Passing parameters

Learn to pass data to widgets via constructor parameters.

Passing data via constructor

Data is passed into a StatelessWidget through constructor parameters. Parameters are usually declared final because the widget is immutable. The required keyword forces non-nullable parameters to be supplied.

class Etiket extends StatelessWidget {
  final String metin;
  const Etiket({super.key, required this.metin});

  @override
  Widget build(BuildContext context) {
    return Text(metin);
  }
}

final and required

final fields cannot be changed after the instance is created; this is the core of immutability. required forces named parameters to be supplied at the call site. Without required, a parameter becomes optional and may be null.

class Kart extends StatelessWidget {
  final String baslik;
  final String? altBaslik;  // optional

  const Kart({
    super.key,
    required this.baslik,
    this.altBaslik,
  });
}

Exercise Print parameter info

In main, print two lines with print: final sabit and required zorunlu.

Show example solution

void main() {
  print('final sabit');
  print('required zorunlu');
}

Test yourself

How is data passed to a StatelessWidget?
- Through setter methods
- Through global variables
- Through constructor parameters
- By defining inside the build method
Show answer

Through constructor parameters — Data is passed to a StatelessWidget via constructor parameters; stored as final fields.

Required parameter

const Etiket({
  super.key,
  ____ this.metin,
});

Show answer

const Etiket({
  super.key,
  required this.metin,
});

Parameter concepts

constructor

Method creating a class instance
final

Immutable field
required

Forces a parameter to be given
this.metin

Assigns parameter to field

Next lesson: const constructor

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