smooth_scroll.js

smooth_scroll.js

/*
参考: [javascript - ScrollTo with animation - Stack Overflow](http://stackoverflow.com/questions/12199363/scrollto-with-animation)
*/

window.requestAnimFrame = (function () {
  return window.requestAnimationFrame ||
    window.webkitRequestAnimationFrame ||
    window.mozRequestAnimationFrame ||
    function (callback) {
      window.setTimeout(callback, 1000 / 60);
    };
})();

/**
 * (description)
 *
 * @export
 * @param {number} scrollTargetY - the target scrollY property of the window
 * @param {number} speed - time in pixels per second
 * @param {string} easing - easing equation to use
 */
export default function scrollToY(scrollTargetY = 0, speed = 2000, easing = "easeOutSine") {
  let scrollY = window.scrollY;
  let currentTime = 0;

  // min time .1, max time .8 seconds
  const time = Math.max(.1, Math.min(Math.abs(scrollY - scrollTargetY) / speed, .8));

  // easing equations from https://github.com/danro/easing-js/blob/master/easing.js
  // var PI_D2 = Math.PI / 2;
  const easingEquations = {
    easeOutSine: function (pos) {
      return Math.sin(pos * (Math.PI / 2));
    },
    easeInOutSine: function (pos) {
      return (-0.5 * (Math.cos(Math.PI * pos) - 1));
    },
    easeInOutQuint: function (pos) {
      if ((pos /= 0.5) < 1) {
        return 0.5 * Math.pow(pos, 5);
      }
      return 0.5 * (Math.pow((pos - 2), 5) + 2);
    }
  };

  // add animation loop
  function tick() {
    currentTime += 1 / 60;

    var p = currentTime / time;
    var t = easingEquations[easing](p);

    if (p < 1) {
      window.requestAnimFrame(tick);

      window.scrollTo(0, scrollY + ((scrollTargetY - scrollY) * t));
    } else {
      window.scrollTo(0, scrollTargetY);
    }
  }

  // call it once to get started
  tick();
}