One of the two will be used. Which one is undefined

objc[37995]: Class PLBuildVersion is implemented in both /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator.sdk/System/Library/PrivateFrameworks/AssetsLibraryServices.framework/AssetsLibraryServices (0x11450e998) and /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator.sdk/System/Library/PrivateFrameworks/PhotoLibraryServices.framework/PhotoLibraryServices (0x114333d38). One of the two will be used. Which one is undefined.

意思是

类PLBuildVersion在/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator.sdk/System/Library/PrivateFrameworks/AssetsLibraryServices.framework/AssetsLibraryServices（0x11450e998）和/ Applications / Xcode中实现 .app / Contents / Developer / Platforms / iPhoneSimulator.platform / Developer / SDKs / iPhoneSimulator.sdk / System / Library / PrivateFrameworks / PhotoLibraryServices.framework / PhotoLibraryServices（0x114333d38） 将使用两者之一。哪一个是不确定的。
类PLBuildVersion在两者中实现。

在模拟器中、发现“One of the two will be used. Which one is undefined.”日志

　　查找资料发现原因：objc runtime 对所用app使用同一个命名空间(flat namespace)，运行机制如下：

1. 首先二进制映像被加载，检查程序依赖关系
2. 每一个二进制映像被加载的同时，程序的objc classes在objc runtime命名空间中注册
3. 如果具有相同名称的类被再次加载，objc runtime的行为是不可预知的。一种可能的情况是任意一个程序的该类会被加载(这应该也是默认动作)