https://developer.apple.com/library/ios/samplecode/UICatalog/Listings/Objective_C_UICatalog_AAPLAlertControllerViewController_m.html#//apple_ref/doc/uid/DTS40007710-Objective_C_UICatalog_AAPLAlertControllerViewController_m-DontLinkElementID_7
原文：http://www.cnblogs.com/wcLT/p/4745943.html