Resolve issue where rebuilds were over-eager.

[slangley]

Resolves Issue #1 By using a static list of children and updating only the entry that changed, instead of reloading the children on every update.

Also uses widgets import instead of material, to allow this to be used in a pure Cupertino app.

[okaryo]

Thanks for checking out the PR!
Anything you have a problem with or unclear about the review content, just ask!

[okaryo]

super.didUpdateWidget(oldWidget); should be called first.

Implementations of this method should start with a call to the inherited method, as in super.didUpdateWidget(oldWidget).

ref: https://api.flutter.dev/flutter/widgets/State/didUpdateWidget.html

[okaryo]

I'd prefer a variable name of _stackKey, which is private.

[okaryo]

Thanks to this modification, the variable _alreadyLoaded is no longer needed, so I thought the whole code could look something like this, but guess what?

I named the function _loadedChildren as _initialChildren.

class _LazyLoadIndexedStackState extends State<LazyLoadIndexedStack> {
  late List<Widget> _children;
  final _stackKey = GlobalKey();

  @override
  void initState() {
    super.initState();

    _children = _initialChildren();
  }

  @override
  void didUpdateWidget(final LazyLoadIndexedStack oldWidget) {
    super.didUpdateWidget(oldWidget);

    _children[widget.index] = widget.children[widget.index];
  }

  @override
  Widget build(final BuildContext context) {
    return IndexedStack(
      key: _stackKey,
      index: widget.index,
      children: _children,
      alignment: widget.alignment,
      textDirection: widget.textDirection,
      sizing: widget.sizing,
    );
  }

  List<Widget> _initialChildren() {
    return widget.children.asMap().entries.map((entry) {
      final index = entry.key;
      final childWidget = entry.value;
      return index == widget.index ? childWidget : widget.unloadWidget;
    }).toList();
  }
}

[okaryo]

I would like to keep the code in Example to the minimum necessary.
If this code is not needed, would it be possible to have it removed along with the print debug(line 7375, 8789, 100~102)?