Golang Interfaces and Dependency Injection

main.go

package main

import (
	"encoding/json"
	"fmt"
	"os"
)

type NotificationState int

const (
	Error NotificationState = iota
	Info
	Unknown
)

type Notification struct {
	State   NotificationState `json:"state"`
	Message string            `json:"message"`
}

type Notifier interface {
	Notify(notification Notification) error
}

type SlackNotifier struct{}

func NewSlackNotifier() *SlackNotifier {
	return &SlackNotifier{}
}

type EmailNotifier struct{}

func NewEmailNotifier() *EmailNotifier {
	return &EmailNotifier{}
}

func (e EmailNotifier) Notify(notification Notification) error {
	b, err := json.Marshal(notification)
	if err != nil {
		return err
	}
	fmt.Printf("Sending Email Notification %s\n", b)
	return nil
}

func (s SlackNotifier) Notify(notification Notification) error {
	b, err := json.Marshal(notification)
	if err != nil {
		return err
	}
	fmt.Printf("Sending Slack Notification %s\n", b)
	return nil
}

type App struct {
	Notifiers []Notifier
}

func NewApp(notifiers ...Notifier) *App {

	return &App{
		Notifiers: notifiers,
	}
}

func (a *App) WithNotifier(notifier Notifier) *App {
	if a.Notifiers == nil {
		a.Notifiers = make([]Notifier, 0)
	}
	a.Notifiers = append(a.Notifiers, notifier)
	return a
}

func (a *App) SendNotification(message string, state int) error {
	notification := Notification{
		Message: message,
	}
	if state == 0 {
		notification.State = Error
	} else {
		notification.State = Info
	}
	errs := make([]error, 0)
	for _, n := range a.Notifiers {
		err := n.Notify(notification)
		if err != nil {
			errs = append(errs, err)
		}
	}
	if len(errs) > 0 {
		return fmt.Errorf("too many errors sending notifications")
	}
	return nil
}

func main() {
	notifier := NewSlackNotifier()
	emailNotifier := NewEmailNotifier()

	app := NewApp(notifier)
	app = app.WithNotifier(emailNotifier)
	err := app.SendNotification("message from app", 1)
	if err != nil {
		fmt.Printf("error sending email notification: %v\n", err)
		os.Exit(1)
	}
	fmt.Println("Done.")
}