I'm in the middle of creating a theming system for my app, where I wish to create a base theme class, with fields for colour and fonts:
abstract class BaseTheme {
late Color backgroundColor;
late Color appBarBackgroundColor;
late Color progressIndicatorColor;
late Color primaryColor;
late Color iconColor;
}
(I have to define them as late to keep the Null Safety happy! These have to be overridden anyway, so no problems there).
I would then implement this base class for each theme, as below:
import 'base_theme.dart';
class LightTheme implements BaseTheme {
Color backgroundColor = Colors.white;
Color appBarBackgroundColor = Color(0xff1a1818);
Color progressIndicatorColor = Color(0xffffc114);
Color primaryColor = Colors.red;
Color iconColor = Colors.white;
}
I would then use these values to populate the ThemeData:
BaseTheme _lightTheme = new LightTheme();
ThemeData _theme = ThemeData(
scaffoldBackgroundColor: _lightTheme.backgroundColor,
// etc...
}
Now, this appears to work fine. However, the page on overriding fields in the Linter for Dart states that overriding fields is bad practice. So, is there a better way to achieve something similar?
What's confusing is that, on that page, one example shows overriding fields when you extend a base class, which is considered bad. However, overriding fields when you implement a base class is good - But in this case, the field type has changed from Base to Derived (ie. they're not the same).
Any pointers (no pun intended) are most welcome!
As the lint recommends, you should avoid overriding fields. Your abstract class should define an interface; as such, it should declare getters and setters and leave the implementation to the concrete, derived classes. Note that this avoids the need for late (which is an implementation detail that does not belong in the abstract base class).
abstract class BaseTheme {
Color get backgroundColor;
set backgroundColor(Color value);
Color get appBarBackgroundColor;
set appBarBackgroundColor(Color value);
// etc.
}
Dart 2.13 allows members to be declared as abstract, so you can use that to avoid the repetitiveness:
abstract class BaseTheme {
abstract Color backgroundColor;
abstract Color appBarBackgroundColor;
// etc.
}
Also see the GitHub issue: overridden_fields documentation is vague.
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