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Published: Mar 9, 2016 License: MIT Imports: 4 Imported by: 0



Package gengen provides goagen with the ability to run user provided generators (*plugins*).

How to Write a goagen Plugin

The only requirement is that the package exposes a global Generate function with the following signature:

func Generate(api *design.APIDefinition) ([]string, error)

where api is the API definition computed from the design DSL. On success Generate should return the path to the generated files. On error the error message gets displayed to the user (and goagen exist with status 1).

The Generate method should take advantage of the APIDefinition IterateXXX methods to iterate through the API resources, media types and types to guarantee that the order doesn't change between two invokation of the function (thereby generating different output even if the design hasn't changed).

They may also take advantage of Metadata. The goa design language allows defining Metadata on a number of definitions: API, Resource, Action, Response and Attribute (which means Type and MediaType as well since these definitions are attributes). A metadata field consists of a key/value pair where both are simple strings. The generator can use these key/value pairs to produce different results, see example below. Metadata has no effect on the buit-in generators. The Output directory is available through the codegen.OutputDir global variable.

Package genresnames is an example of a goagen plugin. It creates a file "names.txt" containing the names of the API resources sorted in alphabetical order. If a resource has a metadata pair with the key "genresnames/name" then the plugin uses the metadata value instead.

Invoke the plugin with:

goagen gen -d <Go package path to design package> --pkg-path=<Go package path to genresnames>

Source code:

package genresnames

import (



// Generate is the function called by goagen to generate the names file.
func Generate(api *design.APIDefinition) ([]string, error) {
	// Make sure to parse the common flags so that codegen.OutputDir gets properly
	// initialized.
	app := kingpin.New("Resource names", "Resource name generator")
	if _, err := app.Parse(os.Args[1:]); err != nil {

	// Now iterate through the resources to gather their names
	names := make([]string, len(api.Resources))
	i := 0
	api.IterateResources(func(res *design.ResourceDefinition) error {
		if n, ok := res.Metadata["genresnames/name"]; ok {
			names[i] = n
		} else {
			names[i] = res.Name
		return nil
	content := strings.Join(names, "\n")

	// Write the output file and return its name
	outputFile := filepath.Join(codegen.OutputDir, "names.txt")
	ioutil.WriteFile(outputFile, []byte(content), 0644)
	return []string{outputFile}, nil



This section is empty.


View Source
var (
	// GenPkgPath contains the path to the third party generator Go package.
	GenPkgPath string

	// GenPkgName contains the name of the third party generator Go package.
	GenPkgName string


This section is empty.


type Command

type Command struct {

Command is the goa generic generator command line data structure. It implements meta.Command.

func NewCommand

func NewCommand() *Command

NewCommand instantiates a new command.

func (*Command) RegisterFlags

func (c *Command) RegisterFlags(r codegen.FlagRegistry)

RegisterFlags registers the command line flags with the given registry.

func (*Command) Run

func (c *Command) Run() ([]string, error)

Run simply calls the meta generator.

Source Files

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