Events: Programs That React to What the User Does
A program does not have to run from top to bottom. A handler sits waiting for a click, a key press or a collision, and runs only when that thing happens.
What a learner can do afterwards
- Attach one handler to a key press and another to a click in the same program, and say which runs when
- Explain why an event-driven program looks like it is doing nothing until the user acts
- Fix a program where the handler was attached to the wrong object or the wrong event
1 · Read
Some programs do not run from top to bottom. A handler is a small set of instructions that sits waiting for one event, like a click or a key press. It runs only when that event happens.
Imagine a game where the left arrow moves your sprite and a click changes its colour. Pressing the key runs the key handler, and clicking runs the click handler. Tapping a button on a page can run code that changes its colour, which tells you it was clicked.
That is why an event program looks like it is doing nothing. It is not stuck. It is listening, and nothing moves until you act.
When the wrong thing happens, check the object first. A click handler on the Stop button never runs when you click Play.
Then check the event. A handler tied to a key press runs on keys, not clicks, so a sound meant for a click plays on a key instead.
A handler can also test before it acts. A Send button should check that the message box holds text, or it sends with nothing typed.
Handlers wait for their event, then run, so nothing happens until you click or press.
Practice items adapted from WeBWorK Open Problem Library problems by University of Missouri contributors (CC BY-NC-SA 3.0). Readings written for this dataset.
2 · Watch
En anglaisTake it off screen
Where it sits
Learn first
Optional connections
This opens up
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
10 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.