使用setTimeout延迟d3转换

Question

I am trying to create a simple plane animation along a spiral curve

However, I encountered issue when trying to delay the transition using d3's delay function, the animation does not seem to go correctly

So I have been contemplating using setTimeout to delay the triggering of the transition function, but could not seem to get it right. Could someone give me a suggestion?

Link to the fiddle . This includes the original transition without setTimeout or any .delay; v15 shows the error with using .delay; and v16 has what I have tried with setTimeout

This is my code for transitioning without setTimeout (which works fine without .delay)

function transitionThis(d,i) {
d3.select(this).transition()
.duration(3000)
//.delay(3000) //causes error in the animation
.ease("exp")
.each("start", function() { d3.select(this).style("opacity", "1"); })    
  .attrTween("transform", translateAlong(path.node()))
  .styleTween("opacity", function () {return d3.interpolate("1", "0");});}

plane.each(transitionThis);

// Returns an attrTween for translating along the specified path element.
function translateAlong(path) {
  var l = path.getTotalLength();
    var t0 = 0;
    return function(i) {
  return function(t) {
    var p0 = path.getPointAtLength(t0 * l);//previous point
    var p = path.getPointAtLength(t * l);////current point
    var angle = Math.atan2(p.y - p0.y, p.x - p0.x) * 180 / Math.PI;//angle for tangent
    t0 = t;
  return "translate(" + p.x + "," + p.y +  ")scale(" + (0.2+t*4) + ")rotate(" + angle +"15"+")";
};
  };
}

and this is my attempt at adding setTimeout

function transitionThis(d,i) {
d3.select(this).transition()
.duration(3000)
//.delay(3000) //causes error in the animation
.ease("exp")
.each("start", function() { d3.select(this).style("opacity", "1"); })    
  .attrTween("transform", translateAlong(path.node()))
  .styleTween("opacity", function () {return d3.interpolate("1", "0");});
}

function delayedFlight() {
var tOut = setTimeout(transitionThis(),3000);
}
plane.each(delayedFlight);

Answer 1

Here's what's happening: for some reason, adding .delay() to the transition makes it so that every once in a while the interpolation function (the one that returns the transform value) gets called two times in a row with the same value of t . For example, here's a subset of the output I got when I inserted console.log(t) :

0.006692062648206693
0.006707542472169953
0.007238969233518517
0.007255714140926161
0.0077049430038543705
0.0077049430038543705 // <- same as previous
0.008568945153864268
0.008588766510821856
0.00899496468906235
0.00899496468906235   // <- same as previous
0.009529833123126597
0.00955187716946928
0.01107410052517391
0.011099716711945125
0.012516716837337842

I'm not why it happens, but technically it's not a bug, just a quirk. However, it causes a sort of bug with your interpolation function: When t0 and t are equal, so are p0 and p and as a result the computed angle is 0 for those intermittent cases. And that's when you see the plane stutter – whenever its rotation is set to 0.

The easiest fix is to make it so that t0 and t are never equal, which can be achieved by intercepting that condition and modifying t0 a bit. Like this:

    // Happens every once in a while
    if (t == t0) {
        // Move t0 "back in time" a bit
        t0 -= .00001;
    }

It's not so pretty, but is probably still better than getting into setTimeouts....

Here's the modified fiddle