The build method
- Flutter course
- Beginner
- Lesson 17/99
- 4 min
Learn what build returns and when it is called.
The build method
A StatelessWidget's build method produces the widget's appearance. It takes a BuildContext parameter and returns a Widget. Flutter calls build when the widget tree needs redrawing. build should be pure: no side effects, depending only on parameters.
@override
Widget build(BuildContext context) {
return Text('Merhaba');
}When is build called?
build is called when the widget is first placed on screen and when its dependencies (Theme, MediaQuery) change. When a parent rebuilds, the child's build is called again too. So build must stay cheap; long tasks should not run inside build.
// build should depend on parameters
@override
Widget build(BuildContext context) {
final theme = Theme.of(context);
return Text('Merhaba', style: theme.textTheme.bodyLarge);
}Practice Print build info
In main, print two lines with print: build donus Widget and build saf fonksiyon.
Starter code
void main() {
// iki satır yazdır:
// build donus Widget
// build saf fonksiyon
}Sample solution
void main() {
print('build donus Widget');
print('build saf fonksiyon');
}Expected output
build donus Widget build saf fonksiyon
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 What does the build method return?
- int
- void
- String
- Widget Correct answer
Explanation: The build method returns a value of type Widget; this widget is drawn to the screen.
2 Return inside build
Widget build(BuildContext context) {
____ Text('Selam');
}Widget build(BuildContext context) {
return Text('Selam');
}Explanation: The build method returns a Widget with the return keyword.
3 build method concepts
| Term | Meaning |
|---|---|
build |
Method returning a Widget |
Widget |
Return type of build |
return |
Keyword yielding a value |
pure function |
Function without side effects |