fx

package main

import (
	"context"
	"log"
	"net/http"
	"os"
	"time"

	"go.uber.org/fx"
)

func NewLogger() *log.Logger {
	logger := log.New(os.Stdout, "[Example] " /* prefix */, 0 /* flags */)
	logger.Print("Executing NewLogger.")
	return logger
}

func NewHandler(logger *log.Logger) http.Handler {
	logger.Print("Executing NewHandler.")
	return http.HandlerFunc(func(http.ResponseWriter, *http.Request) {
		logger.Print("Got a request.")
	})
}

func NewMux(lc fx.Lifecycle, logger *log.Logger) *http.ServeMux {
	logger.Print("Executing NewMux.")
	mux := http.NewServeMux()
	server := &http.Server{
		Addr:    ":8080",
		Handler: mux,
	}
	// If NewMux is called, we know that someone is using the mux. In that case,
	// start up and shut down an HTTP server with the application.
	lc.Append(fx.Hook{
		OnStart: func(context.Context) error {
			logger.Print("Starting HTTP server.")
			go server.ListenAndServe()
			return nil
		},
		OnStop: func(ctx context.Context) error {
			logger.Print("Stopping HTTP server.")
			return server.Shutdown(ctx)
		},
	})

	return mux
}

func Register(mux *http.ServeMux, h http.Handler) {
	mux.Handle("/", h)
}

func main() {
	app := fx.New(
		// Provide all the constructors we need.
		fx.Provide(NewLogger, NewHandler, NewMux),
		// Before starting, register the handler. This forces resolution of all
		// the types Register function depends on: *http.ServeMux and http.Handler.
		// Since the mux is now being used, its startup hook gets registered
		// and the application includes an HTTP server.
		fx.Invoke(Register),
	)

	// In a real application, we could just use app.Run() here. Since we don't
	// want this example to run forever, we'll use Start and Stop.
	startCtx, cancel := context.WithTimeout(context.Background(), 15*time.Second)
	defer cancel()
	if err := app.Start(startCtx); err != nil {
		log.Fatal(err)
	}

	// Normally, we'd block here with <-app.Done().
	http.Get("http://localhost:8080/")

	stopCtx, cancel := context.WithTimeout(context.Background(), 15*time.Second)
	defer cancel()
	if err := app.Stop(stopCtx); err != nil {
		log.Fatal(err)
	}
}

Output:

[Example] Executing NewLogger.
[Example] Executing NewMux.
[Example] Executing NewHandler.
[Example] Starting HTTP server.
[Example] Got a request.
[Example] Stopping HTTP server.