vant/packages/vant-use/src/usePageVisibility/index.ts

import { ref, Ref } from 'vue';
import { inBrowser } from '../utils';

type VisibilityState = 'hidden' | 'visible';

let visibility: Ref<VisibilityState>;

export function usePageVisibility() {
  if (!visibility) {
    visibility = ref<VisibilityState>('visible');

    if (inBrowser) {
      const update = () => {
        visibility.value = document.hidden ? 'hidden' : 'visible';
      };

      update();
      window.addEventListener('visibilitychange', update);
    }
  }

  return visibility;
}