Implement Lazy Tree View

[bdlukaa]

Fixes #135

Usage:

  late List<TreeViewItem> items;

  @override
  initState() {
    super.initState();
    items = [
      TreeViewItem(
        content: const Text('Work Documents'),
        lazy: true,
        onInvoked: (item) async {
          // if it's already populated, return
          if (item.children.isNotEmpty) return;
          // mark as loading
          setState(() => item.loading = true);
          // do your fetching...
          await Future.delayed(const Duration(seconds: 2));
          setState(() {
            item
              // set loading to false
              ..loading = false
              // add the fetched nodes
              ..children.addAll([
                TreeViewItem(content: const Text('XYZ Functional Spec')),
                TreeViewItem(content: const Text('Feature Schedule')),
                TreeViewItem(content: const Text('Overall Project Plan')),
                TreeViewItem(
                    content: const Text('Feature Resources Allocation')),
              ]);
          });
        },
        children: [],
      ),
      TreeViewItem(
        content: const Text('Personal Documents'),
        lazy: true,
        children: [
          TreeViewItem(
            content: const Text('Home Remodel'),
            children: [
              TreeViewItem(content: const Text('Contractor Contact Info')),
              TreeViewItem(content: const Text('Paint Color Scheme')),
              TreeViewItem(content: const Text('Flooring weedgrain type')),
              TreeViewItem(content: const Text('Kitchen cabinet style')),
            ],
          ),
        ],
      ),
    ];
  }

TreeView(
  items: items,
  onItemInvoked: (item) async => debugPrint('$item'),
),

Pre-launch Checklist

• I have run dartfmt on all changed files
• I have updated CHANGELOG.md with my changes
• I have run "optimize/organize imports" on all changed files
• I have addressed all analyzer warnings as best I could
• I have added/updated relevant documentation
• I have run flutter pub publish --dry-run and addressed any warnings

[timsneath]

Splendid! I wonder whether the loading state will flicker if the expansion only takes ~100ms?

In my scenario, no individual node takes very long to populate, it's just that the entire tree is too large to load. I'll try this out and report back.

[bdlukaa]

I wonder whether the loading state will flicker if the expansion only takes ~100ms?

Probably. The loading indicator can be customizable by providing loadingWidget on both TreeView and TreeViewItem if you wish a custom loading indicator!

[timsneath]

Ah, that probably solves my concern. Thanks!

[yepyeniceri]

When I tried this the "super.initState();" is being highlighted. I get the Errormessage "The method 'initState' isn't defined in a superclass of 'Testpage4'."
Testpage4 is an alteration of the example file "tree_view.dart"
What might I do wrong?

[bdlukaa]

the example tree_view isn't a widget, thefore there isn't a initState. You must do that when initializing the list