Expose the Axis of MultiChildRenderObjectWidget's to Their Children

[caseycrogers]

Is there an existing issue for this?

• I have searched the existing issues
• I have read the guide to filing a bug

Use case

There are situations where a child widget will want to know the axis of the nearest enclosing axis-aligned widget (Row, Column, Scrollable, etc.).

The example most relevant to me is in my flutter_gutter package. It has a Gap widget that is just a SizedBox with a non-zero height if the nearest relevant enclosing widget is vertically aligned and non-zero width if the nearest is horizontally aligned. The way it does this is super hacky:
It visits ancestor widgets and checks them against a list of known axis-aligned widget types: Row, Column, Scrollable, etc.
The biggest downside is that Gap will not notice the axis of widgets not explicitly included in my checks, either because I forgot to add them or because they're in an external package (ie BoxyRow/BoxyColumn).

Proposal

Have all axis-aligned widgets expose their axis in a consistent manner. Not entirely sure on the appropriate implementation, but here are two possible approaches:

1. Create abstract class AxisAligned { Axis get axis; } and have the relevant widgets implement this class and override the getter appropriately. Child widgets can crawl their ancestors:

Axis? getAxis(BuildContext context) {
    Axis? axis;
    context.visitAncestorElements((element) {
      final Widget widget = element.widget;
      if (widget is AxisAligned) {
        axis = widget.axis;
        return false;
      }
      return true;
    });
    return axis;
}

1a. Same as above, but abstract class AxisAlignedMultiChildRenderObject extends MultiChildRenderObject...
2. Create an inherited widget called AxisScope equivalent to _ScrollableScope (in this case _ScrollableScope would probably get refactored to be a sub class of AxisScope) that exposes Axis via an inherited Widget and have all the relevant widgets embed the scope in their widget trees:

Axis? getAxis(BuildContext context) {
    // `maybeOf` is just a convenience wrapper around `dependOnInheritedWidget...`
    return AxisScope.maybeOf(context).axis;
}

While the Flutter team of course isn't responsible for external packages, this would at least set a consistent standard and API within the Flutter/flutter repo. Then external packages could update to use this API so that my package and others could use it and know that all Flutter and participating external packages would be properly supported.