CODEWITHSUNDEEP
flutteroopdart
Sai Ram
Sai Ram

Reputation: 4392

what's the use of `State<T extends StatefulWidget>`

I looked at the docs of dart for generics. 

abstract class StringCache {
  String getByKey(String key);
  void setByKey(String key, String value);
}

abstract class ObjectCache {
  Object getByKey(String key);
  void setByKey(String key, Object value);
}

The above two is replaceed by one single generic type T with below code 

abstract class Cache<T> {
  T getByKey(String key);
  void setByKey(String key, T value);
}

Where the use of T is seen clearly. but not sure where the state class uses 

class _CounterState extends State<Counter> {
  int _counter = 0;

  void _increment() {
    setState(() {
      _counter++;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Row(
      children: <Widget>[
        RaisedButton(
          onPressed: _increment,
          child: Text('Increment'),
        ),
        Text('Count: $_counter'),
      ],
    );
  }
}

Upvotes: 1

Views: 1295

Answers (1)

G&#252;nter Z&#246;chbauer
G&#252;nter Z&#246;chbauer

Reputation: 658067

T is a generic type parameter and extends StatefulWidget is a constraint for what types T can be set to.

With 

class _CounterState extends State<Counter> {

T is set to type Counter (which has to be a StatefulWidget).

Passing Counter as type allows you to use

widget.foo

to reference field foo in Counter from _CounterState and you get autocompletion and static type checking.

Upvotes: 3

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