CustomScrollView in Flutter - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a CustomScrollView in Flutter?

A CustomScrollView is a scrollable widget that lets you create custom scroll effects by combining multiple scrollable elements called slivers.

beginner

What is a sliver in the context of CustomScrollView?

A sliver is a portion of a scrollable area that can have custom behavior or layout, like a list, grid, or app bar that reacts to scrolling.

beginner

Name two common slivers used inside a CustomScrollView.

Two common slivers are SliverAppBar (a scrollable app bar) and SliverList (a scrollable list of items).

intermediate

How does CustomScrollView improve UI design compared to a regular ListView?

CustomScrollView allows combining different scrollable elements with custom effects, like collapsing headers or mixed lists and grids, which ListView alone cannot do.

beginner

What property do you use to add slivers inside a CustomScrollView?

You use the slivers property, which takes a list of sliver widgets to build the scrollable content.

Which widget is used to create a scrollable area with custom scroll effects in Flutter?

A CustomScrollView

B ListView

C GridView

D SingleChildScrollView

CustomScrollView allows combining multiple slivers for custom scroll effects.

What does a sliver represent inside a CustomScrollView?

A A scrollable portion with custom layout or behavior

B A button widget

C A fixed header

D A background image

Slivers are scrollable parts that can have special layouts or behaviors.

Which sliver widget provides a scrollable app bar that can expand or collapse?

A SliverList

B SliverGrid

C SliverAppBar

D SliverFillRemaining

SliverAppBar is used for scrollable app bars.

How do you add multiple slivers inside a CustomScrollView?

A Using the children property

B Using the slivers property

C Using the items property

D Using the widgets property

The slivers property takes a list of sliver widgets.

Which of these is NOT a sliver widget?

A SliverList

B SliverGrid

C SliverAppBar

D ListView

ListView is a scrollable widget but not a sliver.

Explain what a CustomScrollView is and why you might use it in a Flutter app.

Think about combining different scrollable parts with special effects.

Describe how you would build a screen with a collapsing app bar and a scrollable list using CustomScrollView.

Focus on which slivers to use and how they work together.

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