D3 Examples - Modifying a Force Layout with buttons

Modifying a Force Layout with buttons
This example modifies a force-layout with transitions for smooth enter and exit. An interesting technique here is the use of transition.attrTween to keep the exiting links connected to the moving nodes, even after the links have been removed from the link selection.
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<!DOCTYPE html>
<div id="option">
    <input name="updateButton" 
           type="button" 
           value="Step 1" 
           onclick="update1()" />
    <input name="updateButton" 
           type="button" 
           value="Step 2" 
           onclick="update2()" />
    <input name="updateButton" 
           type="button" 
           value="Step 3" 
           onclick="update3()" />
    <input name="revertButton" 
           type="button" 
           value="Reset" 
           onclick="resetData()" />
</div>
<svg width="960" height="500"></svg>
<script src="https://d3js.org/d3.v4.min.js"></script>
<script>
​
var svg = d3.select("svg"),
    width = +svg.attr("width"),
    height = +svg.attr("height"),
    color = d3.scaleOrdinal(d3.schemeCategory10);
​
var a = {id: "a"},
    b = {id: "b"},
    c = {id: "c"},
    nodes = [a, b, c],
    links = [];
​
var simulation = d3.forceSimulation(nodes)
    .force("charge", d3.forceManyBody().strength(-1000))
    .force("link", d3.forceLink(links).distance(200))
    .force("x", d3.forceX())
    .force("y", d3.forceY())
    .alphaTarget(1)
    .on("tick", ticked);
​
var g = svg.append("g").attr("transform", "translate(" + width / 2 + "," + height / 2 + ")"),
    link = g.append("g").attr("stroke", "#000").attr("stroke-width", 1.5).selectAll(".link"),
    node = g.append("g").attr("stroke", "#fff").attr("stroke-width", 1.5).selectAll(".node");
​
restart();
​
function update1(){
  links.push({source: a, target: b}); // Add a-b.
  links.push({source: b, target: c}); // Add b-c.
  links.push({source: c, target: a}); // Add c-a.
  restart();
}  
​
function update2(){
  nodes.pop(); // Remove c.
  links.pop(); // Remove c-a.
  links.pop(); // Remove b-c.
  restart();
}
​
function update3(){
    nodes.push(c); // Re-add c.
  links.push({source: b, target: c}); // Re-add b-c.
  links.push({source: c, target: a}); // Re-add c-a.
  restart();
}
  
function resetData(){
  a = {id: "a"},
  b = {id: "b"},
  c = {id: "c"},
  nodes = [a, b, c],
  links = [];
  restart();
}
// d3.timeout(function() {
//   links.push({source: a, target: b}); // Add a-b.
//   links.push({source: b, target: c}); // Add b-c.
//   links.push({source: c, target: a}); // Add c-a.
//   restart();
// }, 1000);
​
// d3.interval(function() {
//   nodes.pop(); // Remove c.
//   links.pop(); // Remove c-a.
//   links.pop(); // Remove b-c.
//   restart();
// }, 2000, d3.now());
​
// d3.interval(function() {
//   nodes.push(c); // Re-add c.
//   links.push({source: b, target: c}); // Re-add b-c.
//   links.push({source: c, target: a}); // Re-add c-a.
//   restart();
// }, 2000, d3.now() + 1000);
​
function restart() {
​
  // Apply the general update pattern to the nodes.
  node = node.data(nodes, function(d) { return d.id;});
​
  node.exit().transition()
      .attr("r", 0)
      .remove();
​
  node = node.enter().append("circle")
      .attr("fill", function(d) { return color(d.id); })
      .call(function(node) { node.transition().attr("r", 8); })
    .merge(node);
​
  // Apply the general update pattern to the links.
  link = link.data(links, function(d) { return d.source.id + "-" + d.target.id; });
​
  // Keep the exiting links connected to the moving remaining nodes.
  link.exit().transition()
      .attr("stroke-opacity", 0)
      .attrTween("x1", function(d) { return function() { return d.source.x; }; })
      .attrTween("x2", function(d) { return function() { return d.target.x; }; })
      .attrTween("y1", function(d) { return function() { return d.source.y; }; })
      .attrTween("y2", function(d) { return function() { return d.target.y; }; })
      .remove();
​
  link = link.enter().append("line")
      .call(function(link) { link.transition().attr("stroke-opacity", 1); })
    .merge(link);
​
  // Update and restart the simulation.
  simulation.nodes(nodes);
  simulation.force("link").links(links);
  simulation.alpha(1).restart();
}
​
function ticked() {
  node.attr("cx", function(d) { return d.x; })
      .attr("cy", function(d) { return d.y; })
​
  link.attr("x1", function(d) { return d.source.x; })
      .attr("y1", function(d) { return d.source.y; })
      .attr("x2", function(d) { return d.target.x; })
      .attr("y2", function(d) { return d.target.y; });
}
​
</script>
​
    