GestureDetector in Flutter - Mini App: Build & Ship

Build: Tap Counter Screen

A simple screen that counts how many times the user taps on a colored box.

Target UI

---------------------
| Tap Counter Screen |
---------------------

[ Tap me! ]

Taps: 0

Display a colored box with the text 'Tap me!'

Detect taps on the box using GestureDetector

Each tap increases the tap count by 1

Show the current tap count below the box

Starter Code

Flutter

import 'package:flutter/material.dart';

class TapCounterScreen extends StatefulWidget {
  @override
  State<TapCounterScreen> createState() => _TapCounterScreenState();
}

class _TapCounterScreenState extends State<TapCounterScreen> {
  int _tapCount = 0;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text('Tap Counter Screen')),
      body: Center(
        // TODO: Add GestureDetector with colored box and tap count display
      ),
    );
  }
}

Task 1

Task 2

Task 3

Task 4

Solution

Flutter

import 'package:flutter/material.dart';

class TapCounterScreen extends StatefulWidget {
  @override
  State<TapCounterScreen> createState() => _TapCounterScreenState();
}

class _TapCounterScreenState extends State<TapCounterScreen> {
  int _tapCount = 0;

  void _incrementCounter() {
    setState(() {
      _tapCount++;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text('Tap Counter Screen')),
      body: Center(
        child: Column(
          mainAxisSize: MainAxisSize.min,
          children: [
            GestureDetector(
              onTap: _incrementCounter,
              child: Container(
                width: 150,
                height: 150,
                color: Colors.blueAccent,
                alignment: Alignment.center,
                child: Text(
                  'Tap me!',
                  style: TextStyle(color: Colors.white, fontSize: 24),
                ),
              ),
            ),
            SizedBox(height: 20),
            Text('Taps: $_tapCount', style: TextStyle(fontSize: 20)),
          ],
        ),
      ),
    );
  }
}

We use a GestureDetector to detect taps on the blue box. When the user taps, the _incrementCounter method runs, increasing the tap count and updating the UI with setState. The box shows 'Tap me!' text, and below it, the current number of taps is displayed. This teaches how to handle simple touch gestures and update the screen accordingly.

Final Result

Completed Screen

---------------------
| Tap Counter Screen |
---------------------

[ Tap me! ]

Taps: 3

User taps the blue box labeled 'Tap me!'

Each tap increases the number shown below by 1

The box visually stays the same but responds to taps

Stretch Goal

Add a double-tap gesture that resets the tap count to zero.

💡 Hint

Use GestureDetector's onDoubleTap property to reset _tapCount and call setState.

Practice

1. What is the primary purpose of the GestureDetector widget in Flutter?

easy

A. To detect user touch gestures like taps and swipes on its child widget

B. To display images and icons on the screen

C. To manage app navigation between screens

D. To style text with different fonts and colors

Solution

1. Step 1: Understand GestureDetector's role

   The GestureDetector widget listens for user touch gestures such as taps, double taps, long presses, and drags on its child widget.

2. Step 2: Compare with other options

   Displaying images and icons, managing app navigation, and styling text are handled by other widgets like Image/Icon, Navigator/Router, and TextStyle, not GestureDetector.

3. Final Answer:

   To detect user touch gestures like taps and swipes on its child widget -> Option A

4. Quick Check:

   GestureDetector = Detects touch gestures [OK]

Hint: GestureDetector listens for user touches on child widgets [OK]

Common Mistakes:

• Confusing GestureDetector with navigation widgets
• Thinking it styles UI elements
• Assuming it displays images

2. Which of the following is the correct way to detect a tap using GestureDetector in Flutter?

easy

A. GestureDetector(onPressed: () { print('Tapped'); }, child: Text('Tap me'))

B. GestureDetector(onClick: () { print('Tapped'); }, child: Text('Tap me'))

C. GestureDetector(onTouch: () { print('Tapped'); }, child: Text('Tap me'))

D. GestureDetector(onTap: () { print('Tapped'); }, child: Text('Tap me'))

Solution

1. Step 1: Identify the correct GestureDetector callback

   The correct callback for detecting a tap is onTap. Other callbacks like onClick, onPressed, and onTouch do not exist on GestureDetector.

2. Step 2: Verify syntax correctness

   GestureDetector(onTap: () { print('Tapped'); }, child: Text('Tap me')) uses the correct syntax with onTap and a child widget, making it valid Flutter code.

3. Final Answer:

   GestureDetector(onTap: () { print('Tapped'); }, child: Text('Tap me')) -> Option D

4. Quick Check:

   Tap detection uses onTap callback [OK]

Hint: Use onTap callback to detect taps in GestureDetector [OK]

Common Mistakes:

• Using onClick or onPressed instead of onTap
• Forgetting to provide a child widget
• Misspelling callback names

3. What will happen when the user taps the text in this Flutter code?

GestureDetector(
  onTap: () => print('Tapped!'),
  child: Text('Click me'),
)

medium

A. The text 'Tapped!' will be printed to the console

B. Nothing happens because onTap is not a valid callback

C. The app will crash due to missing onPressed callback

D. The text will change to 'Tapped!' on screen

Solution

1. Step 1: Understand onTap behavior

   The onTap callback runs the provided function when the user taps the child widget. Here, it prints 'Tapped!' to the console.

2. Step 2: Check other options

   The text does not change on screen automatically without state management like setState. onTap is valid, so something does happen on tap. The app won't crash because onPressed is not required for GestureDetector.

3. Final Answer:

   The text 'Tapped!' will be printed to the console -> Option A

4. Quick Check:

   onTap prints 'Tapped!' on tap [OK]

Hint: onTap runs code on tap, prints to console if print() used [OK]

Common Mistakes:

• Thinking onTap changes UI automatically
• Confusing onTap with onPressed
• Expecting visual changes without setState

4. Identify the error in this Flutter code snippet using GestureDetector:

GestureDetector(
  onTap: print('Tapped'),
  child: Text('Tap me'),
)

medium

A. Missing child widget inside GestureDetector

B. onTap should be a function, not the result of print()

C. GestureDetector does not support onTap callback

D. Text widget cannot be a child of GestureDetector

Solution

1. Step 1: Analyze onTap assignment

   The onTap callback expects a function to call when tapped. Here, print('Tapped') runs immediately and assigns its result (void) to onTap, which is incorrect.

2. Step 2: Correct usage

   onTap should be assigned a function like () => print('Tapped') or (){ print('Tapped'); } to run on tap.

3. Final Answer:

   onTap should be a function, not the result of print() -> Option B

4. Quick Check:

   onTap needs a function, not immediate call [OK]

Hint: Pass a function to onTap, not a direct call [OK]

Common Mistakes:

• Calling print() instead of passing a function
• Omitting child widget
• Using unsupported callbacks

5. You want to detect both tap and long press gestures on a button in Flutter. Which GestureDetector code correctly handles both events?

hard

A. GestureDetector(onPressed: () => print('Tap'), onLongPress: () => print('Long press'), child: Container())

B. GestureDetector(onTap: () => print('Tap'), onDoubleTap: () => print('Long press'), child: Container())

C. GestureDetector(onTap: () => print('Tap'), onLongPress: () => print('Long press'), child: Container())

D. GestureDetector(onTap: () => print('Tap'), onHold: () => print('Long press'), child: Container())

Solution

1. Step 1: Identify correct callbacks for tap and long press

   GestureDetector uses onTap for taps and onLongPress for long presses. GestureDetector(onTap: () => print('Tap'), onLongPress: () => print('Long press'), child: Container()) uses both correctly.

2. Step 2: Check other options for errors

   GestureDetector(onTap: () => print('Tap'), onDoubleTap: () => print('Long press'), child: Container()) uses onDoubleTap instead of onLongPress, which is incorrect. GestureDetector(onPressed: () => print('Tap'), onLongPress: () => print('Long press'), child: Container()) uses onPressed which GestureDetector does not have. GestureDetector(onTap: () => print('Tap'), onHold: () => print('Long press'), child: Container()) uses onHold which is not a valid callback.

3. Final Answer:

   GestureDetector(onTap: () => print('Tap'), onLongPress: () => print('Long press'), child: Container()) -> Option C

4. Quick Check:

   Use onTap and onLongPress for tap and long press [OK]

Hint: Use onTap and onLongPress callbacks together [OK]

Common Mistakes:

• Using onPressed instead of onTap
• Confusing onDoubleTap with onLongPress
• Using non-existent callbacks like onHold