[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);
3}
  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    }
6}
  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
2callback.onLeadCreated(leadId);

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.