Use Listener pattern for incomming webhooks

[eximius313]

Currently If I want to read and handle incomming webhook, I must do this:

JsonMapper mapper = new DefaultJsonMapper();
    WebhookObject webhook = mapper.toJavaObject(jsonCallback, WebhookObject.class);
    webhook.getEntryList().forEach(entry -> {
      entry.getMessaging().forEach(m -> {
        Optional.ofNullable(m.getOptin())
          .map(OptinItem::getRef) //is it an OptIn?
          .map(Optional::of)
          .orElseGet(() -> Optional.ofNullable(m.getReferral()) //is it a Referral?
              .filter(byField(ReferralItem::getSource, "MESSENGER_CODE"::equals))
              .map(ReferralItem::getRef)
           )
          .ifPresent(referral -> {
//handle referral
          });
      });
    });

which is ugly as hell.

But when you think about it..... incomming webhook is just a set of predefined Events https://developers.facebook.com/docs/messenger-platform/webhook#setup

So why not to use Observer (listener) design pattern?

You could provide an Interface:

  public interface WebhookEventListener {
    void echo(final MessagingItem item);
    void optin(final MessagingItem item);
    void referral(final MessagingItem item);
    void payment(final MessagingItem item);
    void postback(final MessagingItem item);
    //...
    void unknown(final MessagingItem item);
  }

and Adapter to ease the implementation:

  public abstract class AbstractWebhookEventListener implements WebhookEventListener {

    @Override
    public void echo(final MessagingItem item) { }

    @Override
    public void optin(final MessagingItem item) { }
    
    @Override
    public void referral(final MessagingItem item) { }

    @Override
    public void payment(final MessagingItem item) { }

    @Override
    public void postback(final MessagingItem item) { }

    @Override
    public void unknown(final MessagingItem item) { }
  }

and then process Webhooks like this:

    JsonMapper mapper = new DefaultJsonMapper();
    final WebhookEventListener optInListener = new AbstractWebhookEventListener() {
      @Override
      public void optin(final MessagingItem item) {
        handle(item.getOptin().getRef());
      }
      @Override
      public void referral(final MessagingItem item) {
        handle(item.getReferral().getRef());
      }
    };
    WebhookObject webhook = Webhook.registerListener(optInListener);
    webhook.process(jsonCallback);

What do you think?

[nbartels]

I'll have a look at this idea ...