element-ui/src/transitions/collapse-transition.js

class Transition {
  beforeEnter(el) {
    if (!el.dataset) el.dataset = {};
    el.dataset.oldPaddingTop = el.style.paddingTop;
    el.dataset.oldPaddingBottom = el.style.paddingBottom;
    el.dataset.oldDisplay = el.style.display;
    el.style.height = '0';
    el.style.paddingTop = 0;
    el.style.paddingBottom = 0;
  }

  enter(el) {
    el.dataset.oldOverflow = el.style.overflow;

    el.style.display = 'block';
    if (el.scrollHeight !== 0) {
      el.style.height = el.scrollHeight + 'px';
      el.style.paddingTop = el.dataset.oldPaddingTop;
      el.style.paddingBottom = el.dataset.oldPaddingBottom;
    } else {
      el.style.height = '';
      el.style.paddingTop = el.dataset.oldPaddingTop;
      el.style.paddingBottom = el.dataset.oldPaddingBottom;
    }

    el.style.overflow = 'hidden';
  }

  afterEnter(el) {
    el.style.display = '';
    el.style.height = '';
    el.style.overflow = el.dataset.oldOverflow;
  }

  beforeLeave(el) {
    if (!el.dataset) el.dataset = {};
    el.dataset.oldPaddingTop = el.style.paddingTop;
    el.dataset.oldPaddingBottom = el.style.paddingBottom;
    el.dataset.oldOverflow = el.style.overflow;

    el.style.display = 'block';
    if (el.scrollHeight !== 0) {
      el.style.height = el.scrollHeight + 'px';
    }
    el.style.overflow = 'hidden';
  }

  leave(el) {
    if (el.scrollHeight !== 0) {
      setTimeout(() => {
        el.style.height = 0;
        el.style.paddingTop = 0;
        el.style.paddingBottom = 0;
      });
    }
  }

  afterLeave(el) {
    el.style.display = el.dataset.oldDisplay;
    el.style.height = '';
    el.style.overflow = el.dataset.oldOverflow;
    el.style.paddingTop = el.dataset.oldPaddingTop;
    el.style.paddingBottom = el.dataset.oldPaddingBottom;
  }
}

export default {
  functional: true,
  render(h, { children }) {
    const data = {
      on: new Transition()
    };

    children = children.map(item => {
      item.data.class = ['collapse-transition'];
      return item;
    });

    return h('transition', data, children);
  }
};