First Flutter app (Hello World) - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the main function in a Flutter app?

The main function is the entry point of a Flutter app. It calls runApp() to start the app and display the UI.

beginner

What widget is used to display simple text in Flutter?

The Text widget is used to show a string of text on the screen.

beginner

What does runApp() do in Flutter?

runApp() takes a widget and makes it the root of the widget tree, showing it on the screen.

beginner

Why do we use MaterialApp in a Flutter app?

MaterialApp provides basic visual structure and styling following Material Design, like app bars and themes.

beginner

How do you center a widget in Flutter?

Use the Center widget to place its child widget in the middle of the screen.

Which function starts a Flutter app?

A startApp()

B runApp()

C mainApp()

D launchApp()

runApp() is the function that launches the Flutter app by inflating the given widget.

Which widget displays text on the screen?

A Text

B Container

C Row

D Scaffold

The Text widget is used to show text in Flutter apps.

What widget centers its child in the available space?

A Center

B Align

C Padding

D Expanded

Center widget places its child in the center of the parent widget.

MaterialApp is used to:

A Display images

B Create a button

C Provide Material Design structure and theming

D Handle user input

MaterialApp sets up the app with Material Design visuals and navigation.

Where do you write the code to start your Flutter app?

A Inside the runApp() function

B Inside the build() method

C Inside the MaterialApp widget

D Inside the main() function

The main() function is the app's entry point where runApp() is called.

Explain the steps to create and run a simple Flutter app that shows 'Hello World' on the screen.

Think about how Flutter apps start and how widgets build the UI.

Describe the role of the MaterialApp widget in a Flutter app and why it is important for beginners.

MaterialApp is like the app's foundation for look and feel.

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