objc[7819]: Class PLBuildVersion is implemented in both /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator.sdk/System/Library/PrivateFrameworks/AssetsLibraryServices.framework/AssetsLibraryServices (0x119b43cc0) and /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator.sdk/System/Library/PrivateFrameworks/PhotoLibraryServices.framework/PhotoLibraryServices (0x1189926f0). One of the two will be used. Which one is undefined.
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：
首先二进制映像被加载，检查程序依赖关系
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
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)
