DART: How to pass an argument to a method that needs to be a const

user_4433208 0
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I'm refactoring a Flutter app for readability, and decided to reduce duplication by moving repeated calls to wrap a widget with Padding by extracting a method. The method looks like this:

Padding _wrapWithPadding(Widget widget, {double horizontal = 8.0, double vertical = 0.0}) {
  return const Padding(padding:
        EdgeInsets.symmetric(horizontal: horizontal, vertical: vertical),
    child: widget);
}

The Dart compiler complains that horizontal, vertical, and widget arguments are not const on the call to the Padding constructor. I understand the problem, but surely there is a way to accomplish removing the duplication of creating a Padding element over and over again?

Is there someway to get the compiler to treat those values as const, or is there another way to accomplish my goal?

dart flutter dry refactoring
2 Answers

If you want the padding to be constant, you should make sure that its child is created by const construction. I don't suggest that.

But alternatively, you can use extension methods

    extension WidgetExtension on Widget {
      Padding addPadding({double horizontal = 8.0, double vertical = 0.0}) {
        return Padding(
            padding:
                EdgeInsets.symmetric(horizontal: horizontal, vertical: vertical),
            child: this);
      }
    }

    Container().addPadding();
user_21053200

This is not feasible with a function.

You can, on the other hand, use a StatelessWidget.

class MyPadding extends StatelessWidget {
  const MyPadding(
    this.widget, {
    Key key,
    this.horizontal,
    this.vertical,
  }) : super(key: key);

  final Widget widget;
  final double horizontal;
  final double vertical;

  @override
  Widget build(BuildContext context) {
    return Padding(
      padding: EdgeInsets.symmetric(horizontal: horizontal ?? 8, vertical: vertical ?? .0),
      child: widget,
    );
  }
}
user_83942650
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