First Flutter app (Hello World)

Introduction

We create a simple app to learn how Flutter shows things on the screen. It helps us understand the basics of making mobile apps.

When you want to test if your Flutter setup works correctly.

When you want to learn how to display text on a mobile screen.

When you want to start a new Flutter project and see a simple output.

When you want to understand the structure of a Flutter app.

Syntax

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 const MaterialApp(
      home: Scaffold(
        body: Center(
          child: Text('Hello World'),
        ),
      ),
    );
  }
}

runApp() starts the app and shows the widget you give it.

MaterialApp sets up basic app design and navigation.

Examples

This shows simple text on the screen.

Flutter

Text('Hello World')

This centers the text in the middle of the screen.

Flutter

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

Scaffold provides a basic page layout with a body area.

Flutter

Scaffold(body: Center(child: Text('Hello World')))

Sample App

This program shows the text "Hello World" centered on a white screen. It uses MaterialApp for app basics, Scaffold for page layout, and Center to put the text in the middle.

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 const MaterialApp(
      home: Scaffold(
        body: Center(
          child: Text('Hello World'),
        ),
      ),
    );
  }
}

Important Notes

Use const for widgets that don't change to make your app faster.

Every Flutter app starts with runApp() to show the first widget.

Widgets are the building blocks of Flutter apps, like LEGO pieces.

Summary

Flutter apps start with runApp() and a widget tree.

Use MaterialApp and Scaffold for basic app structure.

Text is shown using the Text widget inside layout widgets like Center.

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