messenger.go

type Messenger struct {
	// Add this field here
	Client *http.Client

	mux                    *http.ServeMux
	messageHandlers        []MessageHandler
	deliveryHandlers       []DeliveryHandler
	readHandlers           []ReadHandler
	postBackHandlers       []PostBackHandler
	optInHandlers          []OptInHandler
	referralHandlers       []ReferralHandler
	accountLinkingHandlers []AccountLinkingHandler
	token                  string
	verifyHandler          func(http.ResponseWriter, *http.Request)
	verify                 bool
	appSecret              string
}

// Then add this helper method - it is technically NOT threadsafe, but only if you try to change the Client field
// while you are using it to make requests in another goroutine.
func (m *Messenger) httpClient() *http.Client {
  if m.Client != nil {
    return m.Client
  }
  return &http.Client{}
}

// Then in your methods...
func (m *Messenger) ProfileByID(id int64) (Profile, error) {
	// ... all the same
  
  // change this line
  client := m.httpClient()
	resp, err := client.Do(req)
  // ... all the same
}