# Protocol Buffers for Elixir

exprotobuf works by building module/struct definitions from a [Google Protocol Buffer](https://code.google.com/p/protobuf)
schema. This allows you to work with protocol buffers natively in Elixir, with easy decoding/encoding for transport across the
wire.

[![Build Status](https://travis-ci.org/bitwalker/exprotobuf.svg?branch=master)](https://travis-ci.org/bitwalker/exprotobuf)
[![Hex.pm Version](http://img.shields.io/hexpm/v/exprotobuf.svg?style=flat)](https://hex.pm/packages/exprotobuf)

## Features

* Load protobuf from file or string
* Respects the namespace of messages
* Allows you to specify which modules should be loaded in the definition of records
* Currently uses [gpb](https://github.com/tomas-abrahamsson/gpb) for protobuf schema parsing

TODO:

* Clean up code/tests

## Breaking Changes

The 1.0 release removed the feature of handling `import "...";` statements.
Please see [the imports upgrade guide](imports_upgrade_guide.md) for details if you were using this feature.

## Getting Started

Add exprotobuf as a dependency to your project:

```elixir
defp deps do
  [{:exprotobuf, "~> x.x.x"}]
end
```

Then run `mix deps.get` to fetch.

Add exprotobuf to applications list:

```elixir
def application do
  [applications: [:exprotobuf]]
end
```

## Usage

Usage of exprotobuf boils down to a single `use` statement within one or
more modules in your project.

Let's start with the most basic of usages:

### Define from a string

```elixir
defmodule Messages do
  use Protobuf, """
    message Msg {
      message SubMsg {
        required uint32 value = 1;
      }

      enum Version {
        V1 = 1;
        V2 = 2;
      }

      required Version version = 2;
      optional SubMsg sub = 1;
    }
  """
end
```

```elixir
iex> msg = Messages.Msg.new(version: :'V2')
%Messages.Msg{version: :V2, sub: nil}
iex> encoded = Messages.Msg.encode(msg)
<<16, 2>>
iex> Messages.Msg.decode(encoded)
%Messages.Msg{version: :V2, sub: nil}
```

The above code takes the provided protobuf schema as a string, and
generates modules/structs for the types it defines. In this case, there
would be a Msg module, containing a SubMsg and Version module. The
properties defined for those values are keys in the struct belonging to
each. Enums do not generate structs, but a specialized module with two
functions: `atom(x)` and `value(x)`. These will get either the name of
the enum value, or it's associated value.

### Define from a file

```elixir
defmodule Messages do
  use Protobuf, from: Path.expand("../proto/messages.proto", __DIR__)
end
```

This is equivalent to the above, if you assume that `messages.proto`
contains the same schema as in the string of the first example.

### Loading all definitions from a set of files

```elixir
defmodule Protobufs do
  use Protobuf, from: Path.wildcard(Path.expand("../definitions/**/*.proto", __DIR__))
end
```

```elixir
iex> Protobufs.Msg.new(v: :V1)
%Protobufs.Msg{v: :V1}
iex> %Protobufs.OtherMessage{middle_name: "Danger"}
%Protobufs.OtherMessage{middle_name: "Danger"}
```

This will load all the various definitions in your `.proto` files and
allow them to share definitions like enums or messages between them.

### Customizing Generated Module Names

In some cases your library of protobuf definitions might already contain some
namespaces that you would like to keep.
In this case you will probably want to pass the `use_package_names: true` option.
Let's say you had a file called `protobufs/example.proto` that contained:

```protobuf
package world;
message Example {
  enum Continent {
    ANTARCTICA = 0;
    EUROPE = 1;
  }

  optional Continent continent = 1;
  optional uint32 id = 2;
}
```

You could load that file (and everything else in the protobufs directory) by doing:

```elixir
defmodule Definitions do
  use Protobuf, from: Path.wildcard("protobufs/*.proto"), use_package_names: true
end
```

```elixir
iex> Definitions.World.Example.new(continent: :EUROPE)
%Definitions.World.Example{continent: :EUROPE}
```

You might also want to define all of these modules in the top-level namespace. You
can do this by passing an explicit `namespace: :"Elixir"` option.

```elixir
defmodule Definitions do
  use Protobuf, from: Path.wildcard("protobufs/*.proto"),
                use_package_names: true,
                namespace: :"Elixir"
end
```

```elixir
iex> World.Example.new(continent: :EUROPE)
%World.Example{continent: :EUROPE}
```

Now you can use just the package names and message names that your team is already
familiar with.

### Inject a definition into an existing module

This is useful when you only have a single type, or if you want to pull
the module definition into the current module instead of generating a
new one.

```elixir
defmodule Msg do
  use Protobuf, from: Path.expand("../proto/messages.proto", __DIR__), inject: true

  def update(msg, key, value), do: Map.put(msg, key, value)
end
```

```elixir
iex> %Msg{}
%Msg{v: :V1}
iex> Msg.update(%Msg{}, :v, :V2)
%Msg{v: :V2}
```

As you can see, Msg is no longer created as a nested module, but is
injected right at the top level. I find this approach to be a lot
cleaner than `use_in`, but may not work in all use cases.

### Inject a specific type from a larger subset of types

When you have a large schema, but perhaps only care about a small subset
of those types, you can use `:only`:

```elixir
defmodule Messages do
  use Protobuf, from: Path.expand("../proto/messages.proto", __DIR__),
only: [:TypeA, :TypeB]
end
```

Assuming that the provided .proto file contains multiple type
definitions, the above code would extract only TypeA and TypeB as nested
modules. Keep in mind your dependencies, if you select a child type
which depends on a parent, or another top-level type, exprotobuf may
fail, or your code may fail at runtime.

You may only combine `:only` with `:inject` when `:only` is a single
type, or a list containing a single type. This is due to the restriction
of one struct per module. Theoretically you should be able to pass `:only`
with multiple types, as long all but one of the types is an enum, since
enums are just generated as modules, this does not currently work
though.

### Extend generated modules via `use_in`

If you need to add behavior to one of the generated modules, `use_in`
will help you. The tricky part is that the struct for the module you
`use_in` will not be defined yet, so you can't rely on it in your
functions. You can still work with the structs via the normal Maps API,
but you lose compile-time guarantees. I would recommend favoring
`:inject` over this when possible, as it's a much cleaner solution.

```elixir
defmodule Messages do
  use Protobuf, "
    message Msg {
      enum Version {
        V1 = 1;
        V2 = 1;
      }
      required Version v = 1;
    }
  "

  defmodule MsgHelpers do
    defmacro __using__(_opts) do
      quote do
        def convert_to_record(msg) do
          msg
          |> Map.to_list
          |> Enum.reduce([], fn {_key, value}, acc -> [value | acc] end)
          |> Enum.reverse
          |> list_to_tuple
        end
      end
    end
  end

  use_in "Msg", MsgHelpers
end
```

```elixir
iex> Messages.Msg.new |> Messages.Msg.convert_to_record
{Messages.Msg, :V1}
```

## Attribution/License

exprotobuf is a fork of the azukiaapp/elixir-protobuf project, both of which are released under Apache 2 License.

Check LICENSE files for more information.