使用requestAnimationFrame重复的3秒动画

Question

I have an SVG with an animation using requestAnimationFrame and the animation works but what I want it to do is animate over 3 secs and then repeat, so the animation for the circle will animate the dasharray and dashoffsset over 3seconds I don't know how to do this using css because it requires a calculation in javascript using Math.Pi can someone see if they can make this animation work the way I described. codepen.io

HTML

<svg width="20" height="20" viewBox="0 0 20 20">
  <circle cx="10" cy="10" r="7" fill="none" stroke="#e6e6e6" stroke-width="6" />
  <circle id="shape" cx="10" cy="10" r="7" fill="none" stroke="#ff0000" stroke-width="6" stroke-linecap="square" /> 
</svg>

CSS

#shape {
 fill: none;
 stroke: red;
 stroke-width: 6;
 transition: all 4s ease-in-out;
 transform: rotate(-90deg);
 transform-origin: 50%;
}

JavaScript

var endPercentage = 100;

var shape = document.getElementById('shape');
var circumference = Math.PI * parseFloat(shape.getAttribute('r')) * 2;

shape.setAttribute('stroke-dasharray', circumference);
shape.setAttribute('stroke-dashoffset', circumference);

var updatePercentage = function () {
    var dashOffset = parseFloat(getComputedStyle(shape)['stroke-dashoffset']);
    var curPercentage = Math.floor((100 * (circumference - dashOffset)) / circumference) || 0;

    document.getElementById('perc').getElementsByClassName('value')[0].innerText = curPercentage;

    if (curPercentage < endPercentage) {
        window.requestAnimationFrame(updatePercentage);
    }
}

var animateShape = function (percent) {
    var offset = circumference - (circumference * percent / 100);
    shape.setAttribute('stroke-dashoffset', offset);
    window.requestAnimationFrame(updatePercentage);
}

setTimeout(function () {
    animateShape(endPercentage);
}, 0);

Answer 1

You can define pi with:

:root {
  --PI: 3.14159265358979; 
}

...

stroke-dashoffset: calc(2 * var(--PI) * 7);
/* 7 is the radius of your circle */

With a combination of this and calc, you should be able to get a pure CSS solution.

If you still want to go via the JS route, I recommend:

  shape.animate([
    { strokeDashoffset: 0 },
    { strokeDashoffset: circumference }
  ], { duration: 3000, iterations: Infinity });

Warning: polyfill required for older browsers.