What is eventhandler?

An event handler is a small piece of code that “listens” for something happening (an event) in a program-like a button click, a mouse movement, or a key press-and then runs a specific action when that event occurs.

Let's break it down

  • Event: Anything that can happen while a program runs (e.g., clicking a button, loading a page, receiving data).
  • Handler: The function or method that you write to respond to that event.
  • Binding: Connecting the event to its handler so the program knows which code to run when the event fires.

Why does it matter?

Event handlers let programs react to user actions and external changes in real time. Without them, a web page or app would be static and unresponsive, making it impossible to create interactive experiences like forms, games, or dynamic dashboards.

Where is it used?

  • Web pages (JavaScript: button.onclick = function(){...})
  • Mobile apps (Swift: button.addTarget(self, action:#selector(tap), for:.touchUpInside))
  • Desktop software (C#: button.Click += new EventHandler(MyHandler))
  • Server‑side frameworks (Node.js: socket.on('message', handler))
  • IoT devices (Arduino: attachInterrupt(digitalPinToInterrupt(pin), handler, CHANGE))

Good things about it

  • Makes interfaces interactive and user‑friendly.
  • Keeps code organized by separating “what happens” from “when it happens.”
  • Enables asynchronous programming, so the app can stay responsive while waiting for events.
  • Easy to reuse: the same handler can be attached to multiple similar events.

Not-so-good things

  • Too many handlers can clutter code and become hard to maintain.
  • Improperly written handlers may cause performance issues (e.g., heavy calculations on every mouse move).
  • If not removed correctly, they can lead to memory leaks, especially in long‑running applications.
  • Debugging can be tricky because the flow of execution jumps from the main code to the handler when the event fires.