ant-design/components/affix/__tests__/Affix.test.js

import React from 'react';
import { mount } from 'enzyme';
import Affix from '..';
import Button from '../../button';

jest.useFakeTimers();

const events = {};

class AffixMounter extends React.Component {
  componentDidMount() {
    this.container.scrollTop = 100;
    this.container.addEventListener = jest.fn().mockImplementation((event, cb) => {
      events[event] = cb;
    });
  }
  getTarget = () => {
    return this.container;
  }
  render() {
    return (<div
      style={{
        height: 100,
        overflowY: 'scroll',
      }}
      ref={(node) => { this.container = node; }}
    >
      <div
        className="background"
        style={{
          paddingTop: 60,
          height: 300,
        }}
      >
        <Affix
          target={() => this.container}
          ref={ele => this.affix = ele}
        >
          <Button type="primary" >
            Fixed at the top of container
          </Button>
        </Affix>
      </div>
    </div>);
  }
}

describe('Affix Render', () => {
  it('Anchor render perfectly', () => {
    document.body.innerHTML = '<div id="mounter" />';

    const wrapper = mount(<AffixMounter />, { attachTo: document.getElementById('mounter') });
    jest.runAllTimers();

    wrapper.node.affix.refs.fixedNode.parentNode.getBoundingClientRect = jest.fn(() => {
      return {
        bottom: 100, height: 28, left: 0, right: 0, top: -50, width: 195,
      };
    });

    events.scroll({
      type: 'scroll',
    });

    jest.runAllTimers();
    expect(wrapper.node.affix.state.affixStyle).not.toBe(null);
  });
});