问题描述：当运行Xcode8.1时，编译代码成功，可运行，控制台出现如下打印：

objc[924]: Class PLBuildVersion is implemented in both /Applications/Xcode.app/
Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator.sdk/
System/Library/PrivateFrameworks/AssetsLibraryServices.framework/AssetsLibraryServices (0x11827c998) 
and /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator.sdk/
System/Library/PrivateFrameworks/PhotoLibraryServices.framework/
PhotoLibraryServices (0x1180a1d38). One of the two will be used. Which one is undefined.

解决办法：意思是PhotoLibraryServices.framework和AssetsLibraryServices.framework产生了冲突，系统不知道用哪个
Class PLBuildVersion在框架AssetsLibraryServices和框架PhotoLibraryServices里都有定义。在Objective-C里Class名是唯一。两个同名的Class，Xcode会选择其中一个Class来执行。PLBuildVersion在这两个框架里都是一样的，不影响实际运行。