Added IconButtonTheme and apply it to IconButton in M3

[QuncCccccc]

This PR adds IconButtonTheme for IconButton
fixes #107305
fixes #84910

Actual results (on master channel)

M2 Mode with fix	M3 Mode with fix

Pre-launch Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• I read and followed the [Flutter Style Guide], including [Features we expect every widget to implement].
• I signed the [CLA].
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is [test-exempt].
• All existing and new tests are passing.

[HansMuller]

If we don't factor in the icon theme when useMaterial3 is true, then this is a significant breaking change. If the IconButtonTheme overrides IconTheme, then we might avoid a breaking change here, however detecting the difference between an intentional IconTheme override and the default IconTheme created by the Theme widget will be a little bit tricky.

[HansMuller]

I think we can avoid making a breaking change, details are here: IconTheme and IconButton defaults. The gist of the proposal is this:

IconButton’s default size and the color of its Icon descendant are currently defined by the IconTheme. We’d like them to be defined by the new IconButtonTheme instead, if an IconButtonTheme is defined with non-null properties. Otherwise we’d like them to be defined by the defaults for Material 3 rather than the current IconTheme. At least not by the current IconTheme, if the current IconTheme is the default one automatically defined by Theme.

We could check for that case:

  IconThemeData overallIconTheme = IconTheme.of(context);
  IconThemeData? iconTheme = isThemeIconTheme(overallIconTheme)
    ? null 
    : overallIconTheme;

  bool isThemeIconTheme(IconThemeData theme) {
     // Return true if theme is the same as the default 
     // value of Theme.of(context).iconTheme
  }

In this case iconTheme would only be used to define the icon button’s default size and color if it was non-null, i.e. if it wasn’t the default one defined by the overall Theme. That means that if the app has inserted its own IconTheme into the widget tree, we’d use that instead of the M3 defaults. Just to clarify, here’s how we’d resolve each IconButton property, in order of precedence:

1. Widget properties like IconButton.color
2. IconButtonTheme properties
3. IconTheme properties unless they were defined by Theme.of(context).iconTheme
4. Material 3 default IconButtonTheme properties

As always, the first non-null value is the one that’s used. As noted in the code sample, the exception for IconTheme properties is for ones defined by the default value of ThemeData.iconTheme.

[HansMuller]

CC @rydmike - thought you might be interested in this issue since it's related to the one that came up in #107305 (comment)

[rydmike]

Thanks @HansMuller, I like this approach. It will work without breaking past M2 behavior and work as one might expect when using M3 and the new IconButtonTheme as well.

[QuncCccccc]

Thanks a lot for the suggestions. I just pushed some changes based on the comments so both IconTheme and IconButtonTheme should work in M3 now. Please let me know if there're any questions:)

[TahaTesser]

@QuncCccccc
This should close #84910

[HansMuller]

LGTM

[rydmike]

Just verified in master 3.1.0-0.0.pre.2108, that the fix has landed and works. Thanks!