merging old data with new data

Question

I want the old nodes in the force layout to keep their position/momentum/fixedness, when i update the graph with new nodes.

The update adds and removes nodes and links, but the server only sends {name: _}, and doesn't know about the rest, i.e. {x: _, y: _, px: _, py: _, fixed: _}.

Now, I'm manually merging each:

var names = {};
// stores old nodes, indexed by _.name

function merge(nodes) {
    for (var i=0; i < nodes.length; i++) {

        var next = nodes[i];
        var prev = names[nodes[i].name]; // defaults to null
        var node = $.extend(true, {}, next, prev); // merge old into new
        nodes[i] = node;

        names[node.name] = node;
    }
    return nodes;
}

d3.json(function(error, graph){
    merge(graph.nodes)
    force.nodes(graph.nodes);
    ...
}

Is there a better way to do this? Maybe some d3 function that takes the two data (i.e. the old and the new) and outputs one (to be used as the new), that's called by the data join? selection.data() only takes a key.

This example replaces the nodes, rather than merging them.

Related: Problems adding and removing nodes in a force-layout using d3.js

All you should need to do is operate directly on the array you gave to `force.nodes()`. That is, you don't need to set it again, just modify the underlying data structure. — Lars Kotthoff, Dec 01 '13 at 21:56
Yes, so that should work. D3 doesn't provide any functions that do this in a nicer way. — Lars Kotthoff, Dec 02 '13 at 09:45
yeah, i had already written everything, and it does work. this pattern just felt like something D3 might have in its framework. — sam boosalis, Dec 02 '13 at 16:32
In principle, you could use the usual enter/update/exit pattern, but this is really for updating DOM elements, so you would have to add "dummy" DOM elements. — Lars Kotthoff, Dec 02 '13 at 16:36
If that answers your question, I could add it as an answer for reference. — Lars Kotthoff, Dec 02 '13 at 16:56
if the answer to my question is in the negative, by all means. — sam boosalis, Dec 02 '13 at 17:39

score 1 · Answer 1 · answered Dec 02 '13 at 18:05

D3 doesn't provide any functionality to do what you're looking for, but you've already implemented merge, which does the same thing. Basically, you need to modify the data structures that you gave to force.links() and force.nodes(). There is no need to tell the force layout to use them explicitly after making the changes.

merging old data with new data

1 Answers1