Generate Protocol Buffers files for iOS

This guide shows you how to generate the Protocol Buffers source files and library for iOS from scratch.

Generate C++ source files from the proto file

Install Protocol Buffers v3.10.1 on your local machine. Please pay attention to the section titled "Note for Mac users".
Check your installation:
```
protoc --version
```
You should have libprotoc 3.10.1.
In the Cardboard repository, navigate to the proto folder.
Generate the Protocol Buffers source files in C++:
```
protoc --proto_path=. --cpp_out=. cardboard_device.proto
```
You should now have cardboard_device.pb.cc and cardboard_device.pb.h.

Cross-compile the Protocol Buffers library

You will need a cross-compiled Protocol Buffers library to build the Cardboard SDK.

Clone the Protocol Buffers repository into your local machine:
```
git clone https://github.com/protocolbuffers/protobuf.git
```
Navigate into the into Protocol Buffers repository folder. Run the following commands:
```
git checkout v3.10.1
```
```
git submodule update --init --recursive
```
```
./autogen.sh
```

Create environment variables:

build_dir=`pwd`/../libprotobuf/ios
darwin=darwin`uname -r`
protoc=`which protoc`
isysroot=`xcrun --sdk iphoneos --show-sdk-path`
cflags="-Wno-unused-local-typedef -Wno-unused-function -DNDEBUG -g -O0 -pipe -fPIC -fcxx-exceptions"
cxxflags="$cflags -std=c++11 -stdlib=libc++"

Create the output directories:

mkdir -p $build_dir/arch
mkdir -p $build_dir/lib

Configure the 64-bit build:

./configure \
--build=x86_64-apple-$darwin \
--host=arm \
--with-protoc=$protoc \
--disable-shared \
--prefix=$build_dir \
--exec-prefix=$build_dir/arch/arm64 \
"CC=clang" \
"CFLAGS=$cflags -miphoneos-version-min=8.0 -arch arm64 -isysroot $isysroot" \
"CXX=clang" \
"CXXFLAGS=$cxxflags -miphoneos-version-min=8.0 -arch arm64 -isysroot $isysroot" \
LDFLAGS="-arch arm64 -miphoneos-version-min=8.0 -stdlib=libc++" \
"LIBS=-lc++ -lc++abi"

Build the 64-bit library:
```
make -j8
make install
```
Clean the Protocol Buffers directory:
```
make distclean
```

Configure the 32-bit library build:

./configure \
--build=x86_64-apple-$darwin \
--host=armv7-apple-$darwin \
--with-protoc=$protoc \
--disable-shared \
--prefix=$build_dir \
--exec-prefix=$build_dir/arch/armv7 \
"CC=clang" \
"CFLAGS=$cflags -miphoneos-version-min=8.0 -arch armv7 -isysroot $isysroot" \
"CXX=clang" \
"CXXFLAGS=$cxxflags -miphoneos-version-min=8.0 -arch armv7 -isysroot $isysroot" \
LDFLAGS="-arch armv7 -miphoneos-version-min=8.0 -stdlib=libc++" \
"LIBS=-lc++ -lc++abi"

Build the 32-bit library:
```
make -j8
make install
```

Build the fat library:

lipo \
$build_dir/arch/arm64/lib/libprotobuf-lite.a \
$build_dir/arch/armv7/lib/libprotobuf-lite.a \
-create \
-output $build_dir/lib/libprotobuf-lite.a

Remove unused libraries:
```
rm -rf $build_dir/arch
```
Navigate out of the protobuf repository. You should now have the cross-compiled library (libprotobuf-lite.a) in the libprotobuf/ios/lib directory, and all the include files in the include (liprotobuf/ios/include) directory.

Follow the schema below to copy the libprotobuf directory into the proto directory in the Cardboard repository:

proto
├── cardboard_device.pb.cc
├── cardboard_device.pb.h
└── libprotobuf
    └── ios
        ├── include
        │   └── google/*
        └── lib
            └── libprotobuf-lite.a