InputDecorator prefixIconConstraints and suffixIconConstraints

[shihaohong]

Description

Introduces two new parameters that allow the BoxConstraints of the InputDecoration's prefixIcon and suffixIcon to be modifiable. This is particularly useful when isDense is set to true, which allows the icon sizes to be smaller than 48x48px.

Example:

Sample Code:

class TestWidget extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    const double customMinIconSize = 32;
    const BoxConstraints iconConstraints = BoxConstraints(
      minWidth: customMinIconSize,
      minHeight: customMinIconSize,
    );

    return TextField(
      decoration: InputDecoration(
        hintText: 'Test icon size',
        prefixIcon: Icon(Icons.search, size: 26),
        prefixIconConstraints: iconConstraints,
        suffixIcon: Icon(Icons.arrow_right, size: 26),
        suffixIconConstraints: iconConstraints,
        contentPadding: EdgeInsets.symmetric(vertical: 10.0),
        isDense: true,
      ),
    );
  }
}

Alternative considered

This seems to be a tad verbose to define the box constraints separately for prefix and suffix icons. It could simply be controlled by one parameter (ie. iconConstraints). That makes the constraints less flexible since both icons will share the same set of constraints, but if that seems like a more favorable API, please comment on the PR, but I decided to choose flexibility for now. Thanks!

Related Issues

Fixes #21908

Tests

I added the following tests:

• A test to ensure that custom BoxConstraint values are respected

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[gspencergoog]

About the splitting into prefix and suffix: since everything else is split that way, I think it makes sense to also split it that way.

[gspencergoog]

Ideally, this would be an actual {@tool dartpad ...} sample so that people could see it in action and play with it in the web page.

[shihaohong]

I decided to give it a whirl since I've never tried it :)

[shihaohong]

Although I noticed that testing the new feature in a new DartPad instance didn't work because the parameters did not exist on whatever version of Flutter that DartPad uses (I'm guessing stable), so we'll have no guarantee that it'll work via the instructions in snippets/README.md

[gspencergoog]

It will work once DartPad picks up the changes, which should happen in a couple of weeks or so.