Arc Tween

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forked from rcrocker13's block: Arc Tween

<!DOCTYPE html>
<meta charset="utf-8">
<body>
<script src="https://cdnjs.cloudflare.com/ajax/libs/d3/3.5.5/d3.min.js"></script>
<script>

var width = 960,
    height = 500,
    τ = 2 * Math.PI; // https://tauday.com/tau-manifesto

// An arc function with all values bound except the endAngle. So, to compute an
// SVG path string for a given angle, we pass an object with an endAngle
// property to the `arc` function, and it will return the corresponding string.
var arc = d3.svg.arc()
    .innerRadius(176)
    .outerRadius(246)
    .startAngle(0);

// Create the SVG container, and apply a transform such that the origin is the
// center of the canvas. This way, we don't need to position arcs individually.
var svg = d3.select("body").append("svg")
    .attr("width", width)
    .attr("height", height)
  .append("g")
    .attr("transform", "translate(" + width / 2 + "," + height / 2 + ")")

// Add the background arc, from 0 to 100% (τ).
var background = svg.append("path")
    .datum({endAngle: τ})
    .style("fill", "#6b6b6b")
    .attr("d", arc);

// Add the foreground arc in orange, currently showing 12.7%.
var foreground = svg.append("path")
    .datum({endAngle: .127 * τ})
    .style("fill", "orange")
		.style("stroke", "white")
		.style("stroke-width", "2px")
		.style("opacity", .5)
    .attr("d", arc);

// Every so often, start a transition to a new random angle. Use transition.call
// (identical to selection.call) so that we can encapsulate the logic for
// tweening the arc in a separate function below.
setInterval(function() {
  foreground.transition()
      .duration(500)
      .call(arcTween, Math.random() * τ);
}, 500);

// Creates a tween on the specified transition's "d" attribute, transitioning
// any selected arcs from their current angle to the specified new angle.
function arcTween(transition, newAngle) {

// The function passed to attrTween is invoked for each selected element when
  // the transition starts, and for each element returns the interpolator to use
  // over the course of transition. This function is thus responsible for
  // determining the starting angle of the transition (which is pulled from the
  // element's bound datum, d.endAngle), and the ending angle (simply the
  // newAngle argument to the enclosing function).
  transition.attrTween("d", function(d) {

// To interpolate between the two angles, we use the default d3.interpolate.
    // (Internally, this maps to d3.interpolateNumber, since both of the
    // arguments to d3.interpolate are numbers.) The returned function takes a
    // single argument t and returns a number between the starting angle and the
    // ending angle. When t = 0, it returns d.endAngle; when t = 1, it returns
    // newAngle; and for 0 < t < 1 it returns an angle in-between.
    var interpolate = d3.interpolate(d.endAngle, newAngle);

// The return value of the attrTween is also a function: the function that
    // we want to run for each tick of the transition. Because we used
    // attrTween("d"), the return value of this last function will be set to the
    // "d" attribute at every tick. (It's also possible to use transition.tween
    // to run arbitrary code for every tick, say if you want to set multiple
    // attributes from a single function.) The argument t ranges from 0, at the
    // start of the transition, to 1, at the end.
    return function(t) {

// Calculate the current arc angle based on the transition time, t. Since
      // the t for the transition and the t for the interpolate both range from
      // 0 to 1, we can pass t directly to the interpolator.
      //
      // Note that the interpolated angle is written into the element's bound
      // data object! This is important: it means that if the transition were
      // interrupted, the data bound to the element would still be consistent
      // with its appearance. Whenever we start a new arc transition, the
      // correct starting angle can be inferred from the data.
      d.endAngle = interpolate(t);

// Lastly, compute the arc path given the updated data! In effect, this
      // transition uses data-space interpolation: the data is interpolated
      // (that is, the end angle) rather than the path string itself.
      // Interpolating the angles in polar coordinates, rather than the raw path
      // string, produces valid intermediate arcs during the transition.
      return arc(d);
    };
  });
}

</script>

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