CustomScrollView in Flutter - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ ui_behavior

intermediate

What is the visible output of this CustomScrollView code?

Consider this Flutter code snippet using CustomScrollView. What will the user see on the screen?

Flutter

CustomScrollView(
  slivers: [
    SliverAppBar(
      expandedHeight: 150,
      flexibleSpace: FlexibleSpaceBar(
        title: Text('Hello'),
      ),
      pinned: true,
    ),
    SliverList(
      delegate: SliverChildBuilderDelegate(
        (context, index) => ListTile(title: Text('Item $index')),
        childCount: 3,
      ),
    ),
  ],
)

A A scrollable list with a pinned app bar titled 'Hello' that stays visible on top while scrolling through 3 list items.

B A static app bar with title 'Hello' and a non-scrollable list of 3 items below it.

C An error because SliverAppBar cannot be used inside CustomScrollView.

D A scrollable list with no app bar visible and 3 list items.

💡 Hint

Remember that SliverAppBar with pinned:true stays visible when scrolling.

The SliverAppBar with pinned:true stays fixed at the top while the SliverList scrolls beneath it. The FlexibleSpaceBar shows the title 'Hello'.

❓ lifecycle

intermediate

What happens when you scroll up in this CustomScrollView?

Given this CustomScrollView with a SliverAppBar that has floating: true and snap: true, what is the behavior when the user scrolls up slightly?

Flutter

CustomScrollView(
  slivers: [
    SliverAppBar(
      floating: true,
      snap: true,
      expandedHeight: 100,
      flexibleSpace: FlexibleSpaceBar(title: Text('Snap AppBar')),
    ),
    SliverList(
      delegate: SliverChildListDelegate(List.generate(10, (i) => ListTile(title: Text('Item $i')))),
    ),
  ],
)

A The app bar expands and collapses smoothly without snapping.

B The app bar stays hidden until the user scrolls to the top of the list.

C The app bar immediately snaps open and becomes fully visible when the user scrolls up slightly.

D The app bar disappears permanently after scrolling.

💡 Hint

Floating and snap together cause the app bar to appear quickly on scroll up.

With floating:true and snap:true, the app bar snaps fully open immediately when the user scrolls up even a little.

📝 Syntax

advanced

Which option correctly creates a CustomScrollView with a SliverGrid?

Select the code snippet that correctly builds a CustomScrollView containing a SliverGrid with 4 items arranged in 2 columns.

A CustomScrollView(children: [SliverGrid(delegate: SliverChildBuilderDelegate((context, index) => Text('Item $index'), childCount: 4), gridDelegate: SliverGridDelegateWithFixedCrossAxisCount(crossAxisCount: 2))])

B CustomScrollView(slivers: [SliverGrid(delegate: SliverChildBuilderDelegate((context, index) => Text('Item $index'), childCount: 4), gridDelegate: SliverGridDelegateWithFixedCrossAxisCount(crossAxisCount: 2))])

C CustomScrollView(slivers: [SliverGrid(delegate: SliverChildListDelegate([Text('Item 0'), Text('Item 1'), Text('Item 2'), Text('Item 3')]), gridDelegate: SliverGridDelegateWithFixedCrossAxisCount(crossAxisCount: 2))])

D CustomScrollView(slivers: [SliverGrid(delegate: SliverChildBuilderDelegate((context, index) => Text('Item $index')), gridDelegate: SliverGridDelegateWithFixedCrossAxisCount(crossAxisCount: 4))])

💡 Hint

CustomScrollView uses the slivers parameter, not children.

Option B correctly uses slivers with SliverGrid, SliverChildBuilderDelegate with childCount 4, and gridDelegate with 2 columns.

🔧 Debug

advanced

Why does this CustomScrollView code cause a runtime error?

This code throws an error when run. What is the cause?

Flutter

CustomScrollView(
  slivers: [
    SliverList(
      delegate: SliverChildBuilderDelegate(
        (context, index) => ListTile(title: Text('Item $index')),
        childCount: 5,
      ),
    ),
    Container(height: 100, color: Colors.red),
  ],
)

A A Container widget cannot be a direct child of slivers; only sliver widgets are allowed.

B CustomScrollView requires a ScrollController to be set explicitly.

C SliverList must be the last sliver in the list.

D SliverChildBuilderDelegate requires a childCount of zero or null to work properly.

💡 Hint

Check what types of widgets are allowed inside the slivers list.

CustomScrollView's slivers list only accepts sliver widgets. Container is not a sliver and causes a runtime error.

🧠 Conceptual

expert

How many items are visible initially in this CustomScrollView with SliverFixedExtentList?

Given this code, how many list items will be visible on the screen initially if the screen height is 600 pixels?

Flutter

CustomScrollView(
  slivers: [
    SliverFixedExtentList(
      itemExtent: 120,
      delegate: SliverChildBuilderDelegate(
        (context, index) => ListTile(title: Text('Item $index')),
        childCount: 10,
      ),
    ),
  ],
)

A 4 items

B 10 items

C 6 items

D 5 items

💡 Hint

Divide screen height by itemExtent to find visible items count.

Each item is 120 pixels tall. 600 / 120 = 5 items fit exactly on screen initially.

Practice

1. What is the main purpose of a CustomScrollView in Flutter?

easy

A. To combine multiple scrollable widgets called slivers into one scrollable area.

B. To create a fixed header without scrolling.

C. To display only a single scrollable list.

D. To handle user input events like taps and swipes.

Solution

1. Step 1: Understand the role of CustomScrollView

   CustomScrollView allows combining different scrollable widgets called slivers into one scrollable area.

2. Step 2: Compare options with this understanding

   Only To combine multiple scrollable widgets called slivers into one scrollable area. correctly describes this purpose; others describe unrelated features.

3. Final Answer:

   To combine multiple scrollable widgets called slivers into one scrollable area. -> Option A

4. Quick Check:

   CustomScrollView = combine slivers [OK]

Hint: Remember: CustomScrollView = multiple slivers scrolling together [OK]

Common Mistakes:

• Thinking CustomScrollView only shows one list
• Confusing CustomScrollView with fixed headers
• Assuming it handles input events directly

2. Which of the following is the correct way to start a CustomScrollView with a sliver list in Flutter?

easy

A. CustomScrollView(children: [SliverList(delegate: SliverChildBuilderDelegate((context, index) => Text('Item $index'), childCount: 10))])

B. CustomScrollView(slivers: [SliverList(delegate: SliverChildBuilderDelegate((context, index) => Text('Item $index'), childCount: 10))])

C. CustomScrollView(children: [ListView.builder(itemCount: 10, itemBuilder: (context, index) => Text('Item $index'))])

D. CustomScrollView(slivers: [ListView.builder(itemCount: 10, itemBuilder: (context, index) => Text('Item $index'))])

Solution

1. Step 1: Check CustomScrollView property for slivers

   CustomScrollView uses the slivers property to accept sliver widgets, not children.

2. Step 2: Identify correct sliver list syntax

   SliverList requires a delegate like SliverChildBuilderDelegate to build items lazily.

3. Final Answer:

   CustomScrollView(slivers: [SliverList(delegate: SliverChildBuilderDelegate((context, index) => Text('Item $index'), childCount: 10))]) -> Option B

4. Quick Check:

   Use slivers property with SliverList [OK]

Hint: Use slivers: [] property, not children: [] for CustomScrollView [OK]

Common Mistakes:

• Using children instead of slivers property
• Putting ListView inside CustomScrollView slivers
• Not using SliverChildBuilderDelegate for SliverList

3. What will be the visible output when running this Flutter code snippet?

CustomScrollView(
  slivers: [
    SliverAppBar(
      expandedHeight: 100,
      flexibleSpace: FlexibleSpaceBar(title: Text('Header')),
      pinned: true,
    ),
    SliverList(
      delegate: SliverChildListDelegate([
        Container(height: 50, color: Colors.red),
        Container(height: 50, color: Colors.green),
        Container(height: 50, color: Colors.blue),
      ]),
    ),
  ],
)

medium

A. An error because SliverAppBar cannot be used inside CustomScrollView.

B. A static screen with only the header 'Header' visible and no scrolling.

C. A scrollable screen with three colored boxes but no header visible.

D. A scrollable screen with a pinned header titled 'Header' and three colored boxes stacked vertically.

Solution

1. Step 1: Understand SliverAppBar behavior

   SliverAppBar with pinned: true stays visible at top when scrolling.

2. Step 2: Analyze SliverList content

   SliverList shows three containers stacked vertically with specified colors and heights.

3. Final Answer:

   A scrollable screen with a pinned header titled 'Header' and three colored boxes stacked vertically. -> Option D

4. Quick Check:

   SliverAppBar pinned + SliverList visible = A scrollable screen with a pinned header titled 'Header' and three colored boxes stacked vertically. [OK]

Hint: Pinned SliverAppBar stays visible while scrolling content below [OK]

Common Mistakes:

• Thinking header disappears on scroll
• Assuming no scrolling because of pinned header
• Believing SliverAppBar causes error inside CustomScrollView

4. Identify the error in this Flutter code using CustomScrollView:

CustomScrollView(
  children: [
    SliverList(
      delegate: SliverChildBuilderDelegate(
        (context, index) => Text('Item $index'),
        childCount: 5,
      ),
    ),
  ],
)

medium

A. SliverList cannot be used inside CustomScrollView.

B. Missing itemCount in SliverChildBuilderDelegate.

C. Using children instead of slivers property in CustomScrollView.

D. Text widget cannot be used inside SliverChildBuilderDelegate.

Solution

1. Step 1: Check CustomScrollView properties

   CustomScrollView expects sliver widgets inside the slivers property, not children.

2. Step 2: Verify other parts

   SliverChildBuilderDelegate has childCount, SliverList is valid, and Text is allowed inside delegate.

3. Final Answer:

   Using children instead of slivers property in CustomScrollView. -> Option C

4. Quick Check:

   CustomScrollView uses slivers, not children [OK]

Hint: CustomScrollView needs slivers: [], not children: [] [OK]

Common Mistakes:

• Using children property instead of slivers
• Confusing SliverList with ListView
• Thinking Text widget is invalid inside SliverChildBuilderDelegate

5. You want to create a scrollable screen with a header that shrinks on scroll, a grid of images, and a list of text items below. Which combination of slivers inside a CustomScrollView will achieve this?

hard

A. SliverAppBar (with flexibleSpace), SliverGrid, SliverList

B. SliverPersistentHeader, ListView, GridView

C. AppBar, GridView.builder, ListView.builder

D. SliverToBoxAdapter, SliverFixedExtentList, SliverFillRemaining

Solution

1. Step 1: Identify slivers for header, grid, and list

   SliverAppBar with flexibleSpace creates a shrinking header. SliverGrid shows a grid of items. SliverList shows a scrollable list.

2. Step 2: Check options for correct sliver usage

   SliverAppBar (with flexibleSpace), SliverGrid, SliverList uses all correct slivers inside CustomScrollView. Others mix non-sliver widgets or incorrect combinations.

3. Final Answer:

   SliverAppBar (with flexibleSpace), SliverGrid, SliverList -> Option A

4. Quick Check:

   Header + grid + list = SliverAppBar + SliverGrid + SliverList [OK]

Hint: Use SliverAppBar for header, SliverGrid for grid, SliverList for list [OK]

Common Mistakes:

• Using ListView/GridView inside CustomScrollView slivers
• Not using SliverAppBar for shrinking header
• Confusing SliverPersistentHeader with SliverAppBar