First Flutter app (Hello World) - Mini App: Build & Ship

Build: Hello World Screen

This screen shows a simple greeting message in the center.

Target UI

---------------------
|                   |
|                   |
|    Hello World    |
|                   |
|                   |
---------------------

Display a centered text that says 'Hello World'.

Use a Scaffold with an AppBar titled 'Welcome'.

Text should be large and easy to read.

Starter Code

Flutter

import 'package:flutter/material.dart';

void main() {
  runApp(const MyApp());
}

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      home: Scaffold(
        appBar: AppBar(
          title: const Text('Welcome'),
        ),
        body: Center(
          // TODO: Add your code here
        ),
      ),
    );
  }
}

Task 1

Task 2

Solution

Flutter

import 'package:flutter/material.dart';

void main() {
  runApp(const MyApp());
}

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      home: Scaffold(
        appBar: AppBar(
          title: const Text('Welcome'),
        ),
        body: const Center(
          child: Text(
            'Hello World',
            style: TextStyle(fontSize: 24),
          ),
        ),
      ),
    );
  }
}

We use a MaterialApp as the root widget to provide material design styling. Inside it, a Scaffold gives us a basic page layout with an AppBar titled 'Welcome'. The main content is centered using Center, and inside it, a Text widget displays 'Hello World' with a font size of 24 to make it easy to read.

Final Result

Completed Screen

---------------------
| Welcome           |
|-------------------|
|                   |
|    Hello World    |
|                   |
---------------------

The screen shows a top app bar with the title 'Welcome'.

The text 'Hello World' is centered on the screen.

No interactive elements on this screen.

Stretch Goal

Change the text color to blue and make it bold.

💡 Hint

Use the TextStyle property with color: Colors.blue and fontWeight: FontWeight.bold.

Practice

1. What is the main function used to start a Flutter app?

easy

A. startApp()

B. runApp()

C. beginApp()

D. launchApp()

Solution

1. Step 1: Identify the Flutter app entry point

   Flutter apps always start by calling the runApp() function to load the widget tree.

2. Step 2: Confirm the correct function name

   The function runApp() is the official way to start the app and display widgets on screen.

3. Final Answer:

   runApp() -> Option B

4. Quick Check:

   App start function = runApp() [OK]

Hint: Remember Flutter apps always start with runApp() [OK]

Common Mistakes:

• Using startApp() instead of runApp()
• Confusing with main() function
• Trying to use launchApp()

2. Which of the following is the correct way to display the text 'Hello World' centered on the screen in Flutter?

easy

A. Center(Text('Hello World'))

B. Text('Hello World').center()

C. Center(child: Text('Hello World'))

D. Text.center('Hello World')

Solution

1. Step 1: Understand widget nesting in Flutter

   Widgets are nested by passing child widgets as parameters, so Center(child: Text('Hello World')) is correct.

2. Step 2: Check syntax correctness

   Options A, B and C are invalid: A misses the named parameter 'child', B and C use invalid methods.

3. Final Answer:

   Center(child: Text('Hello World')) -> Option C

4. Quick Check:

   Center widget uses child: Text(...) [OK]

Hint: Use Center(child: Text('...')) to center text [OK]

Common Mistakes:

• Omitting 'child:' label
• Using invalid method calls on Text
• Trying to call center() on Text widget

3. What will be displayed on the screen when running this Flutter code?

void main() {
  runApp(MaterialApp(
    home: Scaffold(
      body: Center(
        child: Text('Hello Flutter'),
      ),
    ),
  ));
}

medium

A. Centered text 'Hello Flutter'

B. A blank screen

C. An error message

D. Text 'Hello Flutter' aligned to top-left

Solution

1. Step 1: Analyze widget tree structure

   The MaterialApp provides app structure, Scaffold gives a page layout, Center centers its child, and Text shows the string.

2. Step 2: Determine the visible output

   The Text widget with 'Hello Flutter' is centered on the screen inside Scaffold's body.

3. Final Answer:

   Centered text 'Hello Flutter' -> Option A

4. Quick Check:

   Center widget centers Text = centered text [OK]

Hint: Center widget centers its child widget on screen [OK]

Common Mistakes:

• Thinking Scaffold body is empty
• Assuming default alignment is top-left
• Expecting error without syntax issues

4. Identify the error in this Flutter code snippet:

void main() {
  runApp(MaterialApp(
    home: Scaffold(
      body: Center(
        child: Text('Hello World')
      ),
    ),
  ));
}

medium

A. No error, code is correct

B. Missing semicolon after Text widget

C. Missing closing parenthesis for Center widget

D. Missing comma after Text widget

Solution

1. Step 1: Check widget parameter separators

   In Flutter, Center has only one named parameter (child: Text(...)); no comma is needed after a single parameter.

2. Step 2: Verify parentheses and semicolons

   Parentheses are balanced and semicolons are correctly placed after runApp call. Code runs fine.

3. Final Answer:

   No error, code is correct -> Option A

4. Quick Check:

   Single named param needs no trailing comma [OK]

Hint: No comma needed for single widget param [OK]

Common Mistakes:

• Thinking comma is required after single child
• Confusing semicolon with comma
• Ignoring Dart's flexible comma syntax

5. You want to create a Flutter app that shows 'Hello World' in the center with a blue background. Which code snippet correctly achieves this?

hard

A. runApp(MaterialApp(home: Scaffold(body: Center(child: Text('Hello World'))), backgroundColor: Colors.blue)));

B. runApp(MaterialApp(home: Scaffold(body: Center(child: Text('Hello World')), backgroundColor: Colors.blueAccent)));

C. runApp(MaterialApp(home: Scaffold(body: Center(child: Text('Hello World')), backgroundColor: Colors.blueGrey)));

D. runApp(MaterialApp(home: Scaffold(backgroundColor: Colors.blue, body: Center(child: Text('Hello World')))));

Solution

1. Step 1: Understand Scaffold background color property

   Scaffold uses backgroundColor property to set the screen background color. It must be inside Scaffold's constructor.

2. Step 2: Check correct color usage

   runApp(MaterialApp(home: Scaffold(backgroundColor: Colors.blue, body: Center(child: Text('Hello World'))))); correctly uses Colors.blue inside Scaffold. A places backgroundColor on MaterialApp (invalid). C uses blueGrey (greyish), D uses blueAccent (bright accent).

3. Final Answer:

   runApp(MaterialApp(home: Scaffold(backgroundColor: Colors.blue, body: Center(child: Text('Hello World'))))); -> Option D

4. Quick Check:

   Scaffold backgroundColor = Colors.blue [OK]

Hint: Set backgroundColor inside Scaffold, not MaterialApp [OK]

Common Mistakes:

• Putting backgroundColor in MaterialApp
• Using invalid color names
• Forgetting to wrap Text in Center