Pass method as parameter to a widget

There are a few predefined types that already exist.

VoidCallback

If you want to create a parameter something like this:

onPressed: () { },

Then you can define it in your class like so:

class MyWidget extends StatelessWidget {

  MyWidget({Key key, this.onPressed}) : super(key: key);

  final VoidCallback onPressed;

  // ...
}

Notes

The typedef is defined in the source code like this:

typedef VoidCallback = void Function();

The asynchronous version is AsyncCallback.

typedef AsyncCallback = Future<void> Function();

ValueSetter

If you want to create a parameter something like this:

onPressed: (value) { },

Then you can define it in your class like so:

class MyWidget extends StatelessWidget {

  MyWidget({Key key, this.onPressed}) : super(key: key);

  final ValueSetter<String> onPressed;

  // ...
}

Notes

The typedef is defined in the source code like this:

typedef ValueSetter<T> = void Function(T value);

If you want to specify that the function only gets called when there is a change then use ValueChanged instead.

typedef ValueChanged<T> = void Function(T value);

The asynchronous version is AsyncValueSetter.

typedef AsyncValueSetter<T> = Future<void> Function(T value);

ValueGetter

If you want to create a parameter something like this:

onPressed: () => value,

Then you can define it in your class like so:

class MyWidget extends StatelessWidget {

  MyWidget({Key key, this.onPressed}) : super(key: key);

  final ValueGetter<String> onPressed;

  // ...
}

Notes

The typedef is defined in the source code like this:

typedef ValueGetter<T> = T Function();

The asynchronous version is AsyncValueGetter.

typedef AsyncValueGetter<T> = Future<T> Function();

Define your own type

As you can see from all of the examples above, everything is just a typedef for a Function. So it is easy enough to make your own.

Say you want to do something like this:

onEvent: (context, child) => value,

Then you would make the typedef like this:

typedef MyEventCallback = int Function(BuildContext context, Widget widget);

And use it like this:

class MyWidget extends StatelessWidget {

  MyWidget({Key key, this.onEvent}) : super(key: key);

  final MyEventCallback onEvent;

  // ...
}

See the documentation for more.

Use the VoidCallback type, like this. Check the line comments on the code as well for more information:

class Button extends StatelessWidget {
  final String text;
  final VoidCallback callback; // Notice the variable type

  Button(this.text, this.callback);

  @override
  Widget build(BuildContext context) {
    return Container(
      height: 50,
      child: SizedBox(
        width: double.infinity,
        child: RaisedButton(
          onPressed: callback, // Simply put the function name here, DON'T use ()
          child: Padding(
              padding: EdgeInsets.symmetric(vertical: 13),
              child: Text(
                text,
                style: TextStyle(fontWeight: FontWeight.bold),
              )),
          color: COLOR_BLUE,
          textColor: Colors.white,
          shape:
              RoundedRectangleBorder(borderRadius: BorderRadius.circular(25)),
        ),
      ),
    );
  }
}