{
// Compute the dimensions; the width is provided by Observable.
const height = Math.min(500, width / 2);
const outerRadius = height / 2 - 10;
const innerRadius = outerRadius * 0.75;
// https://tauday.com/tau-manifesto
const tau = 2 * Math.PI;
// 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.
const svg = d3.create("svg")
.attr("viewBox", [0, 0, width, height]);
const g = svg.append("g").attr("transform", "translate(" + width / 2 + "," + height / 2 + ")");
// 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.
const arc = d3.arc()
.innerRadius(innerRadius)
.outerRadius(outerRadius)
.startAngle(0);
// Add the background arc, from 0 to 100% (tau).
const background = g.append("path")
.datum({endAngle: tau})
.style("fill", "#ddd")
.attr("d", arc);
// Add the foreground arc in orange, currently showing 12.7%.
const foreground = g.append("path")
.datum({endAngle: 0.127 * tau})
.style("fill", "orange")
.attr("d", arc);
// Every so often, start a transition to a new random angle. The attrTween
// definition is encapsulated in a separate function (a closure) below.
const interval = d3.interval(function() {
foreground.transition()
.duration(750)
.attrTween("d", arcTween(Math.random() * tau));
}, 1500);
// Stop the interval when this block of code updates
invalidation.then(() => interval.stop());
// Returns a tween for a transition’s "d" attribute, transitioning any selected
// arcs from their current angle to the specified new angle.
function arcTween(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).
// https://d3js.org/d3-transition/modifying#transition_attrTween
return 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.
const 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);
};
};
}
// Return the svg node to be displayed.
return svg.node();
}
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