示例-旋转-spinner改进:从暂停的地方开始

最下面调用draw()的时候，(!startTime)会运行2次。


<!DOCTYPE html>
<html lang="en-US">
  <head>
    <meta charset="utf-8">
    <title>My test page</title>
<style>
html {
  background-color: white;
  height: 100%;
}

body {
  height: inherit;
  background-color: red;
  margin: 0;
  display: flex;
  justify-content: center;
  align-items: center;
}

div {
  display: inline-block;
  font-size: 10rem;
}

</style>
  </head>
  <body>
    <div>↻</div>
    <script>
const spinner = document.querySelector('div');
let rotateCount = 0;
let tmpRotateCount = 0;
let startTime = null;
let rAF;
let spinning = true;
function draw(timestamp) {
  if (!startTime){// && !!timestamp) {
    startTime = timestamp;
    console.log('----------')
    console.log('timestamp: '+ timestamp);
    console.log('startTime: '+ startTime);
    console.log('rotateCount: ' + tmpRotateCount);
    if (tmpRotateCount > 0){
      startTime =  startTime - tmpRotateCount * 10
      console.log('new startTime: ' + startTime);
    }
    console.log('end----------')
  }

  //console.log(timestamp);
  //timestamp不停的在增加，不是调用draw()的时间。
  rotateCount = (timestamp - startTime) / 10;
  rotateCount %= 360;

  spinner.style.transform = `rotate(${rotateCount}deg)`;
  rAF = requestAnimationFrame(draw);

}
draw();

//window.addEventListener('click', function() {
document.body.addEventListener('click', function() {
  if (spinning) {
    cancelAnimationFrame(rAF);
    spinning = false;
    startTime = null;
  }
    else {
    startTime = null;
    tmpRotateCount = rotateCount;
    draw();
    spinning = true;
    }
  });
    </script>
  </body>
</html>
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