TreeSliver & associated classes

[Piinks]

FYI for Reviewers: Much of the API surface matches that of the 2D TreeView in flutter/packages#6592. If it changes here, it should change there, and vice versa.

📜 Design Document

This adds classes and associated callbacks and controllers for TreeSliver. Core components:

• TreeSliver
• RenderTreeSliver
• TreeSliverNode
• TreeSliverController
• TreeSliverStateMixin
• TreeSliverIndentationType

Fixes #114299

sliver_0.mov

sliver_1.mov

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• I followed the breaking change policy and added Data Driven Fixes where supported.
• All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

[Piinks]

For tomorrow:

• Move SliverTree back to widgets from material once Update icon tree shaker to allow system font fallback #147202 lands
• More name collisions, see Google testing

[bleroux]

LGTM for the examples!

Spotted minor typos in the non-examples files.

Outdated

[TahaTesser]

LGTM!

[baumths]

Hello team, please consider adding generics to the callbacks and builders of the TreeSliver and TreeView (from two_dimensional_scrollables) so users can avoid casting the TreeSliverNode.content in each callback. Example:

TreeSliver<MyItem>(
  tree: myTree,
  treeNodeBuilder: (BuildContext context, TreeSliverNode<Object?> node, _) {
    return MyTreeSliverNodeTile(
      // Casting as I need to access some state of MyItem deeper down in the widget tree
      item: node.content as MyItem,
  },
  // In the `TreeView` widget, I'm using this callback to add focus/hover/active decorations to the whole width of a node
  treeRowExtentBuilder: (TreeSliverNode<Object?> node, _) {
    final item = node.content as MyItem;
    return item.preferredHeight;
  },
  onNodeToggle: (TreeSliverNode<Object?> node) {
    final item = node.content as MyItem;
    myStorage.persistMyItemExpansionState(item.id, node.isExpanded);
  },
);

If I just change the type to TreeSliverNode<MyItem> I get the following error message:
The argument type 'MyTreeSliverNodeTile Function(BuildContext, TreeSliverNode<MyItem>, AnimationStyle)' can't be assigned to the parameter type 'Widget Function(BuildContext, TreeSliverNode<Object?>, AnimationStyle)'.

[baumths]

Suggested change

	typedef TreeSliverNodeBuilder = Widget Function(
	typedef TreeSliverNodeBuilder<T> = Widget Function(

[baumths]

Suggested change

	TreeSliverNode<Object?> node,
	TreeSliverNode<T> node,

[baumths]

Suggested change

	final TreeSliverNodeBuilder treeNodeBuilder;
	final TreeSliverNodeBuilder<T> treeNodeBuilder;

[baumths]

Suggested change

	typedef TreeSliverRowExtentBuilder = double Function(
	typedef TreeSliverRowExtentBuilder<T> = double Function(

[baumths]

Suggested change

	TreeSliverNode<Object?> node,
	TreeSliverNode<T> node,

[baumths]

Suggested change

	typedef TreeSliverNodeCallback = void Function(TreeSliverNode<Object?> node);
	typedef TreeSliverNodeCallback<T> = void Function(TreeSliverNode<T> node);

[baumths]

Suggested change

	final TreeSliverRowExtentBuilder treeRowExtentBuilder;
	final TreeSliverRowExtentBuilder<T> treeRowExtentBuilder;

[baumths]

Suggested change

	final TreeSliverNodeCallback? onNodeToggle;
	final TreeSliverNodeCallback<T>? onNodeToggle;