[Proposal] Better support for ThemeExtensions

[bdlukaa]

We can take advantage of Material's theme extensions to replace the current mix theming. Currently, we define global variables and attach value to them in a MixThemeData, so that we can use globally.

Proposal

A callback based function theme<T extends ThemeExtension>((T)) that can be called inside a Mix. The mixer will look into every attribute for a theme and perform the operation based on the current context.

Something like:

final mix = Mix(
  bgColor(theme<MyColors>((myColors) => myColors.brandColor)),
  textColor(theme<MyColors>((myColors) => myColors.danger))),
);

// This is from the official example. Link attached below
@immutable
class MyColors extends ThemeExtension<MyColors> {
  const MyColors({
    required this.brandColor,
    required this.danger,
  });

  final Color? brandColor;
  final Color? danger;

  ...
}

Other things I have considered
Making theme more readable and usable with something like

final mix = Mix(
  bgColor(theme<MyColors>().brandColor),
  textColor(theme<MyColors>().danger),
);

But I can't seem to find a way where theme would have access to the current context

Additional context
https://api.flutter.dev/flutter/material/ThemeData/extensions.html

[leoafarias]

@bdlukaa will review in detail. A few things to consider is that Mix should not be dependent on only Material design tokens, they also will need to be able to use them and/or create custom ones for their own design system.

[leoafarias]

@bdlukaa I like this approach and could simplify a lot of our theming logic. I wonder if we can use this for spacing, sizes and so on. Could you do a POC for the Space theme data, and we can validate the implementation

[leoafarias]

@bdlukaa while taking a closer look into this, I don't think ThemeExtensions replace the Mix Tokens, especially if you are trying to accomplish something like theme<MyColors>().danger, I am going to go through a bit and see if I can make any advance on the separation of Mix Tokens and the ThemeExtensions

[leoafarias]

We have rewritten large parts of Mix, which better supports a lot of Material Theme features. However, I have renamed this to highlight better support for ThemeExtensions.