Lazy IndexedStack

README.md

Moved to https://github.com/AlexV525/dartpad_workshops/tree/main/implement_lazy_indexed_stack permanentely.

Oh...I do have a null-safe version. I'll update it then.

Thanks for the code! Here is an alternate version, that accounts for the child list size changing. It also doesn't accept a null index at all, which cleans up the code a little:

import 'package:flutter/material.dart';

/// A lazy-loading [IndexedStack] that loads [children] accordingly.
class LazyIndexedStack extends StatefulWidget {
  const LazyIndexedStack({
    Key? key,
    this.alignment = AlignmentDirectional.topStart,
    this.textDirection,
    this.sizing = StackFit.loose,
    this.index = 0,
    this.children = const [],
  }) : super(key: key);

  final AlignmentGeometry alignment;
  final TextDirection? textDirection;
  final StackFit sizing;
  final int index;
  final List<Widget> children;

  @override
  _LazyIndexedStackState createState() => _LazyIndexedStackState();
}

class _LazyIndexedStackState extends State<LazyIndexedStack> {
  late List<bool> _activated = _initializeActivatedList();

  List<bool> _initializeActivatedList() => List<bool>.generate(widget.children.length, (int i) => i == widget.index);

  @override
  void didUpdateWidget(covariant LazyIndexedStack oldWidget) {
    if (oldWidget.children.length != widget.children.length) {
      _activated = _initializeActivatedList();
    }
    super.didUpdateWidget(oldWidget);
  }

  @override
  Widget build(BuildContext context) {
    // Mark current index as active
    _activated[widget.index] = true;
    final children = List.generate(_activated.length, (i) {
      return _activated[i] ? widget.children[i] : const SizedBox.shrink();
    });
    return IndexedStack(
      alignment: widget.alignment,
      sizing: widget.sizing,
      textDirection: widget.textDirection,
      index: widget.index,
      children: children,
    );
  }
}

Other tweaks:

• moved the buildChildren method inside of build, as its only 3 lines when using list.generate
• Removed the check for _activatedList[widget.index] , looking it up is likely slower than just storing it

Thanks for the code! Here is an alternate version, that accounts for the child list size changing. It also doesn't accept a null index at all, which cleans up the code a little

Hi @esDotDev. Thanks for the feedback!
I've setup a workshop with the implementation at https://github.com/AlexV525/dartpad_workshops/tree/main/implement_lazy_indexed_stack, and I guess I'll redirect the gist to the workshop recently. Can you review it and see if your updates are still working with the workshop version?