GridView.builder in Flutter - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ ui_behavior

intermediate

What is the output of this GridView.builder code?

Consider this Flutter code snippet that builds a grid of colored squares. What will you see on the screen?

Flutter

GridView.builder(
  gridDelegate: SliverGridDelegateWithFixedCrossAxisCount(crossAxisCount: 3),
  itemCount: 6,
  itemBuilder: (context, index) {
    return Container(
      color: index % 2 == 0 ? Colors.red : Colors.blue,
      margin: EdgeInsets.all(4),
    );
  },
)

A A single column list of 6 red squares.

B A grid with 2 columns and 3 rows, all squares red.

C A grid with 3 columns and 2 rows, alternating red and blue squares starting with red.

D A grid with 3 columns and 2 rows, all squares blue.

💡 Hint

Look at the crossAxisCount and the color condition using index % 2 == 0.

The crossAxisCount: 3 means 3 columns. With 6 items, it forms 2 rows. The color alternates red for even indexes and blue for odd indexes, starting with red at index 0.

❓ lifecycle

intermediate

When does the itemBuilder function get called in GridView.builder?

In Flutter's GridView.builder, when is the itemBuilder function called?

A Every time an item scrolls into view, to build that item widget lazily.

B After the grid is fully rendered, to update all items.

C Only for the first visible row of items.

D Only once for all items when the grid is first built.

💡 Hint

Think about how Flutter optimizes building large lists or grids.

GridView.builder builds items lazily. It calls itemBuilder only for items that are visible or about to be visible, improving performance.

📝 Syntax

advanced

Which option correctly sets a fixed aspect ratio for grid items?

You want each grid item to have a width twice its height. Which gridDelegate code achieves this?

A SliverGridDelegateWithFixedCrossAxisCount(crossAxisCount: 2, childAspectRatio: 2.0)

B SliverGridDelegateWithFixedCrossAxisCount(crossAxisCount: 2, childAspectRatio: 0.5)

C SliverGridDelegateWithMaxCrossAxisExtent(maxCrossAxisExtent: 100, childAspectRatio: 0.5)

D SliverGridDelegateWithMaxCrossAxisExtent(maxCrossAxisExtent: 100, childAspectRatio: 2.0)

💡 Hint

Remember childAspectRatio is width divided by height.

Setting childAspectRatio: 2.0 means width is twice the height. Using SliverGridDelegateWithFixedCrossAxisCount with this ratio fixes the aspect ratio for all items.

🔧 Debug

advanced

Why does this GridView.builder cause a runtime error?

Look at this code snippet:

GridView.builder(
  gridDelegate: SliverGridDelegateWithFixedCrossAxisCount(crossAxisCount: 3),
  itemCount: 5,
  itemBuilder: (context, index) {
    if (index == 4) return null;
    return Container(color: Colors.green);
  },
)

Why does it cause an error?

A Because <code>crossAxisCount</code> must be equal to <code>itemCount</code>.

B Because <code>itemBuilder</code> returned null, which is not allowed.

C Because <code>itemCount</code> is less than the number of items built.

D Because <code>Container</code> cannot be used inside <code>GridView.builder</code>.

💡 Hint

What does itemBuilder expect to return?

itemBuilder must always return a non-null widget. Returning null causes a runtime error.

🧠 Conceptual

expert

How does GridView.builder improve performance compared to GridView.count?

Choose the best explanation for why GridView.builder is more efficient than GridView.count for large data sets.

A <code>GridView.builder</code> disables scrolling by default, improving speed.

B <code>GridView.builder</code> uses less memory by caching all items, <code>GridView.count</code> does not cache.

C <code>GridView.builder</code> automatically paginates data, <code>GridView.count</code> does not.

D <code>GridView.builder</code> builds only visible items lazily, while <code>GridView.count</code> builds all items at once.

💡 Hint

Think about how many widgets are created initially.

GridView.builder creates widgets only for items visible on screen, saving CPU and memory. GridView.count builds all widgets at once, which can be slow for many items.

Practice

1. What is the main purpose of GridView.builder in Flutter?

easy

A. To create a grid with static images only

B. To create a fixed grid with all items built at once

C. To create a list view with fixed height items

D. To create a scrollable grid that builds items only when needed

Solution

1. Step 1: Understand GridView.builder behavior

   GridView.builder builds grid items lazily, meaning it creates items only when they scroll into view.

2. Step 2: Compare with other grid types

   Other grid types like GridView.count build all items at once, which is less efficient for large lists.

3. Final Answer:

   To create a scrollable grid that builds items only when needed -> Option D

4. Quick Check:

   GridView.builder = lazy building [OK]

Hint: Remember builder means items build on demand [OK]

Common Mistakes:

• Confusing GridView.builder with GridView.count
• Thinking it builds all items at once
• Assuming it only works with fixed item counts

2. Which of the following is the correct way to specify the number of columns in GridView.builder?

easy

A. gridDelegate: SliverGridDelegateWithFixedCrossAxisCount(crossAxisCount: 3)

B. columns: 3

C. crossAxisCount: 3

D. gridColumns: 3

Solution

1. Step 1: Identify gridDelegate usage

   GridView.builder requires a gridDelegate to control layout, such as SliverGridDelegateWithFixedCrossAxisCount.

2. Step 2: Check correct property for columns

   The crossAxisCount property inside SliverGridDelegateWithFixedCrossAxisCount sets the number of columns.

3. Final Answer:

   gridDelegate: SliverGridDelegateWithFixedCrossAxisCount(crossAxisCount: 3) -> Option A

4. Quick Check:

   Columns set via gridDelegate.crossAxisCount [OK]

Hint: Use gridDelegate with crossAxisCount for columns [OK]

Common Mistakes:

• Trying to set columns directly on GridView.builder
• Using invalid property names like columns or gridColumns
• Omitting gridDelegate entirely

3. What will be the output of this Flutter code snippet?

GridView.builder(
  gridDelegate: SliverGridDelegateWithFixedCrossAxisCount(crossAxisCount: 2),
  itemCount: 4,
  itemBuilder: (context, index) {
    return Text('Item $index');
  },
)

medium

A. A grid with 2 columns and 4 items labeled Item 0 to Item 3

B. A list with 4 items labeled Item 0 to Item 3

C. A grid with 4 columns and 2 items labeled Item 0 to Item 1

D. An error because itemBuilder returns Text widgets

Solution

1. Step 1: Analyze gridDelegate and itemCount

   The gridDelegate sets 2 columns, and itemCount is 4, so 4 items will be arranged in 2 columns.

2. Step 2: Understand itemBuilder output

   Each itemBuilder call returns a Text widget labeled with the index, from 0 to 3.

3. Final Answer:

   A grid with 2 columns and 4 items labeled Item 0 to Item 3 -> Option A

4. Quick Check:

   2 columns, 4 items, Text widgets labeled [OK]

Hint: Check crossAxisCount and itemCount carefully [OK]

Common Mistakes:

• Confusing columns with itemCount
• Thinking itemBuilder cannot return Text widgets
• Assuming a list instead of a grid

4. Identify the error in this GridView.builder code:

GridView.builder(
  gridDelegate: SliverGridDelegateWithFixedCrossAxisCount(crossAxisCount: 3),
  itemBuilder: (context, index) {
    return Container(color: Colors.blue);
  },
)

medium

A. itemBuilder returns Container instead of Text

B. Missing itemCount property causing infinite items

C. gridDelegate is incorrectly defined

D. crossAxisCount cannot be 3

Solution

1. Step 1: Check required properties of GridView.builder

   GridView.builder requires an itemCount to limit the number of items; without it, it builds infinite items.

2. Step 2: Verify other properties

   gridDelegate and itemBuilder are correctly defined; returning Container is valid.

3. Final Answer:

   Missing itemCount property causing infinite items -> Option B

4. Quick Check:

   itemCount missing = infinite build [OK]

Hint: Always include itemCount to avoid infinite items [OK]

Common Mistakes:

• Forgetting itemCount in GridView.builder
• Thinking itemBuilder must return Text only
• Misunderstanding crossAxisCount limits

5. You want to create a grid that shows only even numbers from 0 to 10 using GridView.builder. Which code snippet correctly implements this?

hard

A. GridView.builder(
gridDelegate: SliverGridDelegateWithFixedCrossAxisCount(crossAxisCount: 2),
itemCount: 5,
itemBuilder: (context, index) {
return Text('Number $index');
},
)

B. GridView.builder(
gridDelegate: SliverGridDelegateWithFixedCrossAxisCount(crossAxisCount: 2),
itemCount: 11,
itemBuilder: (context, index) {
if (index % 2 == 0) return Text('Number $index');
else return Container();
},
)

C. GridView.builder(
gridDelegate: SliverGridDelegateWithFixedCrossAxisCount(crossAxisCount: 2),
itemCount: 6,
itemBuilder: (context, index) {
return Text('Number ${index * 2}');
},
)

D. GridView.builder(
gridDelegate: SliverGridDelegateWithFixedCrossAxisCount(crossAxisCount: 2),
itemCount: 6,
itemBuilder: (context, index) {
return Text('Number $index');
},
)

Solution

1. Step 1: Determine how many even numbers from 0 to 10

   Even numbers are 0, 2, 4, 6, 8, 10 -- total 6 numbers.

2. Step 2: Check itemCount and itemBuilder logic

   GridView.builder( gridDelegate: SliverGridDelegateWithFixedCrossAxisCount(crossAxisCount: 2), itemCount: 6, itemBuilder: (context, index) { return Text('Number ${index * 2}'); }, ) uses itemCount 6 and multiplies index by 2 to get even numbers correctly.

3. Step 3: Analyze other options

   GridView.builder( gridDelegate: SliverGridDelegateWithFixedCrossAxisCount(crossAxisCount: 2), itemCount: 11, itemBuilder: (context, index) { if (index % 2 == 0) return Text('Number $index'); else return Container(); }, ) creates empty containers for odd indexes, wasting space. The other options do not correctly generate even numbers.

4. Final Answer:

   GridView.builder with itemCount: 6 and Text('Number ${index * 2}') -> Option C

5. Quick Check:

   Use index * 2 with correct itemCount [OK]

Hint: Multiply index by 2 to get even numbers [OK]

Common Mistakes:

• Using itemCount 11 and skipping odd items wastes space
• Not adjusting index to generate even numbers
• Returning empty containers for unwanted items