copyWith() method of ThemeData class in Dart-CodePudding

when trying to understand the functionality of copyWith() method in ThemeData class in Dart I created the function but when I use it it doesn't change the first value that I have passed before at the opposite of my expectation and I don't understand what I've missed in my code which causes this problem

My function:

enum Color {
  RED,
  BLUE,
  GREEN,
  YELLOW,
  BLACK,
}

enum Brightness {
  LIGHT,
  DARK,
}

class ThemeData {
  final Color accentColor;
  final Color backgroundColor;
  final Color buttonColor;
  final Brightness brightness;

  ThemeData(
      {this.accentColor = Color.BLACK,
      this.backgroundColor = Color.BLUE,
      this.brightness = Brightness.LIGHT,
      this.buttonColor = Color.RED});

  ThemeData copyWith(
      {Color? accentColor,
      Color? backgroundColor,
      Color? buttonColor,
      Brightness? brightness}) {
    return ThemeData(
        accentColor: accentColor ?? this.accentColor,
        backgroundColor: backgroundColor ?? this.backgroundColor,
        buttonColor: buttonColor ?? this.buttonColor,
        brightness: brightness ?? this.brightness);
  }
}

Input:

import 'Copywith.dart';

void main() {
  var theme = ThemeData();
  theme.copyWith(accentColor: Color.RED);
  print(theme.accentColor);
}

Output:

Color.BLACK

CodePudding user response：

This is exactly what copyWith is supposed to do - It does not change the instance upon which it is called, but rather creates a new instance which shares identical field values to the original, other than those values which are specified as arguments to copyWith.

To demonstrate using a tweaked version of your example:

import 'Copywith.dart';

void main() {
  var original = ThemeData();
  var changed = original.copyWith(accentColor: Color.RED);
  print(original.accentColor); // Output: Color.BLACK
  print(changed.accentColor); // Output: Color.RED
}