First install v3 of the Protocol Buffers compiler (protoc), by cloning its Git repository and following these installation instructions (the ones titled C++; don't miss the note for Mac users).
Then clone this repository and execute the following commands from the root directory where it was cloned.
Compile the gRPC plugins for protoc:
make plugins
Create a symbolic link to the compiled plugin binary somewhere in your $PATH:
ln -s `pwd`/bins/opt/grpc_objective_c_plugin /usr/local/bin/protoc-gen-objcgrpc
(Notice that the name of the created link must begin with "protoc-gen-" for protoc to recognize it as a plugin).
If you don't want to create the symbolic link, you can alternatively copy the binary (with the appropriate name). Or you might prefer instead to specify the plugin's path as a flag when invoking protoc, in which case no system modification nor renaming is necessary.
Finally, run protoc with the following flags to generate the client library for your .proto files:
protoc --objc_out=. --objcrpc_out=. *.proto
This will generate a pair of .pbobjc.h/.pbobjc.m files for each .proto file, with the messages and enums defined in them. And a pair of .pbrpc.h/.pbrpc.m files for each .proto file with services defined. The latter contains the code to make remote calls to the specified API.
This is the recommended approach.
You need to create a Podspec file for the generated library. This is simply a matter of copying an example like this one to the directory where the source files were generated. Update the name and other metadata of the Podspec as suitable.
Once your library has a Podspec, refer to it from your Podfile using :path as described here.
You need to compile the generated .pbpbjc.* files (the enums and messages) without ARC support, and the generated .pbrpc.* files (the services) with ARC support. The generated code depends on v0.3+ of the Objective-C gRPC runtime library and v3.0+ of the Objective-C Protobuf runtime library.
These libraries need to be integrated into your project as described in their respective Podspec files: