requestAnimation API

window.requestAnimationFrame() 方法用来页面重绘之前，通知浏览器调用指定的函数，以满足开发者的需求。

如果想得到连贯的逐帧动画，函数中必须重新调用 requestAnimationFrame() 。

requestID = window.requestAnimationFrame(callback);
/*
    Firefox 23 / IE 10 / Chrome / Safari */
requestID = window.mozRequestAnimationFrame(callback);
/* Firefox < 23 */
requestID = window.webkitRequestAnimationFrame(callback);
/* Oldversions Chromes / webkit */

callback : 每次重新绘制，都会调用这个回调函数。这个回调函数会收到一个参数，这个 DOMHighResTimeStamp 类型的参数指示当前时间距离开始触发 requestAnimationFrame de 回调函数的时间。

返回值 requestId 是一个长整型的非零数值。作为一个唯一的标识符，可以将该值传递给 window.cancelAnimationFrame() 来取消这个回调函数。

优势

• 经过浏览器优化，动画更流畅
• 窗口没激活时，动画将停止，节省计算资源
• 更省电，尤其对移动端

function animation() {
  /* some any code */ requestAnimationFrame(
    animation,
  ); /* some animation code */
} /* request animation */
requestAnimationFrame(animation);

有时候需要控制，就需要句柄：

let globalID;
function animation() {
  /* some any code */
  globalID = requestAnimationFrame(animation);
  /* some animation code*/
}
/* request animation*/
globalID = requestAnimationFrame(animation);
/* stop*/
cancelAnimationFrame(globalID);

例 1

初始 div 宽度为 1 px ，在 step() 函数中将进度加 1 ，然后更新到 div 宽度上，在进度达到 100 之前，一直重复这个过程：

window.requestAnimationFrame =
  window.requestAnimationFrame ||
  window.mozRequestAnimationFrame ||
  window.webkitRequestAnimationFrame ||
  window.msRequestAnimationFrame;
var start = null;
var ele = document.getElementById('test');
var progress = 0;
function step(timestamp) {
  progress += 1;
  ele.style.width = progress + '%';
  ele.innerHTML = progress + '%';
  if (progress < 100) {
    requestAnimationFrame(step);
  }
}
requestAnimationFrame(step);
document.getElementById('run').addEventListener(
  'click',
  function () {
    ele.style.width = '1px';
    progress = 0;
    requestAnimationFrame(step);
  },
  false,
);

例 2

首先在页面中插入一个 <canvas id="motion"> 标签；然后设计一块画布，在画布上随机生成一个圆点，颜色、运动方向随机；接着从画布中央随机向四周运动，同时逐步增大半径；最后调用 requestAnimationFrame() 方法，传递回调函数生成 animate() ，从而设计随机粒子放射运动的效果。

(function () {
  'use strict';
  var vendors = ['webkit', 'moz'];
  for (var i = 0; i < vendors.length && !window.requestAnimationFrame; ++i) {
    var vp = vendors[i];
    window.requestAnimationFrame = window[vp + 'RequestAnimationFrame'];
    window.cancelAnimationFrame =
      window[vp + 'CancelAnimationFrame'] ||
      window[vp + 'CancelRequestAnimationFrame'];
  }
  if (
    /iP(ad|hone|od)._OS 6/.test(window.navigator.userAgent) || // iOS6 is buggy
    !window.requestAnimationFrame ||
    !window.cancelAnimationFrame
  ) {
    var lastTime = 0;
    window.requestAnimationFrame = function (callback) {
      var now = Date.now();
      var nextTime = Math.max(lastTime + 16, now);
      return setTimeout(function () {
        callback((lastTime = nextTime));
      }, nextTime - now);
    };
    window.cancelAnimationFrame = clearTimeout;
  }
})();
var getRandomColor = function () {
  return '#' + ((Math.random() * 0xffffff) << 0).toString(16);
};
var canvas = document.getElementById('motion'),
  c = canvas.getContext('2d'),
  particles = {},
  particleIndex = 0,
  particleNum = 0.2;
canvas.width = window.innerWidth;
canvas.height = window.innerHeight;
function Particle() {
  this.x = canvas.width / 2;
  this.y = canvas.height / 2;
  this.vx = Math.random() * 6 - 3;
  this.vy = Math.random() * 4 - 2;
  this.growth = (Math.abs(this.vx) + Math.abs(this.vy)) * 0.007;
  particleIndex++;
  particles[particleIndex] = this;
  this.id = particleIndex;
  this.size = Math.random() * 1;
  this.color = getRandomColor();
}
Particle.prototype.draw = function () {
  this.x += this.vx;
  this.y += this.vy;
  this.size += this.growth;
  if (this.x > canvas.width || this.y > canvas.height) {
    delete particles[this.id];
  }
  c.fillStyle = this.color;
  c.beginPath();
  c.arc(this.x, this.y, this.size, 0 * Math.PI, 2 * Math.PI);
  c.fill();
};
function animate() {
  requestAnimationFrame(animate);
  c.fillStyle = '#000';
  c.fillRect(0, 0, canvas.width, canvas.height);
  if (Math.random() > particleNum) {
    new Particle();
  }
  for (var i in particles) {
    particles[i].draw();
  }
}
requestAnimationFrame(animate);

例 3

window.requestAnimFrame = (function () {
  return (
    window.requestAnimationFrame ||
    window.webkitRequestAnimationFrame ||
    window.mozRequestAnimationFrame ||
    window.oRequestAnimationFrame ||
    window.msRequestAnimationFrame ||
    function () {
      window.setTimeout(callback, 1000 / 60);
    }
  );
})();
var canvas, context;
init();
animate();
function init() {
  canvas = document.createElement('canvas');
  canvas.style.left = 0;
  canvas.style.top = 0;
  canvas.width = 210;
  canvas.height = 210;
  context = canvas.getContext('2d');
  document.body.appendChild(canvas);
}
function animate() {
  requestAnimFrame(animate);
  draw();
}
function draw() {
  var time = new Date().getTime() * 0.002;
  var x = Math.sin(time) * 96 + 105;
  var y = Math.cos(time * 0.9) * 96 + 105;
  context.fillStyle = 'pink';
  context.fillRect(0, 0, 255, 255);
  context.fillStyle = 'rgb(255,0,0)';
  context.beginPath();
  context.arc(x, y, 10, 0, Math.PI * 2, true);
  context.closePath();
  context.fill();
}