[Apex] How do I implement and use callbacks or event handlers in Apex?

To implement and use callbacks or event handlers in Apex, you can follow these steps:

  1. Define an interface: First, create an interface that defines the callback method. For example, suppose you want to create a callback for when a lead is created. You can define an interface like this:
1public interface LeadCreationCallback {
2    void onLeadCreated(Id leadId);
  1. Implement the interface: In your Apex class that needs to invoke the callback, implement the interface. For example:
1public class LeadProcessor implements LeadCreationCallback {
2    public void onLeadCreated(Id leadId) {
3        // Handle the lead creation event
4        // ...
5    }
  1. Create a callback object: In your code where the event occurs, create an instance of the callback interface and assign it to a variable. For example:
1// Create a callback object
2LeadCreationCallback callback = new LeadProcessor();
  1. Invoke the callback method: When the desired event occurs, invoke the callback method. For example:
1// Invoke the callback method

By following these steps, you can implement and use callbacks or event handlers in Apex. The callback pattern allows for loose coupling between components and enables decoupled communication between them.