The event handling system is a critical part of an application and if it fails, you are left with a broken application. Event listeners are responsible for listening to the events and handling them accordingly while the rest of the event handling work is done by the system.
The event listener is a mechanism which allows the user to interact with the application in a non-blocking manner. An event listener can subscribe to events provided by the application and respond to it. There are different kinds of event listeners such as listeners, handlers, and observers.
EventHandler is an event-based programming language and EventListener is a program that listens to events.
Event source is a mechanism for listening for events. A typical event source consists of an event listener that does not consume any resources, has no knowledge about the content and sends an event in response to a request. An example of such a system is the default web browser if you are using Firefox or Chrome.
Event handling is a very important topic for developers and IT professionals because this task requires several skills. One of the most common use cases for event handling is with software-as-a-service (SaaS) applications, where the software must provide multiple services to customers in response to different events.