Protoc compiler Dart plugin

This application provides a plugin for protoc compiler which generates pure Dart library to deal with protobufs.

Please, do not forget that generated libraries depend on runtime support library which can be found here.

How to build and use

Note: currently the workflow is POSIX-oriented.

To build standalone protoc plugin:

run pub install to install all dependecies
run make build-plugin. That will create a file out/protoc-gen-dart which is a plugin
Now you can use it either by adding into PATH or passing directly with protoc's --plugin option.

Please, remember that the plugin is pure Dart script and requires the presence of dart executable in your PATH.

When both the dart executable and out/protoc-gen-dart are in the PATH the protocol buffer compiler can be invoked to generate like this:

$ protoc --dart_out=. test.proto

Options to control the generated Dart code

The protocol buffer compiler accepts options for each plugin. For the Dart plugin, these options are passed together with the --dart_out option. The individial options are separated using comma, and the final output directive is separated from the options using colon. Pass options <option 1> and <option 2> like this:

--dart_out="<option 1>,<option 2>:."

Option for setting the name of field accessors

The following message definition has the field name has_field.

message MyMessage {
  optional string has_field = 1;
}

This poses the problem, that the Dart class will have a getter and a setter called hasField. This conflicts with the method hasField which is already defined on the superclass GeneratedMessage.

To work around this problem the option field_name can be used. Option field_name takes two values separated by the vertical bar. The first value is the full name of the field and the second value is the name of the field in the generated Dart code. Passing the following option:

--dart_out="field_name=MyMessage.has_field|HasFld:."

Will generate the following message field accessors:

String get hasFld => getField(1);
void set hasFld(String v) { setField(1, v); }
bool hasHasFld() => hasField(1);
void clearHasFld() => clearField(1);

Using protocol buffer libraries to build new libraries

The protocol buffer compiler produces one library for each .proto file it compiles. In some cases this is not exactly what is needed, e.g one would like to create new libraries which exposes the objects in these libraries or create new librares combining object definitions from several .proto libraries into one.

The best way to aproach this is to create the new libraries needed and re-export the relevant protocol buffer classes.

Say we have the file m1.proto with the following content

message M1 {
  optional string a;
}

and m2.proto containing

message M2 {
  optional string b;
}

Compiling these to Dart will produce two libraries in m1.pb.dart and m2.pb.dart. The following code shows a library M which combines these two protocol buffer libraries, exposes the classes M1 and M2 and adds som additional methods.

library M;

import "m1.pb.dart";
import "m2.pb.dart";

export "m1.pb.dart" show M1;
export "m2.pb.dart" show M2;

M1 createM1() => new M1();
M2 createM2() => new M2();

Hacking

You need to have protoc installed.

Linux Aptitude: apt-get install protobuf-compiler
Mac homebrew: brew install protobuf

Remember to run the tests. That is as easy as make run-tests.

The default way of running the Dart protoc plugin is through the generated out/protoc-gen-dart script. However when run this way the Dart code is assembled into one large Dart file using dart2dart. To run with the actual source in the repository create an executable script called protoc-gen-dart with the following content:

#! /bin/bash
dart bin/protoc_plugin.dart

When running protoc just ensure that this script is first when PATH is searched. If the script is in the current directory run protoc like this:

$ PATH=.:$PATH protoc --dart_out=. test.proto

It is also possible to call the script something else than protoc-gen-dart and then refer directly to it using the --plugin option. If the script is called dart-plugin run protoc like this:

$ protoc --plugin=protoc-gen-dart=./plugin --dart_out=. test.proto