const and ListView.builder
- Flutter course
- Expert
- Lesson 9/39
- 4 min
Learn the performance impact of const widgets and builders.
const widget
const widgets are created at compile time and are not recreated during rebuild. This avoids unnecessary build and paint work. const Text('Merhaba') uses the same object every frame.
const Text('Merhaba')
// same object every rebuildListView.builder
ListView.builder only builds the items visible on screen; for long lists it is much more efficient than ListView. It takes itemCount and itemBuilder parameters.
ListView.builder(
itemCount: 1000,
itemBuilder: (context, index) => Text('$index'),
)Practice Create a ListView.builder
In main, create a ListView.builder widget: itemCount: 10, itemBuilder: (context, i) => Text('$i'). Print the widget's runtimeType.
Starter code
import 'package:flutter/material.dart';
void main() {
// ListView.builder oluştur: itemCount 10, itemBuilder (context, i) => Text('$i')
// runtimeType yazdır
}Sample solution
import 'package:flutter/material.dart';
void main() {
final widget = ListView.builder(
itemCount: 10,
itemBuilder: (context, i) => Text('$i'),
);
print(widget.runtimeType);
}Expected output
ListView
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 When is a const widget created?
- Anew on every rebuild
- Never
- At compile time, reused on rebuild Correct answer
- Only at runtime
Explanation: A const widget is created once at compile time; during rebuild the same object is reused, so build and paint cost drops to zero.
2 Use ListView.builder
ListView.____(itemCount: 100, itemBuilder: (context, i) => Text('$i'))ListView.builder(itemCount: 100, itemBuilder: (context, i) => Text('$i'))Explanation: ListView.builder builds only the visible items; separated is for lists with dividers, custom for sliver-based lists.
3 const and ListView.builder matches
| Term | Meaning |
|---|---|
const |
Compile-time constant |
ListView |
List widget |
builder |
Builds visible items |
itemCount |
Total item count |