What is StatefulWidget?

Learn that StatefulWidget is used for UI that changes over time.

What is StatefulWidget?

A StatefulWidget is used for interfaces that can change over time. Unlike StatelessWidget, it stores variable data inside a State object. Views that update with user interaction — counters, checkboxes, forms — are written with StatefulWidget. The widget itself stays immutable, but the State object is mutable.

class Sayac extends StatefulWidget {
  const Sayac({super.key});

  @override
  State<Sayac> createState() => _SayacState();
}

class _SayacState extends State<Sayac> {
  int deger = 0;
  // ...
}

Widget and State

A StatefulWidget consists of two parts: an immutable Widget and a mutable State. The Widget stays the same across rebuilds; the State holds variables updated via setState. Even if the widget is replaced, Flutter preserves the State to prevent data loss.

// Widget: immutable
class Sayac extends StatefulWidget {
  const Sayac({super.key});
  @override
  State<Sayac> createState() => _SayacState();
}

// State: mutable
class _SayacState extends State<Sayac> {
  int deger = 0;  // can change
}

Exercise Print stateful info

In main, print two lines with print: stateful degisir and state mutable.

Show example solution

void main() {
  print('stateful degisir');
  print('state mutable');
}

Test yourself

When is StatefulWidget used?
- Only when displaying text
- When no data will ever come from a server
- When the view never changes
- When there is data that changes with user interaction
Show answer

When there is data that changes with user interaction — StatefulWidget is used for data that changes with user interaction (counter, checkbox, form); StatelessWidget is for static views.

State class definition

class _SayacState extends ____&lt;Sayac&gt; {
  int deger = 0;
}

Show answer

class _SayacState extends State&lt;Sayac&gt; {
  int deger = 0;
}

StatefulWidget concepts

StatefulWidget

For UIs that change over time
mutable

State object can change
State

Object holding variable data
immutable Widget

Stays the same across rebuilds

Next lesson: createState and State

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