flutter simple tree: lazy build subtree nodes

[javaone199]

Problem description

This is a feature request for flutter-simple-treeview.

TreeView( treeController: TreeController(allNodesExpanded: false), nodes: [...])

It requires all the widgets for the whole tree nodes need to be built in advance. For a node that has descendants, building descendant nodes should be delayed until the node is expanded. For a big tree with complex nodes, experience seconds of delays.

Expected behavior

When a node is expanded and its children widgets have not been created, call a builder to build its children.

TreeView( treeController: TreeController(allNodesExpanded: false), nodes: [...], childrenBuilder: (TreeNode parent) { // return a list of TreeNodes. })

TreeNode should have a value (Object) that can be used to reference the data for building the TreeNode so that in the builder method the children of the data can be obtained.