编辑时报错：
Undefined symbols for architecture x86_64:
"_OBJC_CLASS_$_AFHTTPSessionManager", referenced from:
_OBJC_CLASS_$_HttpClient in HttpClient.o
"_OBJC_CLASS_$_AFSecurityPolicy", referenced from:
objc-class-ref in HttpClient.o
"_OBJC_CLASS_$_SSKeychain", referenced from:
objc-class-ref in UserManager.o
"_OBJC_METACLASS_$_AFHTTPSessionManager", referenced from:
_OBJC_METACLASS_$_HttpClient in HttpClient.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
问题已得到解决，解决方法如下：~/Library/Developer/Xcode/DerivedData 关掉xcode，删掉DerivedData这个文件夹里的全部东西 然后重新打开项目clean>build
