iOS6后自主卸载view对象。另外viewWillUnload与viewDidUnload这2个方法就没有意义了，把想关代码都移到上述代码中即可。