What is event?
An event is a signal that something happened - like a button being clicked, a file finishing download, or a sensor detecting motion. In software, events are messages that tell the program “this thing occurred,” so the program can respond accordingly.
Let's break it down
- Source: The part of the system that creates the event (e.g., a mouse, a server, a timer).
- Event object: A small package of data that describes what happened (type, time, extra details).
- Listener/handler: Code that waits for the event and decides what to do when it arrives.
- Dispatch: The process of sending the event from the source to the listeners.
Why does it matter?
Events let programs react instantly without constantly checking (polling) for changes. This makes software faster, more efficient, and easier to organize, especially when many things can happen at once.
Where is it used?
- Web pages (click, hover, form submit)
- Mobile apps (touch, swipe, GPS update)
- Server‑side systems (incoming network request, file change)
- IoT devices (sensor reading, button press)
- Game engines (collision, score update)
Good things about it
- Responsiveness: UI updates immediately when the user interacts.
- Decoupling: Sources and handlers can be written independently, improving code maintainability.
- Scalability: Multiple listeners can react to the same event, enabling complex workflows.
- Resource efficient: No need for constant checking loops, saving CPU and battery.
Not-so-good things
- Complex debugging: When many events fire, it can be hard to trace the flow of execution.
- Potential for “event storms”: Too many events in a short time can overload the system if not throttled.
- Order uncertainty: In asynchronous environments, events may arrive out of the expected order, requiring extra handling.
- Memory leaks: Forgetting to remove listeners can keep objects alive longer than needed.