text: export function requestAnimationFrame() {
      return window.requestAnimationFrame || 
              window.webkitRequestAnimationFrame || 
              window.mozRequestAnimationFrame    || 
              window.oRequestAnimationFrame      || 
              window.msRequestAnimationFrame     || 
              function(cb) {
                  return window.setTimeout(cb, 1000 / 60);    // 每秒60帧肉眼感觉不出卡顿
              };
  }
  
  export function cancelAnimationFrame() {
      return window.cancelAnimationFrame  ||
              window.webkitCancelAnimationFrame ||
              window.mozCancelAnimationFrame    ||
              window.oCancelAnimationFrame      ||
              window.clearTimeout;
  }