ValueListenableBuilder that wraps an Offstage widget and exposes whether it will be offstage before building completes

offstage_value_listenable_builder.dart

import 'package:flutter/material.dart';

typedef OffstageEvaluator<T> = bool Function(T value);

class OffstageValueListenableBuilder<T> extends ValueListenableBuilder<T> {
  final Key? offstageKey;
  final OffstageEvaluator<T> offstageEvaluator;

  OffstageValueListenableBuilder({
    super.key,
    this.offstageKey,
    required this.offstageEvaluator,
    required super.valueListenable,
    required ValueWidgetBuilder<T> builder,
    super.child,
  }) : super(
            builder: (context, value, child) => _buildOffstage(
                  context,
                  offstageKey,
                  offstageEvaluator,
                  value,
                  builder,
                  child,
                ));

  bool get offstage => offstageEvaluator(valueListenable.value);

  static Widget _buildOffstage<T>(
    BuildContext context,
    Key? offstageKey,
    OffstageEvaluator<T> offstageEvaluator,
    T value,
    ValueWidgetBuilder<T> builder,
    Widget? child,
  ) {
    final offstage = offstageEvaluator(value);
    return Offstage(
      key: offstageKey,
      offstage: offstage,
      child: builder(context, value, child),
    );
  }
}

spaced_list_view.dart

import "package:flutter/material.dart";

import "offstage_value_listenable_builder.dart";

class SpacedListView extends StatelessWidget {
  final double spacing;
  final List<Widget> children;

  const SpacedListView({
    super.key,
    required this.spacing,
    required this.children,
  });

  @override
  Widget build(BuildContext context) {
    return ListView.separated(
      itemBuilder: (context, index) => children[index],
      separatorBuilder: (context, index) =>
          _buildSeparator(context, spacing, children[index + 1]),
      itemCount: children.length,
    );
  }

  Widget _buildSeparator(BuildContext context, double spacing, Widget widget) {
    if (widget is OffstageValueListenableBuilder) {
      return ValueListenableBuilder(
        valueListenable: widget.valueListenable,
        builder: (context, value, child) => widget.offstage
            ? const SizedBox.shrink()
            : SizedBox(height: spacing),
      );
    } else if (widget is Offstage && widget.offstage) {
      return const SizedBox.shrink();
    }
    return SizedBox(height: spacing);
  }
}