Events: on tap, do this — App development for students
Apps wait for events: things that happen, like a tap on a button, a key being typed, a timer ticking or a screen opening. Each event can have a handler — a small block of code that runs every time the event happens. In our code, button.onTap = function () { … } sets the code to run on each tap. To predict what the app shows, run the handler once for every tap and follow the values.
When this, do that — “every event (a tap, a timer, typing) runs its handler code”
Event → handler → action: “on tap, do this”.
The handler runs once for every tap.
Counter: start + taps × step. A limit stops it growing; on = !on flips ON / OFF.
Use it for: Making buttons do something: counters, like buttons, switches and greetings.
Worked example: 👆 The user taps the button 5 times. What number does the label show now?
let stars = 3;
button.onTap = function () {
stars = stars + 2;
label.text = stars;
}; → 13
Work it out in your head or on paper, then tap your answer — or type it and press Check. Stuck? Press “Show me how”.
Right: 0 · Tried: 0
🏁 Lesson 17 test
20 questions. Check each answer, then go to the next one. Get 16 or more right to pass.
🏅 Sign in with Google to earn the
👆 Event Handler Hero badge when you pass — and a certificate when you finish all 8 lessons of
Part 3 · Logic and Building. You can still take the test without signing in.
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