ant-design/components/_util/openAnimation.tsx

import cssAnimation from 'css-animation';
import getRequestAnimationFrame, { cancelRequestAnimationFrame } from './getRequestAnimationFrame';

const reqAnimFrame = getRequestAnimationFrame();

function animate(node, show, done) {
  let height;
  let requestAnimationFrameId;
  return cssAnimation(node, 'ant-motion-collapse', {
    start() {
      if (!show) {
        node.style.height = `${node.offsetHeight}px`;
        node.style.opacity = 1;
      } else {
        height = node.offsetHeight;
        node.style.height = 0;
        node.style.opacity = 0;
      }
    },
    active() {
      if (requestAnimationFrameId) {
        cancelRequestAnimationFrame(requestAnimationFrameId);
      }
      requestAnimationFrameId = reqAnimFrame(() => {
        node.style.height = `${show ? height : 0}px`;
        node.style.opacity = show ? 1 : 0;
      });
    },
    end() {
      if (requestAnimationFrameId) {
        cancelRequestAnimationFrame(requestAnimationFrameId);
      }
      node.style.height = '';
      node.style.opacity = '';
      done();
    },
  });
}

const animation = {
  enter(node, done) {
    return animate(node, true, done);
  },
  leave(node, done) {
    return animate(node, false, done);
  },
  appear(node, done) {
    return animate(node, true, done);
  },
};

export default animation;