How does one store "pointers" to nested nodes in an immutable tree in React/Javascript?

Question

I have a immutable nested tree (mori, immutable-js et al) consisting of arbitrary nodes, think file browser. The tree gets rendered by React. If a component representing a node receives focus, I'd like to:

• Show an input field on the node component to change e.g. the node name.
• Show a global panel that contains UI components to edit additional properties of the currently focused node, depending on the type of the selected node.

A simplistic state object could look like this:

{
    root: {
        type: 'folder'
        name: 'Root',
        focused: false,
        children: [
            {
                type: 'text',
                name: 'Essay',
                focused: false
            },
            {
                type: 'folder',
                name: 'Images',
                focused: false,
                children: [
                    {
                        type: 'image',
                        name: 'Paris',
                        focused: true
                    }
                ]
            }
        ]
    },
    currentlyFocusedNode: <Reference to Paris node>
}

Now, how would I keep a valid reference to the currently focused node? If I stored a Paris node reference at currentlyFocusedNode, it would be out of sync as soon as any other part of the app modifies the node (e.g. the inline name input from above). I thought about storing the path to the focused node, which I could use to retrieve the current reference:

currentlyFocusedNode: ['root', 'children', 0, 'children', 0]

But this seems very shaky to me as well, because even a simple node move operation on the tree could leave this path pointing to the wrong or even a non-existing node.

How are those kind of things handled with immutable data structures? Or am I not thinking "immutable" enough at all?

Answer 1

Your question does not have an answer as asked. And it seems you had a sense of that when you said

Or am I not thinking "immutable" enough at all?

First you say you are using an immutable data structure. Meaning it can't be changed.
Then you say:

If I stored a Paris node reference at currentlyFocusedNode, it would be out of sync as soon as any other part of the app modifies the node

With immutable data structures things don't get modified. What you have is an old version of the data and a new version of the data. Neither will ever change.

So the question should really be something like:

How do I identify when 2 nodes represent the same data?

Answer is use an ID.

Another good question might be:

Given a reference to an old node, tree, and newData, how can I update the tree and keep track of the node?

This depends largely on how the rest of the code works and what parts you have access to. E.g. you could have something like this:

function updateNodeInTree(nodeId, newData, inTree){
  oldNode = findNode(nodeId, inTree);
  newNode = merge(oldNode, newData);
  newTree = merge(inTree, {path: {to: newNode}});
  return [newTree, newNode]; 
}

But if you don't have access to where the tree is updated, you may have to settle for:

newTree = updateTree(oldTree, someData);
newNode = findNode(nodeId, newTree);

Answer 2

Give each node a unique ID. Maintain a focused_id value.

Any component rendering parts of a node can check if
this.props.node.id === this.props.focused_id
and render appropriately.

Answer 3

This is where Flux architecture pattern comes in.

I would handle it like this:

1. Save the root and currentlyFocusedNode in a TreeStore where currentlyFocusedNode either saves null or references an object in the tree.
2. When a node is clicked, an event is dispatched (using AppDisptacher) with eventType: 'nodeClicked' and payload containing the object (reference) which was clicked.
3. TreeStore listens to AppDispatcher and makes changes to currentlyFocusedNode
4. React component listens to changes in TreeStore and re-renders whenever there is a change.