# Controlling the TreeView

The TreeView uses a controller to manage the data. This allows the user to manipulate the data outside of the Flutter widget build method.

# Controller Class

The TreeViewController class defines the controller needed to display the nodes in the TreeView. The controller has two properties: children and selectedKey.

# Handle Node Tap

In most use cases, you will want to perform an action when a user taps a node. The TreeView class provides the onNodeTap property to allow you to perform an action when a node is tapped. The Function recieves the key of the node tapped. You can then use the TreeViewController to retrieve the actual node instance using the getNode method.

# Expanding Nodes

Each node controls its own expanded state. When a user expands a node, the onExpansionChanged method on the TreeView is called. You should use these method to change the state of the expanded node so that tree display is appropriately updated.

	onExpansionChanged: (key, state) {
	    Node node = _treeViewController.getNode(key);
	    if (node != null) {
	      List<Node> updated = _treeViewController.updateNode(key, node.copyWith(expanded: expanded));
	      setState(() {
	        _treeViewController = _treeViewController.copyWith(children: updated);

This can also be useful if you want to conditionally expand the node or if you want to load tree nodes only when the user expands the node. This could is especially useful for large datasets.

# Updating the TreeView

In order to update the display of the TreeView, the children have to be updated. This should happen using whatever state management mechanism you have in place. Unless you are completely replacing the entire children list, adding, updating, and deleting nodes can involve a number of steps. The TreeViewController tries to make this process simple by providing a number of methods that perform these steps in one method call.

For example, to update a specific node, you would have to perform these steps:

  1. Retrieve the node by key
  2. Update the properties of the node
  3. Retrieve the list of nodes from the controller
  4. Recursively propagate through the nodes to update the node as specified by the key
  5. Get the new list of nodes to be saved to the controller

The updateNode method on the controller performs all of these steps with one call. It returns an updated list of nodes that you can then save to the controller.

Each similar method has a corresponding method that returns an updated controller vs a list of nodes. In the case of updateNode there is withUpdateNode.

The TreeViewController has the following convenience methods: addNode, collapseAll, collapseToNode, deleteNode, expandAll, expandToNode, getNode, getParent, loadJSON, loadMap, toggleNode, updateAll, withAddNode, withCollapseAll, withCollapseToNode, withDeleteNode, withExpandAll, withExpandToNode, getNode, getParent, loadJSON, loadMap, withToggleNode, withUpdateAll.

Use the API reference to learn more about available methods.

# API Reference

View the TreeViewController properties and methods here (opens new window).

Last Updated: 7/5/2021, 1:07:29 PM