website/sl/sl.go

package sl

import (
	"fmt"
	"log"
)

type provider struct {
	initialized bool
	service     Service
}

type ServiceLocator struct {
	providers map[string]*provider
	hooks     map[string][]Service
}

type Service interface {
	Initialize(l *ServiceLocator) error
}

func New() *ServiceLocator {
	return &ServiceLocator{
		providers: map[string]*provider{},
		hooks:     map[string][]Service{},
	}
}

func getTypeName[T any]() string {
	var v T
	return fmt.Sprintf(`%T`, &v)[1:]
}

// Inject will set the implementation for "S" to "value" (the service will be initialized when needed after all of its dependencies)
func Inject[S Service](l *ServiceLocator, value S) {
	key := getTypeName[S]()
	log.Printf(`injecting value of type %T for interface %s`, value, key)

	l.providers[key] = &provider{false, value}
}

// InjectValue will set the implementation for "S" to "value" and mark this service as already initialized (as this is just a constant)
func InjectValue[S Service](l *ServiceLocator, value S) S {
	key := getTypeName[S]()
	log.Printf(`injecting value of type %T for interface %s`, value, key)

	l.providers[key] = &provider{true, value}
	return value
}

// Use will retrive from the service locator the implementation set for the type "T" and initialize the service if yet to be initialized
func Use[T Service](l *ServiceLocator) (T, error) {
	provider, ok := l.providers[getTypeName[T]()]
	if !ok {
		var zero T
		return zero, fmt.Errorf(`no injected value for type "%T"`, zero)
	}

	if provider.initialized {
		service := provider.service.(T)
		return service, nil
	}

	log.Printf(`initializing %T`, provider.service)
	if err := provider.service.Initialize(l); err != nil {
		var zero T
		return zero, err
	}

	provider.initialized = true
	service := provider.service.(T)
	return service, nil
}