Oops! I Got Flutter Keys Wrong – Are You Making the Same Mistakes?

Oops! I Got Flutter Keys Wrong – Are You Making the Same Mistakes?

This article discusses common mistakes when using Flutter keys and provides solutions for using ValueKey, UniqueKey, and PageStorageKey correctly. Avoid debugging headaches by understanding the proper use of each key type.

Ever wondered why there is always a key parameter in every widget you create in Flutter?

At first, I ignored it too – thinking it wasn’t important.

It took me months (and lots of bugs) to realize the real cause: I was misusing or ignoring Flutter keys.

Today, I want to share some key points I wish I knew earlier.

What are Keys, Really?

In Flutter, keys tell the framework which widgets are related to which old widgets during a rebuild.

Without keys, Flutter guesses. With keys, Flutter knows exactly which widget to keep, update, move, or discard.

Each type of key has its specific use case, and using the wrong one can lead to unexpected behavior. This is what tripped me up – I often used keys that seemed similar, only to end up in a loop of errors and chaos.

1. ValueKey

A ValueKey uses a specific value (like an ID or name) to uniquely identify a widget.

My Mistake:

ListView(
 children: items.map((item) =>
 ListTile(title: Text(item.name))
 ).toList(),
);

When an item was added or removed, Flutter couldn’t correctly match the widget to its data, causing mismatched information to appear on the screen.

Solution:

By adding ValueKey(item.id), Flutter was able to match each widget to its correct data, ensuring proper updates even with dynamic changes.

ListView(
 children: items.map((item) =>
 ListTile(key: ValueKey(item.id), title: Text(item.name))
 ).toList(),
);

2. UniqueKey

A UniqueKey generates a new, unique key every time it’s used, forcing Flutter to treat the widget as fresh each time.

My Mistake:

AnimatedList(
 key: _listKey,
 initialItemCount: _items.length,
 itemBuilder: (context, index, animation) {
 return FadeTransition(
 opacity: animation,
 child: ListTile(
 title: Text(_items[index]),
 ),
 );
 },
);

In the above example we are applying animation to the list tile widget. As Flutter reused the same widgets during the animation, this can led to flickering or layout glitches when the items were updated.

Solution:

By adding UniqueKey(), the developer ensured that each widget was treated as a new widget. Here's a complete example with an animation to demonstrate the fix:

AnimatedList(
 key: _listKey,
 initialItemCount: _items.length,
 itemBuilder: (context, index, animation) {
 return FadeTransition(
 opacity: animation,
 child: ListTile(
 key: UniqueKey(), // Ensuring the widget is treated as unique
 title: Text(_items[index]),
 ),
 );
 },
);

3. PageStorageKey

PageStorageKey preserves the state of widgets (like scroll position) when navigating between pages or tabs.

My Mistake:

ListView.builder(
 itemBuilder: (context, index) => ListTile(title: Text('Item $index')),
);

Switching tabs caused the list to reset its scroll position to the top.

Solution:

By using PageStorageKey, we ensured that the scroll position was saved and restored when switching tabs.

ListView.builder(
 key: PageStorageKey('pagekey'),
 itemBuilder: (context, index) => ListTile(title: Text('Item $index')),
);

Conclusion

• ValueKey: Ideal for distinguishing widgets with the same type or structure but different data, ensuring correct updates and maintaining state.
• UniqueKey: Useful for forcing Flutter to treat a widget as unique, ensuring it is recreated every time, preventing UI issues during dynamic updates or animations.
• PageStorageKey: Helps in saving the state of a widget across page transitions, particularly useful for preserving scroll positions in lists or other stateful widgets.

Got a key bug disaster or a “genius” Flutter tip? Drop it below!

If you somehow liked this, hit clap and follow – because, obviously, that’s what keeps me going. 😆.