fx

Automatic cobra command registration with fx

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Cobra is a popular Go package for creating CLIs. It provides a lot of functionality for creating commands, subcommands, and flags. However, it can be tedious to manually register all of your commands.

fx is a Go package that provides a dependency injection framework. It can be used to automatically register your application components, including but not limited to Cobra commands.

In this blog post, I will show you how you can use fx to automatically register your Cobra commands.

This code was written 5 minutes ago, but works, and I imagine it could help folks bootstrap more complex CLIs rapidly.

The problem

When you create a Cobra command, you need to manually register it with the root command. This can be tedious, especially if you have a lot of commands.

For example, the following code registers a command called hello:

func NewHelloCmd() *cobra.Command {
  cmd := &cobra.Command{
    Use:   "hello",
    Short: "Say hello",
    Run: func(cmd *cobra.Command, args []string) {
      cmd.Println("Hello, world!")
    },
  }
  return cmd
}

func main() {
  rootCmd := &cobra.Command{
    Use:   "myapp",
    Short: "My application",
  }
  rootCmd.AddCommand(NewHelloCmd())
  rootCmd.Execute()
}Code language: Go (go)

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Dependency injection in go using fx, and replacing services for test

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I’m writing a new go application and ended up giving fx (by uber) a try for dependency injection. The getting started docs were brilliant for my use case (creating an API), but the examples for how to inject mock services for tests were lacking, so I decided to write some code examples of how I am currently using fx in tests.

General app setup

I set up my whole application in a app package, which pulls in all services that are needed.

Most of these are provided as constructor functions, with some more dynamic registration of routes as is done in the getting started docs.

package app

func New(additionalOpts ...fx.Option) *fx.App {
	return fx.New(
		fx.Provide(
			config.NewDefault,
			ses.NewSES,
			ses.NewEmailSender,
			mysql.NewGormGB,
			middleware.NewLimiterFactory,
			middleware.NewAuth,
			AsRoute(handlers.UserLogin),
			AsRoute(handlers.ListStuff),
			fx.Annotate(
				router.NewGinRouter,
				fx.ParamTags(`group:"routes"`),
			),
			server.NewHTTPServer,
		),
		fx.Invoke(func(*http.Server) {}),
		fx.Options(additionalOpts...),
	)
}Code language: Go (go)

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