requestAnimationFrame 实现JS动画

requestAnimationFrame会随着浏览器绘制窗口的频率来绘制动画以达到最好的用户体验

// let raf = (function(){
//   return  window.requestAnimationFrame       ||
//           window.webkitRequestAnimationFrame ||
//           window.mozRequestAnimationFrame    ||
//           function( callback ){
//             window.setTimeout(callback, 1000 / 60);//每秒实现60帧的动画效果最好
//           };
// })();
let raf = window.requestAnimationFrame;
let box = document.getElementsByClassName('box')[0];
let w = 100;
let t = 0;
function loop(){
    box.style.width = w + 'px';
    if(w < 500){
        w += 1;
        box.style.width = w + 'px';

    }
    t = raf(loop)
}

loop()
box.addEventListener('click',()=>{//实现点击暂停动画，再次点击继续运行的效果
    if(t){
        window.cancelAnimationFrame(t);
        t = 0;
    }else{
        t = raf(loop)
    }
})