在ios8中使用viewDidLayoutSubviews，应用正常运行，没有问题，但是应用在ios7上运行的时候，报错，导致应用崩溃，错误信息类似：
Cannot find executable for CFBundle 0x78f8f220 </Library/Developer/CoreSimulator/Profiles/Runtimes/iOS 7.1.simruntime/Contents/Resources/RuntimeRoot/System/Library/AccessibilityBundles/GeoServices.axbundle
Assertion failure in -[UIView layoutSublayersOfLayerl]....unrecognizeSelector.....
在viewDidLayoutSubviews方法末尾添加[self.view layoutsubviews]即可解决