Dismissible for swipe actions in Flutter - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ ui_behavior

intermediate

What happens when you swipe a Dismissible widget?

Consider a Flutter app with a Dismissible widget wrapping a ListTile. What is the default behavior when the user swipes the item from right to left?

Flutter

Dismissible(
  key: Key('item1'),
  child: ListTile(title: Text('Swipe me')),
  onDismissed: (direction) {
    print('Item dismissed');
  },
)

A Nothing happens; the swipe gesture is ignored.

B The item is removed from the list and onDismissed callback is called.

C The item is highlighted but remains visible; onDismissed is not called.

D The app crashes because Dismissible requires a background widget.

💡 Hint

Think about what Dismissible is designed to do when swiped.

The Dismissible widget detects swipe gestures and removes the widget from the tree, calling the onDismissed callback to notify the app.

📝 Syntax

intermediate

Which code correctly implements a Dismissible with a red background on swipe?

You want to show a red background behind the item when swiped. Which code snippet correctly sets this up?

A

Dismissible(
  key: Key('item2'),
  background: Container(color: Colors.red),
  child: ListTile(title: Text('Swipe me')),
)

B

Dismissible(
  key: Key('item2'),
  background: Colors.red,
  child: ListTile(title: Text('Swipe me')),
  onDismissed: (_) {},
)

C

)
,}{ )_( :dessimsiDno  
,))'em epiwS'(txeT :eltit(eliTtsiL :dlihc  
,)der.sroloC :roloc(reniatnoC :dnuorgkcab  
,)'2meti'(yeK :yek  
(elbissimsiD

D

Dismissible(
  key: Key('item2'),
  background: Container(color: Colors.red),
  child: ListTile(title: Text('Swipe me')),
  onDismissed: (_) {},
)

💡 Hint

The background property expects a widget, not a color directly.

The background property must be a widget, such as a Container with a color. Option D passes a color directly, which is invalid. Option D uses a string instead of a color constant. Option D misses the required onDismissed callback.

❓ lifecycle

advanced

What happens if you reuse the same Key for multiple Dismissible widgets?

In a list of Dismissible widgets, what is the effect of assigning the same Key to more than one Dismissible?

A Flutter throws an exception because keys must be unique among siblings.

B The widgets work fine; keys are optional for Dismissible.

C Only the first Dismissible responds to swipe; others ignore gestures.

D The app silently ignores duplicate keys and behaves normally.

💡 Hint

Keys help Flutter identify widgets uniquely in the widget tree.

Keys must be unique among sibling widgets to avoid conflicts. Reusing keys causes Flutter to throw an exception during build.

❓ navigation

advanced

How to navigate after dismissing an item in a Dismissible?

You want to navigate to a new screen after an item is dismissed. Where should you place the navigation code?

A Inside the build method of the Dismissible widget.

B Inside the background widget of the Dismissible.

C Inside the onDismissed callback after removing the item from the list.

D Inside the initState method of the parent widget.

💡 Hint

Think about when the dismissal action completes.

The onDismissed callback is called after the swipe completes and the item is removed. This is the right place to trigger navigation.

🔧 Debug

expert

Why does the Dismissible widget not dismiss when swiped?

You have this code but swiping the item does not remove it or call onDismissed: Dismissible( key: Key('item3'), child: ListTile(title: Text('Swipe me')), onDismissed: (direction) { print('Dismissed'); }, ) What is the most likely cause?

A The parent list is not updating its data source to remove the dismissed item.

B The key is missing from the Dismissible widget.

C The onDismissed callback is empty and does nothing.

D The child widget is not a ListTile.

💡 Hint

Dismissible removes the widget visually but the list must update to reflect the change.

Dismissible only removes the widget from the UI temporarily. The underlying data list must be updated to remove the item; otherwise, the widget will reappear on rebuild.