Slider widget in Flutter - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a Slider widget in Flutter?

A Slider widget lets users select a value from a range by moving a thumb along a horizontal line. It is useful for choosing numbers like volume or brightness.

beginner

Which property of Slider controls the current selected value?

The value property holds the current position of the slider thumb, representing the selected value.

beginner

How do you update the Slider's value when the user moves the thumb?

Use the onChanged callback to get the new value and update the state variable that holds the slider's value.

beginner

What do the min and max properties of Slider do?

They set the minimum and maximum values the slider can select, defining the range of the slider.

intermediate

Can a Slider be disabled? How?

Yes, by setting onChanged to null, the Slider becomes disabled and the user cannot move the thumb.

What type of value does the Slider widget's value property expect?

A int

B double

C String

D bool

The Slider's value is a double because it can represent decimal numbers within the range.

Which callback is used to respond when the user moves the slider thumb?

A onChanged

B onPressed

C onTap

D onHover

The onChanged callback is called with the new value when the slider thumb moves.

How do you make a Slider inactive or disabled?

A Set min and max to the same value

B Set value to 0

C Set onChanged to null

D Set enabled property to false

Setting onChanged to null disables the Slider, preventing user interaction.

What happens if you set min to 0 and max to 100 in a Slider?

A Slider range is from 0 to 100

B Slider range is from 1 to 99

C Slider range is unlimited

D Slider will not work

The slider allows selecting any value between 0 and 100 inclusive.

Which widget is best to use for selecting a value by sliding horizontally?

A Checkbox

B Switch

C Radio

D Slider

Slider is designed for horizontal sliding to select a value from a range.

Explain how to create a basic Slider widget in Flutter that lets the user pick a value between 0 and 10.

Think about how to store the current value and update it when the user moves the slider.

Describe how you can disable a Slider and why you might want to do that.

Consider when you want to stop the user from changing the slider temporarily.

Practice

1. What is the main purpose of the Slider widget in Flutter?

easy

A. To display a list of items vertically

B. To let users select a value by sliding a thumb along a line

C. To create a button that changes color when pressed

D. To show an image with zoom controls

Solution

1. Step 1: Understand the Slider widget function

   The Slider widget allows users to pick a numeric value by moving a thumb along a horizontal line.

2. Step 2: Compare with other widget purposes

   Other options describe different widgets: lists, buttons, images, which are unrelated to Slider.

3. Final Answer:

   To let users select a value by sliding a thumb along a line -> Option B

4. Quick Check:

   Slider = select value by sliding [OK]

Hint: Slider is for picking values by sliding [OK]

Common Mistakes:

• Confusing Slider with ListView or Button widgets
• Thinking Slider is for displaying images
• Assuming Slider is a static display widget

2. Which of the following is the correct way to create a basic Slider with a value of 0.5, minimum 0, maximum 1, and a simple onChanged callback?

easy

A. Slider(value: 0.5, min: 0, max: 1, onChanged: (v) => print(v))

B. Slider(value: 0.5, minimum: 0, maximum: 1, onChange: (v) => print(v))

C. Slider(val: 0.5, min: 0, max: 1, onChanged: (v) => print(v))

D. Slider(value: 0.5, min: 0, max: 1)

Solution

1. Step 1: Check property names

   The Slider widget uses value, min, max, and onChanged properties exactly as in Slider(value: 0.5, min: 0, max: 1, onChanged: (v) => print(v)).

2. Step 2: Validate callback presence

   Slider(value: 0.5, min: 0, max: 1, onChanged: (v) => print(v)) includes a valid onChanged callback. Slider(value: 0.5, min: 0, max: 1) misses it, causing an error.

3. Final Answer:

   Slider(value: 0.5, min: 0, max: 1, onChanged: (v) => print(v)) -> Option A

4. Quick Check:

   Correct property names and onChanged callback = Slider(value: 0.5, min: 0, max: 1, onChanged: (v) => print(v)) [OK]

Hint: Use exact property names: value, min, max, onChanged [OK]

Common Mistakes:

• Using wrong property names like minimum or onChange
• Omitting onChanged callback causing errors
• Using val instead of value

3. What will be printed when the user moves the slider thumb to the middle position in this code?

double sliderValue = 0.0;
Slider(
  value: sliderValue,
  min: 0,
  max: 10,
  divisions: 10,
  label: (value) => value.round().toString(),
  onChanged: (double value) {
    print(value);
  },
)

medium

A. 5.0

B. 0.5

C. 10.0

D. 1.0

Solution

1. Step 1: Understand slider range and divisions

   The slider ranges from 0 to 10 with 10 divisions, so each step is 1 unit.

2. Step 2: Calculate middle position value

   The middle position is halfway, so value = (max + min) / 2 = (10 + 0) / 2 = 5.0.

3. Final Answer:

   5.0 -> Option A

4. Quick Check:

   Middle of 0 to 10 with 10 steps = 5.0 [OK]

Hint: Middle value = (min + max) / 2 with divisions [OK]

Common Mistakes:

• Confusing divisions with decimal steps
• Using 0.5 instead of 5.0 for middle
• Ignoring divisions and assuming continuous values

4. Identify the error in this Slider code snippet:

Slider(
  value: 5,
  min: 0,
  max: 10,
  onChanged: null,
)

medium

A. min must be greater than max

B. value must be a string, not a number

C. onChanged is null, making the slider non-interactive

D. Slider requires a label property

Solution

1. Step 1: Check onChanged property requirement

   The Slider widget requires a non-null onChanged callback to be interactive. Setting it to null disables the slider.

2. Step 2: Validate other properties

   Value is correctly a number within min and max. Label is optional.

3. Final Answer:

   onChanged is null, making the slider non-interactive -> Option C

4. Quick Check:

   onChanged null = disabled slider [OK]

Hint: Always provide a non-null onChanged callback [OK]

Common Mistakes:

• Setting onChanged to null disables slider
• Confusing value type as string
• Thinking label is mandatory

5. You want a Slider that lets users pick only whole numbers from 1 to 5 and shows the current value as a label. Which code snippet correctly implements this?

hard

A. Slider(value: 3, min: 1, max: 5, label: (v) => v.round().toString(), onChanged: (v) => print(v))

B. Slider(value: 3, min: 1, max: 5, divisions: 5, label: (v) => v.round().toString(), onChanged: (v) => print(v))

C. Slider(value: 3, min: 0, max: 5, divisions: 5, label: (v) => v.round().toString(), onChanged: (v) => print(v))

D. Slider(value: 3, min: 1, max: 5, divisions: 4, label: (v) => v.round().toString(), onChanged: (v) => print(v))

Solution

1. Step 1: Set min, max, and divisions for whole numbers

   To allow whole numbers from 1 to 5, min=1, max=5, and divisions=4 (steps between 1 and 5) are correct.

2. Step 2: Include label and onChanged callback

   Label shows current value, and onChanged is required for interactivity.

3. Final Answer:

   Slider(value: 3, min: 1, max: 5, divisions: 4, label: (v) => v.round().toString(), onChanged: (v) => print(v)) -> Option D

4. Quick Check:

   Divisions = max - min = 4 for whole steps [OK]

Hint: Divisions = max - min for whole number steps [OK]

Common Mistakes:

• Setting divisions equal to max instead of max-min
• Omitting divisions for discrete steps
• Setting min to 0 when range starts at 1