Radio buttons in Flutter - UI Render Trace

Component - Radio buttons

This UI component shows a group of radio buttons. Users can select only one option at a time. It is useful for choosing one item from a small list.

Widget Tree

Scaffold
├─ AppBar
│  └─ Text
└─ Column
   ├─ ListTile
   │  ├─ Radio
   │  └─ Text
   ├─ ListTile
   │  ├─ Radio
   │  └─ Text
   └─ ListTile
      ├─ Radio
      └─ Text

The Scaffold provides the basic screen structure with an AppBar at the top showing a title. The body is a Column that stacks three ListTiles vertically. Each ListTile contains a Radio button and a Text label next to it.

Render Trace - 5 Steps

Step 1: Scaffold

Step 2: Column

Step 3: ListTile (first)

Step 4: ListTile (second)

Step 5: ListTile (third)

State Change - Re-render

Trigger:User taps on a radio button

Before

selected option is 1

→

After

selected option changes to tapped radio button's value (e.g., 2)

Re-renders:Entire StatefulWidget subtree containing the Column and ListTiles re-renders to update radio button selection

UI Quiz - 3 Questions

What happens when you tap a radio button in this UI?

A Nothing changes visually

B All radio buttons become selected

C The tapped radio button becomes selected and others deselect

D The app crashes

Key Insight

Radio buttons let users pick one choice from a small set. Grouping them with the same groupValue ensures only one can be selected at a time. Using ListTile with Radio and Text creates a clean, accessible row for each option.

Practice

1.

What is the main purpose of a Radio button in Flutter?

easy

A. To display an image

B. To let the user enter text input

C. To navigate between screens

D. To allow the user to select only one option from a group

Solution

1. Step 1: Understand Radio button usage

   Radio buttons are designed to let users pick one choice from a small set of options.

2. Step 2: Compare with other UI elements

   Text input is for typing, images display pictures, navigation moves screens, so these are not correct.

3. Final Answer:

   To allow the user to select only one option from a group -> Option D

4. Quick Check:

   Radio button = single choice selector [OK]

Hint: Radio buttons always pick one from many [OK]

Common Mistakes:

• Confusing radio buttons with checkboxes
• Thinking radio buttons accept text input
• Assuming radio buttons navigate screens

2.

Which of the following is the correct way to declare a Radio button with value 1 and groupValue 2?

Radio( ... )

easy

A. Radio(1, 2, onChanged: (val) {})

B. Radio(value: 1, groupValue: 2, onChanged: (val) {})

C. Radio(value: '1', groupValue: '2', onChanged: (val) {})

D. Radio(value = 1, groupValue = 2, onChanged = (val) {})

Solution

1. Step 1: Check Dart named parameter syntax

   Flutter widgets use named parameters with colons, e.g., value: 1, groupValue: 2.

2. Step 2: Validate parameter types

   Values should be integers here, so strings are incorrect. Also, positional parameters are not allowed.

3. Final Answer:

   Radio(value: 1, groupValue: 2, onChanged: (val) {}) -> Option B

4. Quick Check:

   Use named parameters with colons [OK]

Hint: Use colons for named parameters in Flutter widgets [OK]

Common Mistakes:

• Using equals sign (=) instead of colon (:)
• Passing values as strings instead of correct type
• Using positional parameters instead of named

3.

What will be the selected radio button after this code runs?

int selected = 2;

Column(
  children: [
    Radio(value: 1, groupValue: selected, onChanged: (val) {}),
    Radio(value: 2, groupValue: selected, onChanged: (val) {}),
    Radio(value: 3, groupValue: selected, onChanged: (val) {}),
  ],
)

medium

A. The radio button with value 2

B. The radio button with value 1

C. The radio button with value 3

D. No radio button selected

Solution

1. Step 1: Understand groupValue and value relation

   The radio button whose value matches groupValue is selected.

2. Step 2: Check the values

   groupValue is 2, so the radio with value 2 is selected.

3. Final Answer:

   The radio button with value 2 -> Option A

4. Quick Check:

   Selected = groupValue match [OK]

Hint: Selected radio matches groupValue [OK]

Common Mistakes:

• Confusing value and groupValue roles
• Assuming first radio is selected by default
• Ignoring groupValue in selection

4.

Identify the error in this Flutter code snippet for radio buttons:

int selected = 1;

Radio(
  value: 1,
  groupValue: selected,
  onChanged: null,
)

medium

A. value and groupValue must be strings

B. selected should be a boolean

C. onChanged should not be null to allow selection

D. Radio widget cannot be used inside Column

Solution

1. Step 1: Check onChanged parameter

   onChanged must be a function to update selection; null disables interaction.

2. Step 2: Validate other parameters

   value and groupValue can be int; selected as int is correct; Radio can be inside Column.

3. Final Answer:

   onChanged should not be null to allow selection -> Option C

4. Quick Check:

   onChanged null disables radio [OK]

Hint: onChanged must be non-null for radio to work [OK]

Common Mistakes:

• Setting onChanged to null disables selection
• Confusing data types for value/groupValue
• Thinking Radio can't be inside layout widgets

5.

You want to create a group of radio buttons for colors: Red, Green, Blue. The selected color should update the UI when tapped. Which code snippet correctly implements this behavior?

class ColorSelector extends StatefulWidget {
  @override
  State<ColorSelector> createState() => _ColorSelectorState();
}

class _ColorSelectorState extends State<ColorSelector> {
  String selectedColor = 'Red';

  @override
  Widget build(BuildContext context) {
    return Column(
      children: [
        Radio<String>(
          value: 'Red',
          groupValue: selectedColor,
          onChanged: (val) {
            setState(() {
              selectedColor = val!;
            });
          },
        ),
        Radio<String>(
          value: 'Green',
          groupValue: selectedColor,
          onChanged: (val) {
            setState(() {
              selectedColor = val!;
            });
          },
        ),
        Radio<String>(
          value: 'Blue',
          groupValue: selectedColor,
          onChanged: (val) {
            setState(() {
              selectedColor = val!;
            });
          },
        ),
      ],
    );
  }
}

hard

A. The code correctly updates selectedColor and UI on tap

B. Missing setState call inside onChanged

C. groupValue should be a boolean, not String

D. Radio buttons cannot use String values

Solution

1. Step 1: Check state update on tap

   onChanged calls setState to update selectedColor, which triggers UI refresh.

2. Step 2: Validate types and usage

   groupValue and value are Strings, which is allowed and consistent.

3. Step 3: Confirm UI rebuild

   setState ensures the widget rebuilds with new selectedColor, updating radio selection.

4. Final Answer:

   The code correctly updates selectedColor and UI on tap -> Option A

5. Quick Check:

   setState + matching groupValue = UI update [OK]

Hint: Use setState in onChanged to update selection [OK]

Common Mistakes:

• Forgetting to call setState to refresh UI
• Using wrong types for groupValue
• Assuming Radio buttons can't use strings