When I try to build any project for iOS, the build fails with the message: “clang: error: no such file or directory: ‘/Users/Shared/UnrealEngine/4.4/Engine/Binaries/IOS/UE4Game-Redist-MetalRHI.a’”
Has anyone else experienced this? I’m on a mac, using Xcode 6.
Basically, your error message says that it couldn’t find the metal library in the binaries path.
You need to compile the source from GitHub to use the Metal API.
Doc here : link.
Hopefully Apple release the DNA for beta tech! 
“We are very lucky that Apple has loosened the NDA restrictions for beta tech, because if this was in iOS 7 or earlier we would not have been able to release the code.”
If you are patient enough, I guess that after the official iOS 8 release on September 18th, Epic will upgrade the UE4 binary package with the precompiled Metal lib!
Thanks for the answer, that makes sense.