The build method

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);
}

Exercise Print build info

In main, print two lines with print: build donus Widget and build saf fonksiyon.

Show example solution

void main() {
  print('build donus Widget');
  print('build saf fonksiyon');
}

Test yourself

What does the build method return?
- int
- void
- String
- Widget
Show answer

Widget — The build method returns a value of type Widget; this widget is drawn to the screen.

Return inside build

Widget build(BuildContext context) {
  ____ Text('Selam');
}

Show answer

Widget build(BuildContext context) {
  return Text('Selam');
}

build method concepts

build

Method returning a Widget
Widget

Return type of build
return

Keyword yielding a value
pure function

Function without side effects

Next lesson: Passing parameters

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