Add visual density to the gallery options

[gspencergoog]

Description

This PR adds an option to the gallery options that sets the visual density to one of "standard" (the default), "comfortable", or "compact".

Tests

• Added a test to exercise the new option.

Breaking Change

• No, this is not a breaking change.

[shihaohong]

Is there a way to particularly target this instance of Text('System Default') here and elsewhere? I was thinking something like:

  find.descendant(
    of: find.byType(PopupMenu),
    matching: find.text('System Default')
  )

However, it seems both _PopupMenu and the _VisualDensityItem widgets are private, so I'm not sure if it's possible, but that would improve the process of updating these tests and prevent future bugs/confusion

[gspencergoog]

It's possible, it looks like this:

find.descendant(
  of: find.byWidgetPredicate((Widget widget) => widget.runtimeType.toString() == '_PopupMenu'),
  matching: find.text('System Default')
)

I switched to something like this, but I used 'PopupMenuItem' because the byType finder can't find specialized generics.

[shihaohong]

That's pretty neat, I'll definitely keep this in mind when I need it in the future!

[gspencergoog]

I think this is ready for another look.

[shihaohong]

LGTM!

[flar]

This PR failed the analyzer on linux:

▌04:44:01▐ RUNNING: cd .; bin/flutter analyze --dartdocs --flutter-repo
Analyzing 3 directories...                                      
  error • A value of type 'dynamic' can't be assigned to a variable of type 'GalleryVisualDensityValue' • examples/flutter_gallery/lib/gallery/scales.dart:50:50 • invalid_assignment
  error • A value of type 'Widget' can't be assigned to a variable of type 'MaterialApp' • examples/flutter_gallery/test/drawer_test.dart:73:11 • invalid_assignment
  error • A value of type 'Widget' can't be assigned to a variable of type 'MaterialApp' • examples/flutter_gallery/test/drawer_test.dart:83:11 • invalid_assignment
3 issues found. (ran in 278.8s)
▌04:48:43▐ ELAPSED TIME: 4min 41.944s for bin/flutter analyze --dartdocs --flutter-repo in .

[franciscojma86]

@flar why wasn't this reverted in the end? Just curious since seems like it might have introduced some performance regressions so not sure if it might be related.

[flar]

@franciscojma86
The build failures were fixed by someone else before I merged: #47594

[franciscojma86]

I see, thanks!

[flar]

@flar why wasn't this reverted in the end? Just curious since seems like it might have introduced some performance regressions so not sure if it might be related.

I'm just seeing some performance regressions from around this time so you may be right, but they are subtle enough and the benchmarks themselves are noisy enough that it is hard to point at a specific commit that could have been the cause.